454 mag mpi chuggles/stalls

[tpenfield]

You mentioned a new long block and the previous engine had the same issue. Did the intake manifold come from the old engine to the 'new' long block? (sorry if that was already discussed)

 

[cyclops222]

Post # 59 tells a specific set of problems.
Have you checked for odd colored sparkplugs ? Damaged sparkplug internal parts ? Loose H V cables and connections ?

 

[tpenfield]

Just comparing to my 7.4 LX data files. . . wondering about the IAC position . . . looks high on your engine.

Comparing the data files of the good engine and the 'troublesome' engine may be helpful. Send them as you are able.

 

[vroom ZOOM]

Just comparing to my 7.4 LX data files. . . wondering about the IAC position . . . looks high on your engine.

Comparing the data files of the good engine and the 'troublesome' engine may be helpful. Send them as you are able.

If I can get it to stay running the IAC will go over 100. Check the data files I emailed you with the good and bad engine.

 

[vroom ZOOM]

Just got the troll mode thing figured out. unfortunately that did not point me to my issue. The neutral switch just had some resistance to ground. unplugged it but nothing changed. on to the next thing.

 

[nola mike]

Did you try unplugging the iac? Missed if it's still running ok using cheap injectors?

 

[vroom ZOOM]

Did you try unplugging the iac? Missed if it's still running ok using cheap injectors?

tried putting the cheap injectors in. still wont run. will try the iac right now.

 

[vroom ZOOM]

I just did a brake cleaner test, I unplugged the PCV hose and when the engine went to stall I shot some brake cleaner in there and it immediately picked up. I had to spray A LOT in there before it picked up. Not just a little shot, but a good steady stream! I am now certain that this engine is too lean.

My only question is at the moment, if the computer is saying the injectors are at 3.5ms, are they actually opening for 3.5ms as commanded by the ECM or not?

 

[tpenfield]

It’s a beach day for me today. I’ll take a look at the files this evening .

a good assumption may be that the ECU is responding to the sensor inputs. The question being, what is telling the ECU to make the injector pulse so short?

 

[kd4pbs]

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.

Rigol has some decent ones for a pittance (relatively speaking). I recently picked up their entry-level DHO 4 channel scope, and I still have the older (but in some ways better) DS1054Z. There's no reason a decent o'scope should be thousands of dollars nowadays.

 

[kd4pbs]

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

Yep. All based on existing GMECM tech from the 80s. Automotive ECUs have the advantage of a BLM table... having a closed loop working in their favor. So you take the base fuel table which is burned in a UVEPROM, monitor the INT(egrator) under normal driving conditions, and that creates a persistent (until it loses memory power at least) Block Learn Mode table which is basically a correction factor for the main fuel table.

We're stuck with just the main fuel table with the MEFI when set up as open-loop like we have here. I like to refer to it as the master assumption fuel table since it has no way to verify whether it is correctly fueling the engine or not

 

[kd4pbs]

I was pondering this a bit yesterday, and I do believe you've checked, but spraying some starting fluid around the intake manifold gaskets might identify an issue. I've also used the Gumout brand spray brake cleaner - it's mostly acetone. Up to you, but I'd steer far away from brakleen or other chlorinated cleaners. The tetrachloroethylene in that stuff can make phosgene gas when combusted.

https://www.roadandtrack.com/car-culture/buying-maintenance/a33229282/brakleen-safety-brake-cleaner-august-2020/

 

[tpenfield]

Here are some graphs from both the Good and the Bad engine . . .

Good . . .

.

.
Bad . . .

.

.
I am seeing that on the 'bad' engine the MAP value goes down lower when the engine revs from the initial start and the IAC valve has to open more to keep the engine running. The lower value of the injector pulse may be a reaction to the low manifold pressure, given that the throttle position is at 0%.

I am thinking that the 'bad' engine is being air-starved. . . not sure how, but I'd be checking the throttle body and in particular the air bypass.

 

[nola mike]

%.

I am thinking that the 'bad' engine is being air-starved. . . not sure how, but I'd be checking the throttle body and in particular the air bypass.

So I'm not sure. The MAP is initially appropriate, ~15. Then the IAC keeps opening and the MAP increases, which would indicate more air getting to the sensor via the IAC, throttle open, or leak. The injectors look to be responding appropriately to the MAP and RPMs. If this was unmetered air the MAP wouldn't show the increase. So I'm still thinking IAC open is causing a lean condition/stall.

 

[tpenfield]

It looks like the initial barometric pressure (Baro MAP) is 30 and the 'good' engine goes down to about 12. The 'bad' engine goes to about 8.5. It also looks like the 'bad' engine was able to keep running if/when the IAC moved all the way up to the 155 range, and so the injector pulse stayed up in the 3.5 ms range.

On the runs that the engine stalled, the IAC position only got up to 100 and the injector pulse stayed below 2 ms.

On the tests where the engine continued to run, it appears that more air was let in via the IAC and the MAP was maintained at around 21.

 

[nola mike]

It looks like the initial barometric pressure (Baro MAP) is 30 and the 'good' engine goes down to about 12. The 'bad' engine goes to about 8.5. It also looks like the 'bad' engine was able to keep running if/when the IAC moved all the way up to the 155 range, and so the injector pulse stayed up in the 3.5 ms range.

On the runs that the engine stalled, the IAC position only got up to 100 and the injector pulse stayed below 2 ms.

On the tests where the engine continued to run, it appears that more air was let in via the IAC and the MAP was maintained at around 21.

I'm just looking at post #73. Good engine has initial drop/snap back as engine first fires and settles at 21. This is still higher than I'd think it should be (would be only 11 mmhg vacuum). Bad engine has initial drop, but no snap back, it returns gradually to 26, so only 4 mmhg vacuum at that point. Healthy engine at idle should be > 18.
So why is the MAP increasing? Wonder if the shape of the curves holds a clue? I noticed we don't see what the timing is doing during that time, which could certainly be significant.

 

[tpenfield]

Yes, the thing I’m not in love with about the Diacom is that you can only graph 3 things at a time.

In the recent test- I may be that the Map value increased because the iac opened up.

anyway, I think the OP has the intake/throttle body to look at.