need Some help with LT1 MAF issue

[Fastmax32168]

My 94 Corvette has developed an issue which leaves me scratching my head. When you first start it, before the O2 sensors kick in, everything appears normal. As soon as it goes into closed loop, it begins to add in fuel the longer it runs, until the BLMs max out at 160 after a cell has had enough hits. You can tell it is pig rich by the way it runs and the smell of the exhaust, even though the ECM still thinks it is lean. Now....if I unplug the MAF and put it into speed density mode it does not do this. It runs as normal. The BLMs stay where they should. I have replaced the MAF, as the old one had some corrosion on it, however it has made no difference. . I have searched for vacuum leaks and found none. Suggestions?
Thanks,
Roy

[steveo]

lets see a datalog of the issue occuring. there's a lot of information we don't have such as what your maf airflow is reading, as well as what conditions this occurs in, maybe we can eliminate some possibilities at least

a large air leak between the maf and the intake manifold would be the most common cause if speed density seems to fix it, but that doesn't line up with the fact that you say it runs normally until it hits closed loop. also switching to speed density might be enrichening things enough where it's becoming a band-aid for something.

on a car made in 1994, every wire and also every rubber thing can't be trusted. for example how is your maf and o2 wiring? gm made those wires really thin. you could have one or more that's morphed into a resistor over time.

one cause people overlook for a perceived over-enrichment condition caused by high trims like this - you could have a non-firing cylinder. that cylinder acts like an oxygen pump and floods the o2 sensors, making them think things are quite lean, so they dump a lot more fuel, in turn making the remaining cylinders quite rich. you'd usually be able to tell that as trims would be quite different per-bank, though, and reading plugs should be a smoking gun if it's been running like this for a while.

[LeMarky Dissod]

Check for air leaks between the MAF sensor and the O2 sensors, intake and/or exhaust manifold leaks being the most likely culprits … and pretty much everything steveo said.
H2Opump wiring is also notorious for being made of wires that were too thin & brittle 25 years ago.

[Fastmax32168]

Thanks for the responses. This problem isn't exactly what I thought it was, it was adding fuel everywhere before I replaced the MAF and I thought that problem was back but it appears to be mainly at idle now, and worse on the right bank. I sure would like some of you that are smarter than I am to have a look at this file though since it is all over the place. This is actually my first EE hack file here, I have always used tunercat, but there certainly seems to be a lot more info in this program. A little history here, I have had this car for 15 years, it has a very healthy 383 in it that will propel it to high 11s at our local track. I left it sit with little use for a few years and it was running badly, so I cleaned all the njectors installed a new fuel pump and filter, opti, coil, plugs, wires, and maf sensor to freshen it up. I have new O2s here but haven't put them in yet. Other than the poor idle, it sounds and runs decent driving it around, but as you can see from the file, the tune seems all over the place. I looked hard for air leaks either before combustion and afterwards and really cant find any. Keeping in mind that this same tune would log good a few years ago, on the exact some motor, is it possible that changing around all of these components has affected it to the point where I need to start over again, or has something else gone haywire? Any input here is appreciated. Thanks
Update: I cant get the eedata file to upload and attach. It keeps saying invalid file. I dunno

[kur4o]

You can zip the logs and than try to upload.

[Fastmax32168]

Thanks, let me know if you can open it now

[kur4o]

You are having some troubles at idle, blms jump to 160.

To isolate some of the causes can you command open loop throught the control section of eehack on warm engine.
Just click on the overide AFR checkbox, top left.

If the lean condition is real the 02s will read low and the engine will run like crap and try to die.
If the lean is false the 02 will again read low, but the engine will run great without issues.

This will help us isolate some of the possible reason, leak in intake or leak in exhaust, misfires and so on.

[Fastmax32168]

Very interesting idea and great feature on the EEhack program. I attached a file of my doing as you requested. After a few minutes of running when I figured it out you will be able to see when I disabled AFR and reset the BLM. It did not die, it didnt idle perfect, but it wasnt really worse than when I had the O2s at work. The O2 readings dropped out the bottom, are they actually still functioning or are they just disabled by EEhack? After a few minutes I reenabled them and it began to immediately march its way to 160. I am wondering if it might not have a leak in the valley under the intake, I have just discovered I have negative pressure at the oil cap when doing the strip of paper test. Thanks for the guidance.

[steveo]

you said you've searched for vacuum leaks, have you plugged off every vacuum line including the PCV and see if things change?

that log looks like when i had a big hole in my brake booster.

do you have a log of when your engine was idling healthy? i'd be curious what the intake vacuum looked like vs now.

The O2 readings dropped out the bottom, are they actually still functioning or are they just disabled by EEhack?

still functioning

[kur4o]

Definitely air leak after the maf or Air pump leak leading to fresh air in the exhaust. You can also check ccp solenoid for stuck open with leak on the lines sucking fresh air.

The final test will be to command open loop with maf disconnected.

[Fastmax32168]

I hooked up a smoke machine this morning and found a small leak where my snow performance injection nozzle hooks in. It wasn't very big, but maybe it doesn't take much at idle. I have it mending up now, I haven't tried plugging individual port yet, but if this doesn't take care of it that sounds like the next logical move.

[kur4o]

Looked at the logs and at one point engine tries to die in open loop. Also now right side stays much leaner than left side. I suspect an insufficient fuel delivery. Clogged or dirty injectors can explain the low pulse lean condition.

You can try give it some heavy throttle driving with some cleaning agent in the tank.

[Fastmax32168]

kur4o, That was one of my first thoughts, I started there. Pulled all the injectors and cleaned them manually. Lots of fuel to be found.

[Fastmax32168]

I repaired the small leak I found where my snow performance nozzle went into the boot but it has had no effect on the problem. I replaced the PCV valve because it was sticky, but that also had no effect Here is what I have learned. I unplugged all of the hoses one at a time and plugged the vacuum ports and it has no effect at all on the problem.
Double checked my header hookup near the O2 sensors and cant find a leak. I unplugged the air pump but that had no effect.
The right bank is a constant problem, the left bank is also 80% of the time, sometimes it corrects itself for no reason and at other times follows the right bank exactly.This problem only seems to manifest itself at idle, if I give it even a tiny amount of throttle and hold it there the BLMs seem to remain normal. I get the tiniest wisp of smoke off of the center of the EGR valve when I pressurize the intake with the smoke machine, but I cant get it to suck anything in there when it is running. I could try replacing it, but it aint a cheap WAG part.

[kur4o]

I still think it is some injector problem.

You can try to nail down the problem cylinder that gives the high 02 content in exhaust.

In eehack control window there is some cyl cut buttons, Disable each cylinder for 3-5 seconds than enable for 3-5seconds, than move to next one.

Watching map reading can give us some clue if there is some dead one. Also you can measure the temp of headers for each cylinder. If you have some way colder than the others, we can start switching components to nail the cause of the lean.