454 mag mpi chuggles/stalls

[alldodge]

It is a puzzler and I don't have anymore ideas

 

[kd4pbs]

Grr. That is frustrating, my friend! Nothing much fun about fixing a poorly running open-loop EFI system.
Have you swapped the coolant temperature sensor? I know things can go sideways real quick if the ECM doesn't know the correct engine temperature. And that temp isn't shown in your graphs.

 

[vroom ZOOM]

Grr. That is frustrating, my friend! Nothing much fun about fixing a poorly running open-loop EFI system.
Have you swapped the coolant temperature sensor? I know things can go sideways real quick if the ECM doesn't know the correct engine temperature. And that temp isn't shown in your graphs.

ECT sensor seems to be reading just fine.

 

[kd4pbs]

Hmm... What time span is that? Odd that it stayed at exactly 132.8 that whole time.

 

[vroom ZOOM]

Hmm... What time span is that? Odd that it stayed at exactly 132.8 that whole time.

about 15 seconds. That is an interesting observation though, I will go see if it changes in a longer timeframe.

 

[kd4pbs]

That might not be outside the realm of possibility.
The big thing will be what it reads with the engine cold I guess. That will tell whether or not there's an issue there.

I'm not very familiar with the big block MPI system - does the computer control the timing directly with a crankshaft position sensor with the "distributor" basically being just a switch and a camshaft position sensor like the small block MPI, or is it more like how the EFI systems are set up where one can affect timing by rotating the distributor but the ECM can vary the timing?
I apologize if you stated this (can't remember), but did you swap ignition modules? Coil? This is one of those straw-graspers for sure!

 

[vroom ZOOM]

That might not be outside the realm of possibility.
The big thing will be what it reads with the engine cold I guess. That will tell whether or not there's an issue there.

I'm not very familiar with the big block MPI system - does the computer control the timing directly with a crankshaft position sensor with the "distributor" basically being just a switch and a camshaft position sensor like the small block MPI, or is it more like how the EFI systems are set up where one can affect timing by rotating the distributor but the ECM can vary the timing?
I apologize if you stated this (can't remember), but did you swap ignition modules? Coil? This is one of those straw-graspers for sure!

Yes it is the TBI style system (delco EST). No crank sensor. I swapped entire distributors. Both work on the starboard engine.

 

[vroom ZOOM]

Well, I just honed in on another huge clue. I ran a data log through diacom on the working engine to get some numbers. At idle, the IAC is around 17 counts and the injector pulse width is 3.4 ms. On the broken port engine, the injector pulse width is 2.5 ms, even going down to 1.8! So for some reason, the starboard engine is getting almost DOUBLE the fuel! Which explains why this engine feels so lean! No wonder it doesnt want to run! The question is, what would force the ECM to pull so much fuel out? Why is the ECM leaning the mixture so much?

 

[kd4pbs]

An open loop ECM has to take wild-ask-guesses to properly fuel the engine. When I used to tune ECMs back in ye olde GMECM days, most of my attention was done towards closed loop tuning. Often the open loop parts were almost afterthoughts. Pretty much every sensor is utilized in varying ways to ensure proper fueling, and since there is no O2 sensor feedback, it can't compensate for a slightly out-of-tolerance sensor. MAP, IAT, TPS, CTS, and of course RPM. Thing is, you've pretty much verified all of that. I'm also at a loss. Maybe just compare the sensor data back-to-back between the two engines? I hope it boils down to something crazy simple like a poor ground or wiring issue. Maybe a carpenter bee decided to try to nest in the IAC passageway

 

[vroom ZOOM]

I think it is time for me to invest in an oscilloscope. I want to compare the injector voltage and current waveforms on both engines to see if the injectors are actually opening as commanded by the ECM. Also hook it up to the ecm power and ground feeds and distributor rpm signal to see what is going on.

 

[vroom ZOOM]

Ok, as I dig through this things get super wierd.

in diacom, there is a parameter called "Troll Mode input". I disregarded it at first thinking that it was not changeable and programmed into the ECM. When I put any ECM on the running starboard engine, troll mode is "OFF" all the time. When I put any of the two ECMs onto the portside engine, troll mode is always ON. What the heck.

 

[alldodge]

MEFI's all use fuel tables
The table simply is a X and Y column table.
rpm = A
Temp = B
MAP = C
injector pulse with = D

This is some of my old 509 info

 

[tpenfield]

@vroom ZOOM . . .

I got your Diacom file . . . Not a lot of data from an MEFI3. I am wondering if you set the throttle to about 5% and start the engine, will it stay running.?

 

[vroom ZOOM]

Okay... so I found something else funky

@vroom ZOOM . . .

I got your Diacom file . . . Not a lot of data from an MEFI3. I am wondering if you set the throttle to about 5% and start the engine, will it sty running.?

yes but it will rev to almost 3k then almost stall, then catch and rev up again etc.

Any ideas about how the ecms know which engine they are on and that turns troll mode on or off even though it doesnt exist?

Also today I had to move the boat into another slip. starboard ran fine as usual. Port would stall but docking on one engine with about 2 ft to spare between another boat and the slips was hairy so I unplugged the ECT and it didn't stall on me, I was even able to get it in and out of gear to park the boat. The IAC was working hard though, it was hissing a lot more than the starboard engine, and the port engine still ran really rough.

The starboard exhaust smells a lot richer too, port smells pretty lean.

 

[tpenfield]

I'm seeing the injector pulse width drops below 2.0 ms, which I think is low for an engine not at its operating temperature. So, in a way your engine is being fuel-starved within seconds after starting.

It would be interesting to figure out what other sensor inputs would cause the injector pulse width to be so low.

 

[tpenfield]

It might be good at this point to list out the things that you have not replaced or swapped out with the other engine.

 

[vroom ZOOM]

It might be good at this point to list out the things that you have not replaced or swapped out with the other engine.

Transmission
Mufflers and exhaust plumbing
shift cables
throttle cable
water pump
load anticipation switch
trans temp switch
trans harness
knock sensor
heat exchanger
longblock(inc. heads and valve cover)
analog temperature sender
analog oil pressure switch
engine mounts

 

[tpenfield]

Here is a Diacom reading from my 7.4L LX engine at idle, similar ECT. . .


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Notice the difference in injector pulse and the MAP

 

[vroom ZOOM]

Here is a Diacom reading from my 7.4L LX engine at idle, similar ECT. . .

View attachment 410496
.
Notice the difference in injector pulse and the MAP

On my running engine the pulsewidth is 3.5, and map is around 21. BUT the port engine did decide to idle today at one point, albeit very rough and borderline stalling. I did record the data though. Injector pulsewidth and MAP was at 3.5 and 21, just like on the starboard engine but it was running like CRAP!

This leaves the ONLY difference in diacom readings between the two to be.... THE TROLL MODE INPUT mystery! I am probing all the pins on the ECM to figure out how the heck it knows which engine it is attached to...

 

[tpenfield]

Here are the engine speed, injector pulse, and MAP as a graph.

Overall the injector pulse looks low. MAP also looks fairly low.

Maybe send me a similar file from your 'good' engine for comparison.
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