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Mercruiser 454 7.4 MPI low power, surging, backfires

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freebeerisgood

Member
I am having some surging, backfire and low power issues with my Mercruiser 454 7.4 MPI I/O (Serial 0F803353). It has a tough time staying running at idle, backfires at idle, and it won’t make over 2000 RPM in the water under load, but will rev higher in idle.

The boat sat for 5 years covered on the trailer while I was out of state, and I am in the process of getting it back running - Here is what I have done in order.
  • 60 gallons Fresh gas (tank was stored dry)
  • Changed engine oil & Drive oil
  • New fuel filter
  • New water pump
I ran the engine and it had issues immediately at idle.
  • Filled the fuel filter and VST with B12 fuel injector cleaner, and it cleaned up the idle a bit.
  • Changed Plugs
  • New Rotor cap & button (ignition timing was checked by a mercruiser mechanic)
Ran boat in the water, I was hoping to run it hard and it would clean itself out, no luck. It would not make more than 2000RPM under load, still had a hard time running at idle.
  • Pulled fuel filter, no water in gas.
  • Pulled fuel rail, rebuilt the injectors (O-rings & filter baskets), and cleaned them with on a bench tester with a can of gumout, it cleaned up the way they spray a lot.
  • Pulled VST and cleaned it out, screen looked clear, there was some gunk in the bottom but nothing bad.
  • Put it back together and checked fuel pressure on the fuel rail, it was low @ anywhere from 30-35PSI, wavered a lot.
  • Changed fuel pressure regulator(used mercruiser part), pressure stays steadier while engine is running, but still low at 30 PSI. It's also 30 PSI when the ignition is turned on, before starting.
My next step was to probably find a high pressure fuel pump and pull the VST out and change it. It seems my issues are stemming from low fuel rail pressure, I understand it supposed to be 43PSI?

Before I do, I’m curious if anyone has any other ideas?

Thanks!
Freebeerisgood
 
wouldn't hurt to get the ECU scanned for trouble codes...

Looks like it has the EST distributor and they are known to have issues with the pickup coil...

agreed that low fuel pressure is the primary focus right now.
 
makomark said:
wouldn't hurt to get the ECU scanned for trouble codes...

Looks like it has the EST distributor and they are known to have issues with the pickup coil...

agreed that low fuel pressure is the primary focus right now.
Click to expand...
I appreciate the reply.

For the scan, I agree. Im actually having a tough time finding another mechanic that will find time to let me bring the boat in, they don't seem to want to work on I/Os.

One thing I omitted accidentally while I was typing this out in a hurry was that the distributor was changed (with a “known” good used one) because initial attempt to start had a no spark condition intermittently. I almost bought a new one when the mechanic I brought it to offered the used part for $50 plus install. However, for reasons I wont go into, this mechanic will not get my business anymore.
Since the no ignition issue was initially fixed, Ive been working off the assumption that the other symptoms I am troubleshooting are not ignition related, however, I am happy to be wrong.
 
You likely have the MEFI-1 or -2 ECU...you can use the old mechnic's trick to read any stored codes if you are careful and understand basic electronics...there are plenty of www pages that will show you how to connect a jumper, a LED and a resistor to the diagnostic port connector to read any stored codes.

hopefully, the distributor is fully functional and isn't a contributor...
 
The VST list 37 Psi and spec is 24-38 Psi
Your low but not so low that it's having this much trouble. Regulator will be 805227A1

After sitting for 5 years and low power I would check compression

When I had my 502 built for 530HP I removed the VST and went with Cool fuel to get pressure up to 43
 
makomark said:
You likely have the MEFI-1 or -2 ECU...you can use the old mechnic's trick to read any stored codes if you are careful and understand basic electronics...there are plenty of www pages that will show you how to connect a jumper, a LED and a resistor to the diagnostic port connector to read any stored codes.

hopefully, the distributor is fully functional and isn't a contributor...
Click to expand...

AllDodge said:
The VST list 37 Psi and spec is 24-38 Psi
Your low but not so low that it's having this much trouble. Regulator will be 805227A1

After sitting for 5 years and low power I would check compression

When I had my 502 built for 530HP I removed the VST and went with Cool fuel to get pressure up to 43
Click to expand...

A couple things I just did to continue down this rabbit hole.

1) I threw on a new cap and rotor button just because I had one.
2) Read the codes with a jumper wire & a multi meter's beep - codes 12 & 33 are present. I know 12 is no issue, 33 seems to be MAP sensor related, possibly a vacuum leak
3) Pulled the VST return line, plugged the line with my finger and turned the key on, fuel pump made 59psi, tells me the pump is good and strong(I think). I ran the engine and the fuel pressure running was only 21, but steady as a rock (considering grabbing a second gauge to verify accuracy just because I am OCD). Engine fires up strong, idles great for a short bit (20-45 seconds), then starts to get weird, lower RPM and then dies. 805227A1 is the fuel pressure regulator I used, I am assuming it is good because after replacing, the fuel pressure is similar to the other, but I have plenty of experience with bad parts from my time in aviation, so its in the back of my mind. The fuel pressure in the rail stays steady when the key is turned off, so I assume there is not a stuck injector, also in the back of my mind.

I will verify compression on all 8 at some point shortly.

Right now I am going to work on the code 33 issue -- I need to figure out to verify if the sensor is good using a multi meter, whether is a resistance check or output voltage check. Any thoughts on that? I'll take to google shortly.

I appreciate the help.
 
I think the best (non-scan tool) approach is to measure the MAP sensor output voltage with it connected and a vacuum pump applied to the hose barb...so key ON - engine NOT running...
 
Code 33 can be sensor wire short or open. Your MEFI 1 is not much when it comes to diagnosis, there just is not much programed into the MEFI 1.

It can be the lead, the MAP or the 5V supply voltage coming from the ECM
With key ON see if you have exactly 5V on the MAP gray wire, or TPS
 

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FWIW, the MEFI manual states that other conditions that cause rough or unstable idle can set the 33 code...

If you just unplug the MAP sensor, the ECU will use a default value and control the fuel (pulse widths) using the TPS input...
 
AllDodge said:
Code 33 can be sensor wire short or open. Your MEFI 1 is not much when it comes to diagnosis, there just is not much programed into the MEFI 1.

It can be the lead, the MAP or the 5V supply voltage coming from the ECM
With key ON see if you have exactly 5V on the MAP gray wire, or TPS
Click to expand...

I have just under 5v (4.7) on the gray MAP wire
 
That can be the issue, IF you meter is accurate

The ECM uses the 5V as a reference voltage to determine how every other sensor reports it's findings
 
AllDodge said:
That can be the issue, IF you meter is accurate

The ECM uses the 5V as a reference voltage to determine how every other sensor reports it's findings
Click to expand...
Interesting. I just checked voltage on my trucks battery, which has a scan gauge installed. The scan gauge showed 13.0, and my meter showed 12.71.

I also just slaved in a MAP sensor I found at an auto parts store. The OEM was a GM part, the one I found looked to be a cross reference part from AC Delco. NO CHANGE to the way the engine runs.
 
makomark said:
FWIW, the MEFI manual states that other conditions that cause rough or unstable idle can set the 33 code...

If you just unplug the MAP sensor, the ECU will use a default value and control the fuel (pulse widths) using the TPS input...
Click to expand...
AllDodge said:
Looks like your 5v ref is ok
Click to expand...
Im thinking the 5v is good as well.

I disconnect the MAP and it actually made the idle worse, it almost instantly died. If it's supposed to then get input from the TPS, maybe the TPS has an issue? When the idle is strong, and I give it some throttle, the response is all over the place and very very inconsistent, sometimes there is almost no response, and sometimes it drastic.

I still don't fully understand why I only have 20psi on the fuel rail with a new regulator & a pump that I know can make 59psi.
 
3) Pulled the VST return line, plugged the line with my finger and turned the key on, fuel pump made 59psi, tells me the pump is good and strong(I think).
Click to expand...
I don't see how your finger could hold 59psi
 
Your low pressure has to be the new Reg or injectors are leaking bad

Could test the Reg on the bench using air pressure and adjustable Reg. Apply air the base port and increase air and it should open close to 37
 
AllDodge said:
Your low pressure has to be the new Reg or injectors are leaking bad

Could test the Reg on the bench using air pressure and adjustable Reg. Apply air the base port and increase air and it should open close to 37
Click to expand...
I did not take them and have them bench tested, but at this point I am going to so I know they are not sticking open. I will also check the reg with controlled air pressure.

makomark said:
what the pump makes deadheaded vs what it can maintain in the system are two different things...
Click to expand...
I agree. After I have the injectors bench tested we will see how it goes, if they are not leaking then the fuel pump will probably be next.


Before I ripped the intake off I decided to dig out my timing light and check the timing. I have been working off the assumption that the guy that changed the distributor, adjusted it correctly. I want to verify.

At 800 to 1000 RPM (best place it idles without dying), the timing is a decent way ATDC. The timing marks are long gone, but the groove for 0 is easy to see, and the timing is about 30MM ATDC (left of the groove) - and just so I am clear here, on a balancer that spins clockwise, ATDC is to the left of the groove, if the timing was ~8 BTDC it would be before (right) the 0 degree notch hits TDC. Am I correct on this?

The timing should be 8 degrees BTDC at idle correct? If so, I think I found an issue.

Also, my timing light is very basic, it is not adjustable.
 
First, the bench testing i mentioned was for the regulator not the injectors. You want to find out if it opens at 21 or 37 psi.

Don't have my manual right now but think your timing should be 10 btdc
 
AllDodge said:
First, the bench testing i mentioned was for the regulator not the injectors. You want to find out if it opens at 21 or 37 psi.

Don't have my manual right now but think your timing should be 10 btdc
Click to expand...
Yes, I will use my compressor to test the regulator. I also plan to have a shop bench the injectors in a propper test machine.

I have read both 8 and 10 BTDC. Either way its currently way ATDC. Seem the guy that did my distributor missed something.
 
Looking at the front of the engine, BTDC is to the LEFT of the ZERO line of the timing tab and ATDC is to the RIGHT.
 
AllDodge said:
Meaning it's all good now or something else?
Click to expand...
I should have been more clear - its all good now, well back to 98%. Im 100 RPM short of what its always turned, 4200. Ill check compression when I get some time and check the timing one more time. My throttle is a little sticky and may need an adjustment, I may make up my last bit of RPM there.

Cant say I understand the 20psi on the fuel rail but its performing, fuel flow is exactly like it always has been as various RPMs and trims.
 
Good deal
Your rpm is 400 low and reducing pitch about 2 inch will get you closer. Will also give you better hole shot and may pick up a couple mph. Will also help it run easier
 
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