CAR OK, OT. I need help on my car.I'm stumped.

Justintiime

OT Supporter
Sep 25, 2005
4,031
Roanoke, VA
Symptom, low load cruising my AFR gauge pegs pig rich. The car develops a stumble and is low on power. Any other time it seems to run good, but runs rich...you can smell it in the exaust.

This is a roller 355 engine with AFR heads and the equivalent of an LT1 intake, and OBD2 electronics and PCM.

Cylinders 1 and 2 read about 100* colder than the rest. I've swapped plugs, and injectors ad cylinders 1 and 2 still read cold.

-Scanner shows no codes
-Sensors all read perfect
-Fuel pressure is spot on. Clamped feed and return line with no drop in pressure over an hour
-no raw fuel from regulators
-Injectors all ohm fine both hot, and cold
-Cap and rotor still look new.

Whats left that I can do with my resources is to ohm the plug wires.

I am not able to pull the plugs due to lack of resources, but the plugs are about 3 years old with maybe 20k hard miles. They're stock AC plugs.

I'm lost. I'm not sure what to do. I'm about to either part the fucker out, or do an LS1 swap.

Help??? :(
 
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Justintiime

Justintiime

OT Supporter
Sep 25, 2005
4,031
Roanoke, VA
I dont have a maf. I cant pull the plugs because I dont have the right tools, or a jack, or the place to do it :( I'm thinking of ohming the wires this weekend and other then that im stumped.

I've also thought of getting some tuning software to see if I can datalog the car when its acting up? I dont know if that would help
 
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Justintiime

Justintiime

OT Supporter
Sep 25, 2005
4,031
Roanoke, VA
I've tried another map sensor. Both read the same on the scanner too. Wouldn't a vacuum leak cause other driveability issues?

Also, when cold if I give the car too much gas, it backfires through the throttlebody
 

twistid

Banged By Super Models
Jul 15, 2001
40,382
The Kansass/Oklahomo Border
:dunno: when my van had a vacuum leak, it ran rich... with rough idle, and backfired if you hit the throttle... and it was not a hose... it was at a plastic T... until i got it home, i had to put it in neutral every time i came to a stop... and hold the throttle down, to get the rpms up.
 

deusexaethera

OT Supporter
Jan 27, 2005
18,592
I was gonna suggest a wiring fault that's messing up the transmission of sensor readings to the ECU, or a failing coolant temperature sensor causing the ECU to think the engine is cold all the time, but...:hsugh:
 

MeanGreen

Active Member
Sep 7, 2006
5,670
Annapolis, MD
I've tried another map sensor. Both read the same on the scanner too. Wouldn't a vacuum leak cause other driveability issues?

Also, when cold if I give the car too much gas, it backfires through the throttlebody

if its back firing though the TB, that sounds like a timing issue to me.
 

twistid

Banged By Super Models
Jul 15, 2001
40,382
The Kansass/Oklahomo Border
i had a friend with a fire out the carb issue... he had changed out his intake, and when he put it all back together his timing was off... fire shot out, he dumped beer in it... never ran better.

have you done the firing order swap??? :dunno:
 

liberals=whiner

I would second the vacuum leak. get a can of starting fluid (i think, or maybe its brake cleaner) and spray it around the intake. if the idle picks up you've found it.

Sorry if you know this or have done it already, just want to make sure. The cold backfire could be a symptom of fuel puddling in the intake.

Or if you want Ill give you 3k for the car...
 

Bugalu

OT Supporter
Nov 23, 2003
65,851
Nashville
make sure your ECT sensor is working properly.

though it wouldent explain the 100 degree drop on the two cylinders.

I've seen ECT sensors cause issues like that (although on a toyota)
 
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Justintiime

Justintiime

OT Supporter
Sep 25, 2005
4,031
Roanoke, VA
ect sensor?

I think thats called a "coolant temp sensor" on my car. It was the first sensor I read on the scanner. it read ambient temperature and slowly climbed as the car warmed up.

I'm about to bring it to a guy with a snap on scanner that can do datalogging. He also has a lift so maybe we can pull some plugs.

I know some smart people in my local Pontiac Club chapter so they're all putting their heads together, and all agree that it doesnt make sense lol
 

LT4Justin

Active Member
Aug 21, 2002
1,307
Another easy step in diagnosis would be to go to autozone or wherever and rent some noid lights... you plug them into the injector harness and make sure they flash, which tells you the injectors are being told to fire.

Also, when cold if I give the car too much gas, it backfires through the throttlebody

One of my friends had a stockish 91 GTA years ago that did that... the problem got worse and worse until the car stopped running completely. It ended up being a bad injector. IIRC those motors are batch fire and the one bad injector used all the voltage and killed the whole works.
 
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Justintiime

Justintiime

OT Supporter
Sep 25, 2005
4,031
Roanoke, VA
my car has been converted to OBD2 and is now sequential fire. Do the noid lights go between the injector and the harness, so the car will still run?
 

retorq

What up bitch??
Dec 14, 2006
6,002
Mohave Desert
Popping thru the intake has to be a timing issue, a valve hung open or something allowing flame into the intake path ... I have had a screw in stud back out on my truck, acted similar to what you described although I didn't check cylinder temps, I figured my cam was goign flat. By the time the valve stud was all the way out, it wouldn't rev without popping thru the TBI. I am pretty sure it was the exhaust stud that backed out ... freak thing, never heard of it happening to anyone else before or since.
 

GPK

Groovy
Apr 18, 2007
12,806
ATX
I would second the vacuum leak. get a can of starting fluid (i think, or maybe its brake cleaner) and spray it around the intake. if the idle picks up you've found it.

DO NOT use brake cleaner. I about killed myself one day trying to track down a vac leak on my truck with that shit. A little got down the carb and even though I was outside, that shit made my lungs burn like a mother, I must have coughed for like 15 minutes.
 

RedNeckINTP

I would check the timing chain for excess slop. Take off distributor cap and move crank back and forth by hand. A little slack is fine, a lot will throw your cam timing off bad enough to make systems you describe.
 
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Justintiime

Justintiime

OT Supporter
Sep 25, 2005
4,031
Roanoke, VA
I got my plugs out and they were all black. Replaced then and it seems better. Time will tell if it's "fixed" I'm thinking I may need hotter plugs. Definitely need to buy a jack so I can work on my car in the parking lot at least lol
 

RedNeckINTP

If they are all black then it sounds like it could be a sensor. I'd also look for bad vacuum lines or something unhooked. Some computers (don't know about GM) have a default setting that allows you to limp home if they go bad, the ones I've seen that do that run very rich. Used to be GM mechanic over 20 years, I've forgotten most everything lol.
 
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Justintiime

Justintiime

OT Supporter
Sep 25, 2005
4,031
Roanoke, VA
replaced plugs, cap and rotor. Little better but problem is still there.

Compression check is 160 on each cylinder. Even measured lift of the cam and none of the lobes are worn.

Is it possible for the ECM to just lose its program and need to be reflashed? The stock tune is base off of an Express cargo van. It was then modified for my car. Maybe its running off a stock tune? That could account for everything appearing fine on the scanner, but it running weird I guess.

These problems only occur after the car is all warmed up and hot.
 

retorq

What up bitch??
Dec 14, 2006
6,002
Mohave Desert
Put a vacuum gauge on it and see if it's low or anything. If the sensors are all reading OK it's more then likely something outside the computer .. head gasket, plugged injector, leaking valve ...
 
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Justintiime

Justintiime

OT Supporter
Sep 25, 2005
4,031
Roanoke, VA
leakdown test was fine.

Put in new plugs and cap and rotor and the car ran much better but is still running pig rich. The inevitable happened and running rich is playing hell on my new plugs and now its running the same.

Seems to be fine at idle and good at WOT (albeit I leave a LOT of smoke at WOT) but part throttle crusing is where the AF gauge reads like 11:1 afr. TEHFUX time to put it away for winter

bad MAP? Bad O2 sensors? bad injectors? I'm officially out of ideas.
 

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