5.7 MPI Misfires Under Load

[ksvista]

Ok. So I have a 5.7 MPI that runs great on the trailer but misfires and pops under high rpm and load.

Specifics:
Compression 158-160
Fuel Pressure 41psi
Vacuum 19 and steady
Oil Pressure 60 psi
New cap, rotor, plugs, wires, fuel/water filter, and oil change.

Rinda OBD:

No DTC's

When running the fuel injector test. Rinda only will test #1 and #2. Errors out on #3-#8

When running the misfire test, Rinda only will test #1 and #2. Errors out on #3-#8

 

[Scott Danforth]

Check the contents of your fuel-water filter.

If you have a crab-cap change it

 

[ksvista]

Check the contents of your fuel-water filter.

If you have a crab-cap change it

thanks. already changed

 

[ksvista]

I have a 5.7 MPI ECM555 that runs great on the trailer but misfires and pops under high rpm and load.

Rinda OBD:
No DTC's


Specifics:
Balnce test injectors per Service Manual
Compression 158-160
Fuel Pressure 41psi
Vacuum 19 and steady
Oil Pressure 60 psi
New cap, rotor, plugs, wires, fuel/water filter, and oil change.


update. changed the high-pressure fuel pump today. Still 42 psi at start, 38 psi at idle, 34 psi at 1800 rpm. MerCruiser service manual says 42psi at 1800. Also bypassed the tank with a direct feed. only 1psi difference

 

[Fun Times]

What year is the boat and what is the engine serial number?

 

[QBhoy]

What rpm does it do it ? If it’s not the limiter doing it at about 5100/5200 ish rpm…it will be likely running lean. How recent was that distributor cap ? Was it tightened down carefully onto its seat ? Very easy to crack them and miss seat them home. If it was replaced before winter or any sustained period of not used. I’d never rule it out.
Also could be fuel restriction from the fuel delivery. Especially if you have cool fuel. Would also suggest having a certainty around the health of its ability to feed from an absolutely perfect electrical system.
Can I also ask if it’s a bravo or alpha it’s mated to ? I know that my alpha driven mpi had issues similar that were caused by the shift interrupt switch getting the slightest nudge from the cable assembly next to it. Was evident only at full or near full throttle. Would cut the ignition momentarily

 

[ksvista]

Thanks! Its a 2003 350 MPI with a Bravo III. I did see the service bulletin on the shift interrupt alpha and the 2 degree distributor move.

Still have fuel pressure 42 psi at start, 38 psi at idle, drops to 34 psi at 1800 rpm.

MerCruiser service manual says 42psi at 1800. Not sure?

Also bypassed the tank with a direct feed. only 1psi up

 

[Scott Danforth]

Still have fuel pressure 42 psi at start, 38 psi at idle, drops to 34 psi at 1800

And there is the problem

You are running lean. Should be 43 psi at every RPM

Now find out why

Is it an inlet restriction, crud in the pump assembly, etc

 

[ksvista]

Update. Fuel pressure 42psi under load. Still popping. sounds like through the intake

 

[Fun Times]

Make sure your cap and rotor are seated correctly/properly and not cracked anywhere.

 

[ksvista]

question. what % should the IAC duty cycle be at higher RPM or WOT?

 

[ksvista]

Diacom Rinda readings for IAC duty cycle of mid 90% at 3500rpm?
Should be 0%?
40% at idle?

 

[Fun Times]

Diacom Rinda readings for IAC duty cycle of mid 90% at 3500rpm?
Should be 0%?
40% at idle?

Off memory, the higher number of around 90% sounds about right.

Own a timing light? Try connecting to each wire and see if the flash is consistent in time at all RPM.

Could try pulling off the vacuum line to the fuel regulator and see if there's fuel in the line and if the psi goes a bit higher at idle.

What happens when you do this test? Doing it under load/way?
It's normal to only test 1 and 2 and get nothing on out for this engine model.

When running the misfire test, Rinda only will test #1 and #2. Errors out on #3-#8

 

[ksvista]

Off memory, the higher number of around 90% sounds about right.

Own a timing light? Try connecting to each wire and see if the flash is consistent in time at all RPM.

Could try pulling off the vacuum line to the fuel regulator and see if there's fuel in the line and if the psi goes a bit higher at idle.

What happens when you do this test? Doing it under load/way?
It's normal to only test 1 and 2 and get nothing on out for this engine model.

Thanks. I was just reviewing some of the rinda recorded data and read a few places that the IAC should be 0% at high rpm. Just couldn't confirm.

Was able to test the regulator and fuel pressure under load. All are good 42 no vacuum, 38 w/vacuum, and 42 under load.

Yup. Found in service manual #36 that the ECM555 that I have can only do bank #1 and #2 test (not individual cylinders)

only thing that jumped out was the IAC

 

[ksvista]

2003 350MPI ECM555. Backfire through intake under load.

Diacom Rinda readings for IAC duty cycle of mid 90% at 3500rpm?
Should be 0%?
40% at idle?

 

[vroom ZOOM]

2003 350MPI ECM555. Backfire through intake under load.

Diacom Rinda readings for IAC duty cycle of mid 90% at 3500rpm?
Should be 0%?
40% at idle?

Your IAC readings look perfect. It is near full open at 3500 so if you close the throttle rapidly, the engine won't stall, and has some air to burn the still flowing fuel with. If you are having a backfire under load, check your ignition timing and compression. Could be a hanging valve or timing out of wack.

 

[ksvista]

Your IAC readings look perfect. It is near full open at 3500 so if you close the throttle rapidly, the engine won't stall, and has some air to burn the still flowing fuel with. If you are having a backfire under load, check your ignition timing and compression. Could be a hanging valve or timing out of wack.

thanks!
Specifics:
Compression 158-160
Fuel Pressure 41psi
Vacuum 19 and steady at idle
Oil Pressure 60 psi
New cap, rotor, plugs, wires, fuel/water filter, and oil change.
Rinda OBD:
No DTC's

 

[vroom ZOOM]

thanks!
Specifics:
Compression 158-160
Fuel Pressure 41psi
Vacuum 19 and steady at idle
Oil Pressure 60 psi
New cap, rotor, plugs, wires, fuel/water filter, and oil change.
Rinda OBD:
No DTC's

your numbers look good, however, I recommend to check the timing. I had a boat that would fire out the intake like an AK-47. Timing was off. After setting to the required location, no more issues.

 

[Scott Danforth]

Compression gauge is off as new motor spec is 150psi

fuel pressure is a bit low

What is pressure under loat at WOT?

backfire under load is usually running lean

 

[ksvista]