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454 MAG mpi chugs/stalls at idle - running lean

V

vroomZOOM

Regular Contributor
S/N 0L394105 (serial number is off the identical starboard engine, tags are missing off the problematic port engine)

Hello, trying to figure out a 454 mag mpi horizon that chugs and stalls and won't idle. It will start up every time, and instead of settling to idle it will stall out. I have a feeling it is too lean because:

- It will stay running or at least try to stay running if I unplug the MAP sensor (ECM runs richer AFAIK)
- When it starts, it revs up a lot more than the starboard engine

Here is what I have done so far:
- Complete reman longblock
- New mercruiser exhaust
- Checked the fuel pressure. 45 psi even when the engine stalls.
- Fuel quality. No water found, starboard engine runs just fine from the same pickup and tank using the crossover valve.
- checked for codes. No codes found.
- swapped complete intake with TPS, IAC, IAT and MAP sensor from running engine. No change.
- Brand new OEM fuel injectors
- swapped complete distributor and plug wires from running engine. No change.
- New spark plugs (MR43T)
- tried unplugging the knock sensor. No change.
- ran the engine from separate harness and key switch. No change
- New alternator
- swapped wire harness from running engine. No change. Other engine runs with both harnesses just fine
- Swapped ECM from running engine. No change, other engine runs with both ECMs.
- Checked for shorts in injector wiring
- Tried changing injector fuse. No change.
- Checked the timing and tried to advance and retard the timing, no dice. I know I can get it close enough to run. I took the dizzy in and out of the other engine a million times and it always started and ran. When the engine is running with the map sensor unplugged the timing seems to be bouncing around 8 degrees but sometimes hops up to 20.

Unplugging the MAP or ECT sensors seems to make the engine run better.

Any advice appreciated, Thanks!
 
did the "complete reman long block" ever run as expected after it was installed?

Can you ellaborate on "ran the engine from separate harness and key switch"???
 
makomark said:
did the "complete reman long block" ever run as expected after it was installed?

Can you ellaborate on "ran the engine from separate harness and key switch"???
Click to expand...
No, it never ran right. Previous engine also ran like this. I thought the issue was in the longblock so I got a new one.

Separate harness and key switch aka boat in a box, see picture
DSC_0945.JPG
 
ok...I wish I had your troubleshooting budget....

Unplugging the sensors just causes the ECU to substitute default values. The timing will just around as the ECU is always adjusting it.

Have you verified the alternator output is stable and at a reasonable value (volt-wise)?

Do you have a scan tool or the RINDA software and adapters?

if the relays stayed in the harness when you swapped that, they can be ruled out....if not, changing them would be a good idea.

The only other thing that comes to mind is the handful of sensors being out of calibration...in that condition, they wont set a DTC but may cause the engine to run rough...thats were the scan tool/RINDA sw would be real handy to have. You can get the 'should be' values from the other (good) engine...
 
makomark said:
ok...I wish I had your troubleshooting budget....

Unplugging the sensors just causes the ECU to substitute default values. The timing will just around as the ECU is always adjusting it.

Have you verified the alternator output is stable and at a reasonable value (volt-wise)?

Do you have a scan tool or the RINDA software and adapters?

if the relays stayed in the harness when you swapped that, they can be ruled out....if not, changing them would be a good idea.

The only other thing that comes to mind is the handful of sensors being out of calibration...in that condition, they wont set a DTC but may cause the engine to run rough...thats were the scan tool/RINDA sw would be real handy to have. You can get the 'should be' values from the other (good) engine...
Click to expand...
Yes I have diacom. Here are some snapshots of diacom when I try to start the engine:

1753468876750.png


1753468950038.png


1753469019661.png


All the numbers look ok, MAP reads baro at 30 in hg. Relays stayed in the harness and I also tried swapping them out. I also tried running witht he alternator disconnected but no use.
 
not the battery voltage - the big dips are from the restarts but the peak value looks low to me - most use 13.8VDC as a minimum...

where the traces shown taken from the engine with the old or new alternator?
 
another thing that will help - ensure the battery switches are set so each engine runs and charges its own battery...if there is a house battery, isolate it until the issue is resolved.
 
makomark said:
another thing that will help - ensure the battery switches are set so each engine runs and charges its own battery...if there is a house battery, isolate it until the issue is resolved.
Click to expand...
That is how my boat is set up. The port engine only charges its own battery, and the starboard charges its battery and the house battery. The voltage traces were with the new alternator.
 
I'd be inclined to ensure the battery cables are sound and have no voltage drop through them...

The other thing that comes to mind is the coil and the interconnect between the coil and the ignition module...the factory harnesses have an RF filer in them and I know of a few that have failed...that may be an easy swap check with the other engine...
 
makomark said:
I'd be inclined to ensure the battery cables are sound and have no voltage drop through them...

The other thing that comes to mind is the coil and the interconnect between the coil and the ignition module...the factory harnesses have an RF filer in them and I know of a few that have failed...that may be an easy swap check with the other engine...
Click to expand...
Tried swapping the jumper from the coil to the ICM. No luck there. I put a multimeter between the grounds on both engines and got about 0.03 volts, which tells me the big ground cables are in good shape. will test the positive cables now too.
 
makomark said:
Best to do the test with some current flowing between the measurement points...
Click to expand...
Here is what I tested right now:

ECM positive (orange) to battery positive - 1.2V while cranking, 0.3 volts while it was running for the short period of time. Doesn't go past 0.3 volts when it stalls. Negative about 0.2 volts when running and stall between ECM ground and battery negative.

Injector harness: testing for voltage at the injector fuse, lowest was about 12.7 with engine off and all injectors static ON. Each injector draws about 0.75 amps each when testing at J1-1 and J1-17. i.e. therefore the wiring to the injectors and the injector coils are good. Tried the ECM from the running engine once again and no luck.
 
have you tried alternator case to battery ground with engine running?

also, Alternator output to starter solenoid (battery cable connection) and to ECU +12VDC input...
 
makomark said:
have you tried alternator case to battery ground with engine running?

also, Alternator output to starter solenoid (battery cable connection) and to ECU +12VDC input...
Click to expand...
Alternator case to battery ground is about 0.03 volts difference. Alternator output to ECU is 0.8 volts tops. Alternator positive to battery positive jumps around depending on if cranking or running but never goes more than +- 0.5V
 
After reading/re-reading your initial post, it sounds to me to be a fuel related problem. You have swapped the ignition parts and the other engine runs fine with them. Either a restricted fuel line or bad from stock OEM injectors. Once I found a wire broken inside the insulation going through the harness. Was a bugger and took several days but the colored insulation was fine but wire completely disconnected so that it would run fine and then sputter.

Good Luck Sir.
RK
 
Roadking57 said:
After reading/re-reading your initial post, it sounds to me to be a fuel related problem. You have swapped the ignition parts and the other engine runs fine with them. Either a restricted fuel line or bad from stock OEM injectors. Once I found a wire broken inside the insulation going through the harness. Was a bugger and took several days but the colored insulation was fine but wire completely disconnected so that it would run fine and then sputter.

Good Luck Sir.
RK
Click to expand...
fuel pressure is constant 45psi all the time. Both wire harnesses work fine on the other engine. Both sets of injectors work fine on the other engine.
 
KISS...Keep It Simple Sweetheart !! Fuel & Fire, and Timing. If these are all correct it should run right. I have had a cam lobe wear off on an older pontiac 400. It was popping back thru the carburetor. These are just my (old guy) stories. Bouncing them off you to maybe help think of something different.
I mean unless the crankshaft is cast wrong (which I doubt) this engine should run good. Would like to know what you test or find further plz.

Please keep us updated
Thanks
RK
 
Did you ever find the issue? I'm having the same issue on a reman block, changed so many sensors and pumps, all is showing correctly on the scan tool, except the IAC is at 70 when it should be 45-60...hissing all the time at idle and runs rough.
 
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