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Unsolved Mystery MerCruiser 1999 TBI 5.7L V8

G

GlenDownUnder

New member
Hi everyone.

I'm posting this out of complete dispair after days of serious head scratching. We've been struggling to diagnose a problem despite many hours of work by several mechanics.

We have two MerCruiser 5.7L V8 TBI's in our 1999 34 foot sportscruiser attached to Bravo legs. The current motors are Quiksilver remans, installed around 2019. The starboard motor has about 600 hours on it and has been regulalry used and serviced since install. The original serial number of the problem Starboard motor is: 0L034038

A few months ago, after a reputable mechanic undertook a periodic spark plug change (Installed ITR4A15 plugs) and replaced the steering cooler, the starboard motor started having issues. Until that point it had been running very well, other than usually needing a small amount of throttle to start when cold. We'd hoped the spark plugs may help fix that.

There are three sypmtoms of the problem that we now have, possibly all related...

PROBLEM 1: After the minor works, it wouldn't start properly and when it did start, it wouldn't idle as smoothly as before. I'd describe this idle as 'lumpy' with a bit of hesitation. Every 10 - 15 seconds, it will rev slightly higher for a moment, kind of like a pulse, then settle back down. However, it will rev well in neutral and seems to be okay at higher revs when not under load. No knocking or banging etc. We seemed to fix the starting problem by putting in a new IAC valve but it still wouldn't idle properly.

We have used a OBD scan tool. Here's a video showing the numbers on ScannerPro at idle...

STARBOARD (Rough motor) -
PORT (Good motor) -

Three figures that are noticabley different to the good Port motor (which runs perfectly well) are:
  1. Spark advance is very erratic at idle. It does stabilise when revved higher.
  2. The MAP sensor voltage on the problem Starboard motor is much higher at idle and lower than the good Port motor under load. Note: The good Port figures are within the required spec.
  3. The 'BPW' figure (unsure what this is)
PROBLEM 2: The other big problem is when the motor is under load. Once it hits about 2000rpm under load it starts to knock and possibly detonate with a significant loss in power. This is very repeatable and consistent.

PROBLEM 3: This is less repeatable but when the motor warms up during testing, it is sometimes completely conks out and is then hard to restart. Sometimes it even shuts down at idle when warm. Once cooled down it starts no problem but PROBLEMS 1 & 2 above remain.

Given that we have two motors, it has helped with checking parts. We have 'swapped' some parts from the Starboard motor with a part from the good Port motor to check, other parts have been replaced with new parts. This is what has been done so far, the problem has not changed. nWhen we have 'swapped' a part from the good Port motor, the motors have both performed the same which indicates the part was functionally normally.
  1. Check timing using Merc wokshop manual method
  2. Cleaned & serviced distributor cap
  3. Swapped distributor cap
  4. Swapped distributor and rotor
  5. Swapped ignition leads
  6. Cleaned throttle body
  7. Cleaned and swapped injectors
  8. Swapped coils
  9. Swapped MAP sensors
  10. Swapped TPS
  11. New IAC valve
  12. Checked fuel delivery pressure
  13. New fuel pump
  14. New fuel regulator
  15. New fuel cooler
  16. Swapped engine fuel filters (no debris etc, filters were relatively new)
  17. Checked all vacuum lines and hoses
  18. Swapped all fuel lines, including line to rail
  19. New fuel line tank to fuel filter
  20. Checked Anti-Siphon valve
  21. Cooling system flushed
  22. Ran Starboard motor off good Port fuel tank
  23. Ran Starboard off a jerry can
  24. Swapped ECM's (MEFI 1)
  25. Swapped knock sensors
  26. Cleaned and swapped spark arrestor
  27. Checked earths/resistance on relevant electrics
  28. Pinned out electrical system clean contacts and tested all within spec when engines are running (with a consistent timing light strobe)
  29. Checked oil levels
  30. Checked engine oil quality - not milky, no air bubbles
  31. Fresh 91 unleaded fuel in both tanks
  32. After months of testing, we ended up putting in another new set of spark plugs (NGK BPR6EFS) as the others were going black. This appeared to be carbon build-up, possibly from running at idle alot and/or motor running rich. Maybe something else?
None of the above solved the problem or seemed to make any difference.

We ran compressions (cold motor) and the test results were similar to compressions that were undertaken early last year when the motor was running well. The numbers were about 17% tolerance:
  1. 205
  2. 202
  3. 205
  4. 205
  5. 202
  6. 170
  7. 192
  8. 202
As you can see, alot has been done. What are we missing after all of this?

Thanks very much for taking the time to read!
 
Last edited:
Kind of seems your IGN voltage (Battery) may be a bit too low on the starboard side not allowing the EFI system to function properly.... I see low 12.5 - 12.7 volts running... That more typically ought to be a fully charged battery a rest then you'd typically see 13.2 to the low side, average 13.8 and maybe as high as 14.2 at times running 'all depending on battery state of charge and wire/battery cable condition/s and how the boat 12v power system is rigged up too could play a part of readings.

Possibly faulty alternator, disconnected, damaged, corroded small wire or battery cable on the engine or at the engine starter stud, on/near the alternator, battery switch < both sides, 'all possibly accidently compromised while installing new spark plugs...maybe...

Check 12v right at the back if the alternator and all over the engine both engine running and off...Do check both engines as well so you know.
 
Fun Times said:
Kind of seems your IGN voltage (Battery) may be a bit too low on the starboard side not allowing the EFI system to function properly.... I see low 12.5 - 12.7 volts running... That more typically ought to be a fully charged battery a rest then you'd typically see 13.2 to the low side, average 13.8 and maybe as high as 14.2 at times running 'all depending on battery state of charge and wire/battery cable condition/s and how the boat 12v power system is rigged up too could play a part of readings.

Possibly faulty alternator, disconnected, damaged, corroded small wire or battery cable on the engine or at the engine starter stud, on/near the alternator, battery switch < both sides, 'all possibly accidently compromised while installing new spark plugs...maybe...

Check 12v right at the back if the alternator and all over the engine both engine running and off...Do check both engines as well so you know.
Click to expand...
Thanks very much mate, this is great information. Really appreciate it.

It is a quite big difference between the two motors, especially when they are practically identical.

I looked at the data recordings on the problem motor when it was under load and it sits around 12.0 - 12.1v which is even lower than at idle. I haven't recorded data for the good motor under load, that will be tomorrow's job.

Do you think that low voltage (12.0 - 12.1v) would/could cause the power drop & detonation under load?
 
Insufficient battery voltage will cause the ECU to 'sense' the engine incorrectly which causes sub-optimal outputs which makes the engine run rough, at a minimum.

Fix that issue and then proceed...
 
I just checked the NGK part number and those plugs look a lot longer than the OEM AC Delco MR43LTS plugs. I understand that the AC may be hard to find and some places tell customers that they're impossible to find (they're not), but the cross to NGK Iridium is NGK-7397.

Did you check for codes? Do you have access to a service manual?
 
jimn said:
I just checked the NGK part number and those plugs look a lot longer than the OEM AC Delco MR43LTS plugs. I understand that the AC may be hard to find and some places tell customers that they're impossible to find (they're not), but the cross to NGK Iridium is NGK-7397.

Did you check for codes? Do you have access to a service manual?
Click to expand...
I'm not sure how ScannerPro reports the codes but I think it's just 'flags'. Will look into that further. It gives a few -

Yes, we have been referring to the Mercury service manual #24 to diagnose and check specs.
 
makomark said:
Insufficient battery voltage will cause the ECU to 'sense' the engine incorrectly which causes sub-optimal outputs which makes the engine run rough, at a minimum.

Fix that issue and then proceed...
Click to expand...
Okay so we are now getting good numbers on the voltage, up to 14.1v under load and low's 13's at idle. We also re-checked the timing after this. There is a slight improvement in performance but the problem still persists albeit slightly less.

Anyone have any another ideas on what could be causing this?
 
GlenDownUnder said:
Hi everyone.

I'm posting this out of complete dispair after days of serious head scratching. We've been struggling to diagnose a problem despite many hours of work by several mechanics.

We have two MerCruiser 5.7L V8 TBI's in our 1999 34 foot sportscruiser attached to Bravo legs. The current motors are Quiksilver remans, installed around 2019. The starboard motor has about 600 hours on it and has been regulalry used and serviced since install. The original serial number of the problem Starboard motor is: 0L034038

A few months ago, after a reputable mechanic undertook a periodic spark plug change (Installed ITR4A15 plugs) and replaced the steering cooler, the starboard motor started having issues. Until that point it had been running very well, other than usually needing a small amount of throttle to start when cold. We'd hoped the spark plugs may help fix that.

There are three sypmtoms of the problem that we now have, possibly all related...

PROBLEM 1: After the minor works, it wouldn't start properly and when it did start, it wouldn't idle as smoothly as before. I'd describe this idle as 'lumpy' with a bit of hesitation. Every 10 - 15 seconds, it will rev slightly higher for a moment, kind of like a pulse, then settle back down. However, it will rev well in neutral and seems to be okay at higher revs when not under load. No knocking or banging etc. We seemed to fix the starting problem by putting in a new IAC valve but it still wouldn't idle properly.

We have used a OBD scan tool. Here's a video showing the numbers on ScannerPro at idle...

STARBOARD (Rough motor) -
PORT (Good motor) -

Three figures that are noticabley different to the good Port motor (which runs perfectly well) are:
  1. Spark advance is very erratic at idle. It does stabilise when revved higher.
  2. The MAP sensor voltage on the problem Starboard motor is much higher at idle and lower than the good Port motor under load. Note: The good Port figures are within the required spec.
  3. The 'BPW' figure (unsure what this is)
PROBLEM 2: The other big problem is when the motor is under load. Once it hits about 2000rpm under load it starts to knock and possibly detonate with a significant loss in power. This is very repeatable and consistent.

PROBLEM 3: This is less repeatable but when the motor warms up during testing, it is sometimes completely conks out and is then hard to restart. Sometimes it even shuts down at idle when warm. Once cooled down it starts no problem but PROBLEMS 1 & 2 above remain.

Given that we have two motors, it has helped with checking parts. We have 'swapped' some parts from the Starboard motor with a part from the good Port motor to check, other parts have been replaced with new parts. This is what has been done so far, the problem has not changed. nWhen we have 'swapped' a part from the good Port motor, the motors have both performed the same which indicates the part was functionally normally.
  1. Check timing using Merc wokshop manual method
  2. Cleaned & serviced distributor cap
  3. Swapped distributor cap
  4. Swapped distributor and rotor
  5. Swapped ignition leads
  6. Cleaned throttle body
  7. Cleaned and swapped injectors
  8. Swapped coils
  9. Swapped MAP sensors
  10. Swapped TPS
  11. New IAC valve
  12. Checked fuel delivery pressure
  13. New fuel pump
  14. New fuel regulator
  15. New fuel cooler
  16. Swapped engine fuel filters (no debris etc, filters were relatively new)
  17. Checked all vacuum lines and hoses
  18. Swapped all fuel lines, including line to rail
  19. New fuel line tank to fuel filter
  20. Checked Anti-Siphon valve
  21. Cooling system flushed
  22. Ran Starboard motor off good Port fuel tank
  23. Ran Starboard off a jerry can
  24. Swapped ECM's (MEFI 1)
  25. Swapped knock sensors
  26. Cleaned and swapped spark arrestor
  27. Checked earths/resistance on relevant electrics
  28. Pinned out electrical system clean contacts and tested all within spec when engines are running (with a consistent timing light strobe)
  29. Checked oil levels
  30. Checked engine oil quality - not milky, no air bubbles
  31. Fresh 91 unleaded fuel in both tanks
  32. After months of testing, we ended up putting in another new set of spark plugs (NGK BPR6EFS) as the others were going black. This appeared to be carbon build-up, possibly from running at idle alot and/or motor running rich. Maybe something else?
None of the above solved the problem or seemed to make any difference.

We ran compressions (cold motor) and the test results were similar to compressions that were undertaken early last year when the motor was running well. The numbers were about 17% tolerance:
  1. 205
  2. 202
  3. 205
  4. 205
  5. 202
  6. 170
  7. 192
  8. 202
As you can see, alot has been done. What are we missing after all of this?

Thanks very much for taking the time to read!
Click to expand...
I didn't watch the videos, so I hadn't seen that they showed data in real time, but I would like to know what program you're using to show this. The font is small and blurry, so it's hard to read when it's moving. Try zooming by pressing Ctrl+ but more important, spabilize the laptop and phone.

MEFI I, on a 1999 engine? Are you sure about that? Can you post a photo of the ECM, MAP and Knock sensors?

I'm going to start with one of the obvious comments that you made, but don't understand. Normally, idle ISN'T stable because these use 'spark stabilization' as part of the ECM's program. It's used because people who spend a bunch of money on boats want them to idle smoothly. The manual should show that going into base timing is perfermed by disconnecting a jumper or using the software to disable it- do that before attempting to set the timing. If this is similar to the Indmar engines, base timing is 10°BTDC but even if you're within a couple of degrees, it's close enough to run well- worst case, it experiences a bit of detonation at high RPM or on hard acceleration and in that case, you should see the Knock sensor do it job by pulling 5° out of the advance. AFAIK, total advance should be 40°- if you see it pass that number, find out why.

Don't show data in an unstable video and expect people to tell you what's wrong. Use the Snap Shot feature and show several of those, from a period of about a minute AND hold the phone steadier, so it's easier to focus on the data.

Also, the diagnostics software should allow you to function the IAC- check that out and make sure it does what it should. It should also show the IAC's pintle position while it's closing after key off and when it re-opens to park for next startup.

TBI, right? Check the fuel cone by starting the engine and removing the flame arrestor, then shine a timing light on the spray. At any engine speed, the fuel cone should be complete, with no 'tearing' (gaps).

Are you checking the fuel pressure when you run at all speeds? When I was at Mastercraft training, we were taught to check FP ON THE WATER, not on the trailer:
- KOEO (Key On, engine off)
- Idle
- 2000RPM
- WOT (Wide Open Throttle)

This will obviously require more than one person.
 
jimn said:
I didn't watch the videos, so I hadn't seen that they showed data in real time, but I would like to know what program you're using to show this. The font is small and blurry, so it's hard to read when it's moving. Try zooming by pressing Ctrl+ but more important, spabilize the laptop and phone.

MEFI I, on a 1999 engine? Are you sure about that? Can you post a photo of the ECM, MAP and Knock sensors?

I'm going to start with one of the obvious comments that you made, but don't understand. Normally, idle ISN'T stable because these use 'spark stabilization' as part of the ECM's program. It's used because people who spend a bunch of money on boats want them to idle smoothly. The manual should show that going into base timing is perfermed by disconnecting a jumper or using the software to disable it- do that before attempting to set the timing. If this is similar to the Indmar engines, base timing is 10°BTDC but even if you're within a couple of degrees, it's close enough to run well- worst case, it experiences a bit of detonation at high RPM or on hard acceleration and in that case, you should see the Knock sensor do it job by pulling 5° out of the advance. AFAIK, total advance should be 40°- if you see it pass that number, find out why.

Don't show data in an unstable video and expect people to tell you what's wrong. Use the Snap Shot feature and show several of those, from a period of about a minute AND hold the phone steadier, so it's easier to focus on the data.

Also, the diagnostics software should allow you to function the IAC- check that out and make sure it does what it should. It should also show the IAC's pintle position while it's closing after key off and when it re-opens to park for next startup.

TBI, right? Check the fuel cone by starting the engine and removing the flame arrestor, then shine a timing light on the spray. At any engine speed, the fuel cone should be complete, with no 'tearing' (gaps).

Are you checking the fuel pressure when you run at all speeds? When I was at Mastercraft training, we were taught to check FP ON THE WATER, not on the trailer:
- KOEO (Key On, engine off)
- Idle
- 2000RPM
- WOT (Wide Open Throttle)

This will obviously require more than one person.
Click to expand...
Thanks very much for your time Jimn, all great points.

In regards to the footage, the boat is in a wet pen where it is often unsteady on the water. Did my best to keep it stable. I posted a video instead of snapshots because the still photos wouldn't tell the story of how the numbers are fluctuating. It's small so you can see all of the (Potentially) relevant numbers on the screen at the same time.

If you can't see the numbers, try changing the Youtube video playback settings to HD 1080 rather than default 360. The video is sharp and clear, no one else has had problems viewing it that I'm aware of.

ECM is definitely a MEFI 1. It is a Delco 16220371. Engines may be 1998, boat was delivered 1999.

Base timing for this motor is 8 degrees as per the manual and the figures on the spark arrestor cover. When setting timing we are using the timing plug (That goes on the DLC) to put the ECM in service mode, timing is set at 1200rpm.

Yes, it is TBI. We have watched the spray under the spark arrestor with a timing light and it seems very clean and consistent in a cone shape.

The good Port motor idle is very stable, it doesnt fluctuate and isn't lumpy and doesnt contantly surge like the starboard motor. The starboard motor used to be like that. Sometimes the problem motor almost stalls at idle and you need to manually add throttle to stop the stall. It also lacks alot of power. This big drop in power under load happened after the spark plugs were changed as mentioned above. The boat ran a week earlier and went perfectly well. (The hull had also recently been cleaned so that power drop couldn't possibly be attributed to fouling on the hull, especially when it happened a week between journeys).

The IAC valve value does change so that indicates it is moving. I'll have to check the numbers on it. It is a brand new genuine Mercruiser IAC.

We haven't checked fuel pressure at all of the run speeds you have suggested. I'll also record the data when running. That will have to be a job when I have someone that can help.

I'll try and get pics of the sensors and post those too.

Thanks again.
 
GlenDownUnder said:
Thanks very much for your time Jimn, all great points.

In regards to the footage, the boat is in a wet pen where it is often unsteady on the water. Did my best to keep it stable. I posted a video instead of snapshots because the still photos wouldn't tell the story of how the numbers are fluctuating. It's small so you can see all of the (Potentially) relevant numbers on the screen at the same time.

If you can't see the numbers, try changing the Youtube video playback settings to HD 1080 rather than default 360. The video is sharp and clear, no one else has had problems viewing it that I'm aware of.

ECM is definitely a MEFI 1. It is a Delco 16220371. Engines may be 1998, boat was delivered 1999.

Base timing for this motor is 8 degrees as per the manual and the figures on the spark arrestor cover. When setting timing we are using the timing plug (That goes on the DLC) to put the ECM in service mode, timing is set at 1200rpm.

Yes, it is TBI. We have watched the spray under the spark arrestor with a timing light and it seems very clean and consistent in a cone shape.

The good Port motor idle is very stable, it doesnt fluctuate and isn't lumpy and doesnt contantly surge like the starboard motor. The starboard motor used to be like that. Sometimes the problem motor almost stalls at idle and you need to manually add throttle to stop the stall. It also lacks alot of power. This big drop in power under load happened after the spark plugs were changed as mentioned above. The boat ran a week earlier and went perfectly well. (The hull had also recently been cleaned so that power drop couldn't possibly be attributed to fouling on the hull, especially when it happened a week between journeys).

The IAC valve value does change so that indicates it is moving. I'll have to check the numbers on it. It is a brand new genuine Mercruiser IAC.

We haven't checked fuel pressure at all of the run speeds you have suggested. I'll also record the data when running. That will have to be a job when I have someone that can help.

I'll try and get pics of the sensors and post those too.

Thanks again.
Click to expand...
The reason I'm surprised that it's MEFI I is because Indmar/Mastercraft started using EFI in late-'93, with the Corvette LT-1, followed by TBI in '94. They both used MEFI I, but changed to MEFI II in about '96 or '97. I don't remember the time line for all, but when I went to training in early '98 and we covered MEFI I, II, CEFI III, then MEFI IV (IIRC) in late 2000. The changes between MEFI I and IV were significant.
 
with the reman engines, the 'year' doesn't really matter.

I think you observed the big indicator back when you posted the videos....the difference between the MAP sensors...the starboard engine isn't making enough vacuum to idle very well - its indicating ~ 9-10" Hg....where the port is showing 16-17" Hg... I'd be inclined to focus there.

Reviewing your history of swaptronics, looks like most of the bolt-on pieces were swapped...did you check the vacuum tubing to make sure it was in good shape? The other obvious thing is to recheck the plug wires to make sure they are connected correctly (both ends and in the correct order). Could also be one or more of the plug wires are bad (internally)...wouldn't hurt to given them a continuity check while shaking (wiggling) them. Hooking up a vacuum gauge to verify what the ECU is seeing wouldn't hurt...
 
The other possibility I can imagine is the wiring harness or a connector was damaged during a maintenance operation before the problem started...
If it happened, it's probably minor (hard to see) and will take some effort to find. Something that will adversely impact a sensor value read by the ECU (accuracy) but not so much to trip the DTC check - in other words, you will never see a trouble code (flag?) on the scanner.
 
Looking up your serial number it verifies
MEFI-1
MR43LTS or BPR6EFS gap .045
Timing 8* BTDC using base timing mode
Fuel pressure 30 psi at 1800 RPM (also should be that at idle)

Your voltage should read 13.5 to 14.5V after it has first reved above 1500 RPM
If motor starts but does not rev up automatically and then settle back down the ALT won't be putting out full change voltage

Your compression numbers are way high or that isn't psi or your gauge has taken a dump. A new motor should be 150 psi and being that high I would look for water or gas in the cylinders. Pull plugs and crank it over with coil disconnected to see if anything spits out
 
makomark said:
with the reman engines, the 'year' doesn't really matter.

I think you observed the big indicator back when you posted the videos....the difference between the MAP sensors...the starboard engine isn't making enough vacuum to idle very well - its indicating ~ 9-10" Hg....where the port is showing 16-17" Hg... I'd be inclined to focus there.

Reviewing your history of swaptronics, looks like most of the bolt-on pieces were swapped...did you check the vacuum tubing to make sure it was in good shape? The other obvious thing is to recheck the plug wires to make sure they are connected correctly (both ends and in the correct order). Could also be one or more of the plug wires are bad (internally)...wouldn't hurt to given them a continuity check while shaking (wiggling) them. Hooking up a vacuum gauge to verify what the ECU is seeing wouldn't hurt...
Click to expand...
The year definitely can matter but there's a lot that can vary-

I don't remember seeing anything about whether this is a Vortec or non-Vortec.

- Is it a long block or short block?
- Did the relacemen come with an intake manifold, or is the original being used?
- What acessories and sensors were replaced and what was the quality of the replacements?
- How was the harness treated during the changeover?
- Which type of gasket set was used?
- #6 compression is low- why?

I have seen 170 on compression gauges and used more than one to compare but really, as long as they're within a fairly close range, it shouldn't matter.
 
GlenDownUnder said:
Thanks very much for your time Jimn, all great points.

In regards to the footage, the boat is in a wet pen where it is often unsteady on the water. Did my best to keep it stable. I posted a video instead of snapshots because the still photos wouldn't tell the story of how the numbers are fluctuating. It's small so you can see all of the (Potentially) relevant numbers on the screen at the same time.

If you can't see the numbers, try changing the Youtube video playback settings to HD 1080 rather than default 360. The video is sharp and clear, no one else has had problems viewing it that I'm aware of.

ECM is definitely a MEFI 1. It is a Delco 16220371. Engines may be 1998, boat was delivered 1999.

Base timing for this motor is 8 degrees as per the manual and the figures on the spark arrestor cover. When setting timing we are using the timing plug (That goes on the DLC) to put the ECM in service mode, timing is set at 1200rpm.

Yes, it is TBI. We have watched the spray under the spark arrestor with a timing light and it seems very clean and consistent in a cone shape.

The good Port motor idle is very stable, it doesnt fluctuate and isn't lumpy and doesnt contantly surge like the starboard motor. The starboard motor used to be like that. Sometimes the problem motor almost stalls at idle and you need to manually add throttle to stop the stall. It also lacks alot of power. This big drop in power under load happened after the spark plugs were changed as mentioned above. The boat ran a week earlier and went perfectly well. (The hull had also recently been cleaned so that power drop couldn't possibly be attributed to fouling on the hull, especially when it happened a week between journeys).

The IAC valve value does change so that indicates it is moving. I'll have to check the numbers on it. It is a brand new genuine Mercruiser IAC.

We haven't checked fuel pressure at all of the run speeds you have suggested. I'll also record the data when running. That will have to be a job when I have someone that can help.

I'll try and get pics of the sensors and post those too.

Thanks again.
Click to expand...

I mentioned using the snapshot feature because it's impossible to watch more than one parameter, so this shows ALL from that point in time and the real takeaway is that it shows any that are out of range. THAT'S the main point- are they in range, out of range and how close to the normal range.

I would definitely compare the ITR4A15 plugs with an AC MR43LTS- if the NGK is longer, check the ground electrode. I would check all of them-the gap, condition, evidence of running rich or lean, etc. The plug wires are a possible source for the problems, too- they don't last forever and I have used wires, cap/rotor that come with lifetime warranty because of this in boats AND in my personal vehicles.

While you're getting photos, can you post the ECM, MAP sensor, Knock sensor and wire colors for the relays?

Make sure the MAP sensor tube doesn't have liquid in it- that can cause odd issues, especially if the sensor is mounted low.
 
In this case one of the main things that is incorrect regarding the spark plugs is that the heat range that was installed temporarily by using ITR4A15 is ‘it is/was a bit higher (2 numbers hotter) vs the recommend AC MR43LTS or more widely used after using the AC, NGK-BPR6EFS… With NGK, the higher the number, the cooler the plug range… with this engine model, I’d recommend only using the ACMR43LTS or NGK-BPR6EFS.

1741141531064.jpeg


May I ask, what did you find wrong to repair the lower voltage readings?
 
If MR plugs are hard to find, CR43LTS can be subbed, M stands for Marine, C stands for Commercial vehicle

Some places say the MR are hard to find, but after wasting a trip to one place, I found them less than a mile from my house, at what used to be Quest- it's now Advance Auto Parts.
 
If the problem started after work was done have we doubled checked the work?
It would seem strange a "new" problem pops up after new work was done.
I like the KISS method. Keep It Simple Stupid
 
You are likely chasing a physical air-flow or mechanical problem that is masquerading as a sensor issue. The first thing i see is the MAP-sensor voltage. High voltage means low vacuum At idle, a healthy engine pulls a strong vacuum. Your starboard engine is failing to do so.

If it can't breathe out, it can't pull a vacuum in the intake. This results in the High MAP / High BPW reading.

It could be that the internal exhaust flappers designed to prevent water from washing back into the engine failed. When these age or overheat, the rubber burns off, the metal hinge breaks, and the flapper falls down into the exhaust Y-pipe, causing a massive blockage.
At idle, it chokes. Once you hit 2000 RPM under load, the backpressure becomes so severe that the engine chokes on its own exhaust gases, causing detonation/knocking, massive power loss, and extreme heat buildup (which causes the engine to die when warm).

You also mentioned that the problems started immediately after a mechanic replaced the steering cooler. It could be that during the install the mechanic knocked off a large vacuum line or damaged the intake manifold gasket while working back there. A vacuum leak will also cause high MAP voltage. While vacuum leaks typically cause a high, surging idle rather than a lumpy rich one, the loss of vacuum could be severe enough to trigger your symptoms.

Last thing to check is the ECT sensor, if it has failed and is telling the ECM the engine is at -40 degrees, the ECM will command a massive BPW (fuel dump) to "choke" the cold engine. This causes a lumpy idle and black plugs. Once the engine actually warms up, this extreme over-fueling floods the engine, causing it to die and making it incredibly hard to restart hot.
 
b_lange said:
You are likely chasing a physical air-flow or mechanical problem that is masquerading as a sensor issue. The first thing i see is the MAP-sensor voltage. High voltage means low vacuum At idle, a healthy engine pulls a strong vacuum. Your starboard engine is failing to do so.

If it can't breathe out, it can't pull a vacuum in the intake. This results in the High MAP / High BPW reading.

It could be that the internal exhaust flappers designed to prevent water from washing back into the engine failed. When these age or overheat, the rubber burns off, the metal hinge breaks, and the flapper falls down into the exhaust Y-pipe, causing a massive blockage.
At idle, it chokes. Once you hit 2000 RPM under load, the backpressure becomes so severe that the engine chokes on its own exhaust gases, causing detonation/knocking, massive power loss, and extreme heat buildup (which causes the engine to die when warm).

You also mentioned that the problems started immediately after a mechanic replaced the steering cooler. It could be that during the install the mechanic knocked off a large vacuum line or damaged the intake manifold gasket while working back there. A vacuum leak will also cause high MAP voltage. While vacuum leaks typically cause a high, surging idle rather than a lumpy rich one, the loss of vacuum could be severe enough to trigger your symptoms.

Last thing to check is the ECT sensor, if it has failed and is telling the ECM the engine is at -40 degrees, the ECM will command a massive BPW (fuel dump) to "choke" the cold engine. This causes a lumpy idle and black plugs. Once the engine actually warms up, this extreme over-fueling floods the engine, causing it to die and making it incredibly hard to restart hot.
Click to expand...
I think he's gone- hasn't posted since March, 2025.
 
In case GlenDownUnder is still watching, let us know what happened.

Also, in case someone else has a similar problem:

Opening the throttle is an easy way to verify a bad ECT- hard hot start with too much fuel needs more air, so it starts.

If the flapper is damaged and part of it is stuck in a place where it restricts the flow, the sound of the exhaust will be different- it can also end up where it doesn't restrict the flow of exhaust, as mine did.

If the MAP sensors are the same, swapping them might show that one is faulty. Also, a vacuum pump can be connected to the sensor and pumping a couple of times (no high vacuum) should cause the voltage to vary- the sensor doesn't care what's creating the vacuum,. so if the pressure matches the other engine, it should make a difference on the engine that has a problem, assuming the vacuum isn't correct.

If some kind of liquid is in the tube going to the MAP sensor, it can alter the reading- disconnecting the tube at the sensor while it's running should clear it, but the sensor may need to be removed and positioned so any liquid can drain out. Also, the attachment is from the Mastercraft manual and shows that the sensor shouldn't be cleaned with liquid cleaner or solvent.
 

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