1994 LT1 with AT. Starts right up. Runs great for about 5 minutes. After it gets warm, it stumbles and the idle gets a little erratic. I originally thought the tranny was failing. It seemed to not want to shift when anything above about a quarter throttle. I realized through playing with the throttle in Neutral that the engine is stumbling badly.

The car has 107K miles. When it is cold it shifts crisply and runs great.

I am thinking it has to be a spark issue. Perhaps a bad wire that may arc when it gets warm, or a coil starting to fail when it warms up, perhaps a plug failing at temperature.

It has occurred to me that it could also be fuel related. Perhaps a weak fuel pump.

There is no check engine light, there are no trouble codes. Even when it stumbles.

I figure it needs a tune up anyway, so I plan to start there. Has anyone dealt with this that may be able to suggest something? I am a recent GM owner, and am MUCH more familiar with Ford engines. I know that each engine tends to have its own idiosyncrasies so I am hoping for a voice of experience here.

Merry Christmas everyone.

Casey

 

I would start with the ICM,autozone can check it for free..it might be getting heat soaked.I would put a washer or 2 between the cylinder head and bracket that it is mounted to.

 

The first place to start is with a computer scan. Look specifically at the water temperature. The PCM uses that water temperature, among other things to set the fuel mixture. If the water temperature sensor in the water pump I'd telling the PCM the coolant is cold then it richness the mixture and you get drivability issues. If you can't scan (you can do it yourself with the proper cable and a laptop using software like freescan) then you can measure the resistance of the sensor at various temps. I think shoebox or injuneer has a table defining the correct relationship on their websites.

I guess it could be the ICM but I wouldn't start there from your description....

 

Un plug an O2 see if it runs better when warm. If so you have a control issue. Most likely and O2 or it's wiring.

 

Wouldn't that toss a code and an SES light?

 

Yes it would. However it will run in open loop and as stated before, it finds control problems. There is no other effect to the engine other than SES and open loop control.

For people that don't have scanners or laptops to do the job. It's probably the first thing to do to eliminate the obvious "easy" things to find.

Simply plugging in the O2 when he's done will clear the code.

 

What I actually meant, but didn't do a good job of asking, was; "wouldn't a bad O2 sensor have already set a SES light?" I assumed that since he didn't have a light he didn't have a bad sensor...sluggish maybe but not bad. I know, I know..I shouldn't ASSume...

Thanks,

 

No my wiring was actually burned open and it never showed a code.

 

Seems to be using a lot of fuel too. I know that in the FORD 302 the ICM would be the likely culprit. Mounted to the Distributor on the Mustangs. Since testing it is free and removing it is easy, I might as well eliminate that as a possibility.

CC

 

i also have a 94 lt1 and it was doing the same thing,it ran great cold but would stumble and cut out after it warmed up. i finally got a code out of it and it said MAF failure. so i figured i would try to save $200 and buy cleaner. i still have a SES light but no more stumble. at 180f and wot it wont cut out stumble or any of it any more!!! try it, it worked for me and saved me money and time!!! buy the MAF cleaner and give it a GOOD clean. ignore the instructions of only 10 sprays. spray the thing untill its cleaned from the wires inside to the screen on the other side. im telling you, it worked for me! good luck!

 

i am willing to bet its a spark plug or wire, prob a wire, arching out. these lt1's are notorious for plug wires being improperly mounted/ran and will rub against something easily, eventually arching out to a ground source. i had this same issue a month ago. warm idle and warm driving misfire, and it was bad. almost died a few tims even. new plugs and wires and your set. its ALOT of labor though. maybe somebody could check them? if not ICM could be it. that can get checked at autozone like stated above. most of the time a warm engine issue like this is electrical if you weren't aware already

 

Well, the autozones of the area do not have the proper pigtail for their tester to test the LT1 ICM. Having seen the posts here, I will go ahead and give the MAF cleaning a try, and the plugs and wires. I am also going to replace both temp sensors. The water and the air. This occours as the guage is very sporadic, and I know it could dump fuel if it thinks it is running cool.

If anyone else has suggestions, I am all ears. Thanks everyone. CC

 

Very doubtful that it's a wire causing this issue. If it were a wire it would be missing and breaking up regardless of open or closed loop. His symptoms show a closed loop issue.....

 

while you are cleaning the maf make sure the inlet elbow is seated all the way around the throttle body and that it does not have any splits. when i have to put mine back on a put my hand through to make sure it seats properly around the throttle body especially at the bottom.

check for corrosion on any harness plugs like coil, icm, and opti harness(plugs in to front mount distributor(opti) behind the water pump and on the passenger side of the intake manifold)

check your fuel pressure and check the vacuum line that plugs into the pressure regulator on the back of the manifold for fuel.

 

Speaking of the OPTI, I have had it suggested that it could be the culprit. Anyone had simptoms similar to mine from a failed or failing optispark? I really hope that is not it as they seem to be pretty dang expensive.