5.7 MPI Misfires Under Load

[ksvista]

Sorry. Does this graph look normal?

 

[dubs283]

I would think at idle speed the iac would be cycling at least a bit in the upper % range. Seems stuck near 98% with a sudden drop. Did you adjust throttle position at/near the drop?

An iac is a common failure component on mpi engines and will cause running issues without triggering a fault code. Most common issue is the engine won't hold an idle/dies at idle speed but otherwise performs fine at off idle through wot.

Have you changed the iac?

I'd be curious to see a graph of the tps at various rpms

 

[ksvista]

thanks! TPS info. Does the sensor power look normal?

 

[ksvista]

Thanks. Not sure on the compression gauge. Its new.

However, Fuel Pressure 42psi @ 1800 under load

 

[ksvista]

another sampling. thanks

 

[Scott Danforth]

What does your fuel pressure look like?

Most of the time, backfiring under load is either running lean, or your timing is off

 

[dubs283]

Tps graph for sensor and voltage appear normal. Can tell the throttle remained open/steady during the hesitation event.

The iac sensor chart seems to closely follow the variations in rpm. Might try unplugging the iac once the engine is above 1000 rpm or so. Should not affect the performance at all however it may sound an audio alarm/possibly limit rpm. This check would rule out a faulty iac.

I will again mention that an iac to me has pretty much become a routine maintenance item on older vintage mpi engines (555). The issue with the iac routinely failing is they often get overclocked when operators run the engine at idle/just off idle speeds for extended periods.

An automobile with an automatic transmission has a torque converter that will alleviate the load from the engine at lower rpms. Your boat is essentially a direct drive that gets "back fed" load from the drive and can struggle to maintain even rpms at low speeds, hence the iac constantly working to even out the load/rpms

Mercury solved the failing iac problem by abating it and using the ecm to control throttle plate opening at lower rpms

 

[ksvista]

What does your fuel pressure look like?

Most of the time, backfiring under load is either running lean, or your timing is off

thanks! fuel pressure is in spec 42psi at 1800+. Timing is set to #8 on the distrib and then controlled by the cam pos sensor. this has been a real head scratcher!!

 

[ksvista]

Tps graph for sensor and voltage appear normal. Can tell the throttle remained open/steady during the hesitation event.

The iac sensor chart seems to closely follow the variations in rpm. Might try unplugging the iac once the engine is above 1000 rpm or so. Should not affect the performance at all however it may sound an audio alarm/possibly limit rpm. This check would rule out a faulty iac.

I will again mention that an iac to me has pretty much become a routine maintenance item on older vintage mpi engines (555). The issue with the iac routinely failing is they often get overclocked when operators run the engine at idle/just off idle speeds for extended periods.

An automobile with an automatic transmission has a torque converter that will alleviate the load from the engine at lower rpms. Your boat is essentially a direct drive that gets "back fed" load from the drive and can struggle to maintain even rpms at low speeds, hence the iac constantly working to even out the load/rpms

Mercury solved the failing iac problem by abating it and using the ecm to control throttle plate opening at lower rpms

Thanks. Just ordered a new IAC. Probably get it installed tomorrow and take another run at it. What are your thoughts on the sensor power chart? Normal?

 

[dubs283]

What are your thoughts on the sensor power chart? Normal?

Nothing abnormal looking to me. The voltages follow the sensor movements pretty close and are within the 5 volt range. With no faults present there's no reason to suspect an issue with the 5 volt ref from the ecm. With a graphing/recording of data you look for consistency (which you have) for normal operation. When you see variables in the data that's when you start to suspect certain components, sometimes it takes multiple tests on different components.

Reason I suspect the iac is the reference closely follows/matches the rpm fluctuations. If you disconnect a sensor it will trigger a fault under normal operation. If you disconnect the iac it should trigger a fault but removing it from the system will isolate it and if the engine performs better at higher rpm its most likely the issue. I can't recall that I've ever had a faulty/disconnected iac cause guardian/rpm reduction

 

[Scott Danforth]

Post 3 and 6 have different fuel pressures

What does the fuel pressure go up to when you disconnect the regulator?

 

[WIMUSKY]

Merged threads. Please only create 1 thread per issue....Thx

 

[jcole108]

Clean you spark arrester.

 

[tank1949]

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

AT NIGHT, check for leaking fire on plug wires. You will see it, if there is an issue.

 

[ksvista]

 

[ksvista]

Could this be a misfire/pop

 

[ksvista]

can someone read/interpret my rinda recordings? I can send via e-mail. misfire under load. 2003 350 MPI

 

[alldodge]

Post the file here, folks like @tpenfield , @muc and couple others can have a look and even you can post screen shots

 

[tpenfield]

If need be you can send me an IM, as you might have to email it (done that in the past).

 

[Scott06]

Why dont you let people know what you are working on and what issue you are trying to solve

If you have an MPI engine and you are chasing a possible misfire have you changed the distributor cap which is a know failure point on them.