Intermittent stall long time fixing

[QZilla500]

Ok here goes,
99 Grand Prix 3.8 153,000 on car approx. 80,000 on engine No codes present
Car will stall at idle, coasting to a stop, cruising at 65, or on acceleration (once) If I just restart the car the "trac off" light will stay on unless I cycle the key. Runs fine either way and it runs great when it runs. Most times it fires right back up, others it takes a couple tries. Seems the colder outside the more it acts up. If it doesn't fire and I force it with my foot on the gas, it will run on about three cylinders until I can get it to about 3000 rpm. I have not done it that way lately and just hit the key until it runs on it's own.
Have been through a couple tanks of fuel trying to fix this and much time and $$
Have captured idle stall on scanner. O2S1 goes to .8 to .9 flat lines there and car stalls.
ST and LT fuel trims seem to drop as well. (I may not be explaining properly but I have the numbers)
Thought possible leaky injectors, pulled injectors and rail unit, turned on key no leaks.
Fuel pressure remains constant at 42 psi when stalled, will vary with load. Hooked clear vac line to regulator, no fuel in line. 20 inches of vac. Used 1/2 can carb cleaner spraying for vac leaks.
MAF, TPS, MAP, readings look good, (to me)
MAF was swapped at one point same results.
Front O2 fairly slow, spends a lot more time on (rich ?) side .6 and above at idle,flat lining for a good bit. Will also flat line on the lean side, but not as often.
At 1500 rpm and above switching seems fairly normal.
Changed Front O2, same results. Rear O2 is new as well.
Tail pipes sooty and soot seeping at front doughnut) Ran car (in shop) with exhaust off at manifold, (thought cat might be plugged, MPG seems way low) same readings. At one point while running I pulled regulator vac line and burped throttle and RPM went to 3000 and stayed there until I shut off key.
Possibly due to exhaust being off or regulator maxing out? Hooked vac back on ran ok but picked up a dead spot of sorts just off idle. Let car cool and dead spot went away.
Crankshaft sensor replaced. ICM has been swapped, PCM has been swapped and CASEd.
Checked and cleaned grounds, battery replaced, new fuel filter, had a rear knock sensor code a while back replaced that, pulled on and checked every connector under the hood and dash I could get my hands on.
There is evidence someone may have put in different injectors. For some reason all injectors have been spliced in. Why they did not just plug them in I cannot say. I wonder now if someone tried to hot rod it with bigger injectors. I could see how this would affect idle but why cut out at speed?
I'm new to reading scanner info if you ask me to look for something, can do.
I do not have an enhanced scanner though.
I am pretty much lost at this point....any takers?

[BNaylor]

Welcome to AF.

Thats interesting. What brand fuel injectors were used and what rating in lbs? Sounds like it is running excessively rich. Are there any other mods or is this engine stock?

[Headnsouth]

Sorry QZ I posted the wrong year. For some reason I thought 93.
Geez mod lighten up. : I guess you never made a mistake.

[BNaylor]

Headnsouth,

What is the relevancy of this TSB? Either post relevant info applicable to the year, make and model Grand Prix that will be helpful to the OP or do not post at all. Are we clear?

[tblake]

the OP seems to be on the right track with diagnosis, however just because the injectors arent leaky when they are out and not being worked, doesnt mean you dont have one thats getting "lazy" and not switching on and off fast enough. I hope this makes sense. You already have a fuel pressure guage, now find yourself an "injector balance testor" and read up on injector balance testing, google it. I'd almost bet you will find your problem.

[QZilla500]

Car is fixed.
Think we had two problems going on. One was a broken plug at the ICM
The other was the MAF. While the test MAF was in, the plug caused a stall and threw me off the trail by putting the old MAF back in.
Being new to reading scanner info I really did not know what I was looking at.
I can say that the new MAF now reads .50 at idle, down from .80-1.00 and the LTFT is at around -3 from -7
I never did a good WOT run on record but I've read that may be a better indacator of MAF readings?
Captured it when it got really bad at idle the MAF was over 5.00 and it was like the choke (remember those?) was stuck.
The engine as I stated was replaced. (Prevoius to me) I think it was from a different year and why so many things were spliced in.
Put about 20 miles on and 3 cold starts no stalls.
My son finally gets his Christmas present and I have a new scanner!
Thanks for the replies,
Q

[tblake]

good job, Great Diagnosis!!!! Thanks for letting us know how eveything tuned out. We love follow-ups