stumbles under load at speed
Thread Starter
5th Gear
Joined: Mar 2014
Posts: 5
Likes: 0
stumbles under load at speed
I have purchased a 1994 lt1. it runs smooth until under a load on the highway at around 55-65mph. I can step on the throttle for passing gear and it will sort of stumble and miss then smooth out and take off. any ideas why it acts this way? thanks for any input.
Thread Starter
5th Gear
Joined: Mar 2014
Posts: 5
Likes: 0
it is around 2.5rpm does not happen at lower speed. automatic 700r. if you from a dead start punch it everything is smooth throughout. it is if your cruising at about 65mph then give it more throttle as if your going to pass it will stumble and spit then will clean out and take off smoothly. note at idle you can hear a slight miss. thanks for your input.
Drifting
Joined: Mar 2012
Posts: 1,492
Likes: 61
From: New Jersey
1992 to 1996 have ASR (traction control). Unless you shut it off by pressing the button just above the headlight switch, it is on all the time. That may be causing the "lag" you are experiencing when you floor it. Try shutting it off and see what happens. 
Drifting
Joined: Mar 2012
Posts: 1,492
Likes: 61
From: New Jersey
You can't change the timing on the '92 to '96 - it is controlled by the engine computer. The best thing you can do is get the FSM (factory service manual) for your year. It has diagnostic trees for various problems including yours. Until then, try the following as a starting point.
Do you have the info on the prior maintenance from the prior owner?
Because it could be a maintenance issue or other problem.
So with that in mind -
1. How many miles on the car.
2. How old are the spark plugs, spark plug wires, fuel filter, vacuum lines. Do any required maintenance before going any further with the diagnosis. Clean the throttle body and IAC (idle air control) valve attached to the throttle body.
3. Were there any problems with the car before you bought it that were supposedly fixed.
4. Have you checked the fuel pressure on startup, at idle and checked leakdown after shutting off engine.
5. Have you pulled any spark plugs to check their condition - wet, sooty, tan and checked their gap setting.
6. Have you checked for codes? Go to post #18 of the following thread - http://forums.corvetteforum.com/c4-g...ps-w-pics.html
If so, what has come up.
Start with these and see what you come up with.
Do you have the info on the prior maintenance from the prior owner?
Because it could be a maintenance issue or other problem.
So with that in mind -
1. How many miles on the car.
2. How old are the spark plugs, spark plug wires, fuel filter, vacuum lines. Do any required maintenance before going any further with the diagnosis. Clean the throttle body and IAC (idle air control) valve attached to the throttle body.
3. Were there any problems with the car before you bought it that were supposedly fixed.
4. Have you checked the fuel pressure on startup, at idle and checked leakdown after shutting off engine.
5. Have you pulled any spark plugs to check their condition - wet, sooty, tan and checked their gap setting.
6. Have you checked for codes? Go to post #18 of the following thread - http://forums.corvetteforum.com/c4-g...ps-w-pics.html
If so, what has come up.
Start with these and see what you come up with.
Drifting
Joined: Mar 2012
Posts: 1,492
Likes: 61
From: New Jersey
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Le Mans Master
Joined: Jan 2006
Posts: 5,620
Likes: 206
From: Orlando FL
You may be right and I agree. I hesitated saying it because sometimes people go off and change it before trying to diagnose it. It very well could be the opti but he should do the diagnostics to rule everything else out before going through that process to replace it (hopefully with a good one).
People tend to tip toe around these days before they say opti. Many other troubleshooting steps have already been indicated which is correct to do. Not that many symptoms stated in the initial post but my thinking was that it is a possibility of the opti. I don't jump on opti either, but just my opinion from what I feel here.
That bucking or roughness up a expressway speed when high gear is used and where the engine is at low RPM and under load. That just seems to be a key factor. You better believe before I did my first opti I did hours of troubleshooting and looked at everything.
Drifting
Joined: Mar 2012
Posts: 1,492
Likes: 61
From: New Jersey
I heard that.
People tend to tip toe around these days before they say opti. Many other troubleshooting steps have already been indicated which is correct to do. Not that many symptoms stated in the initial post but my thinking was that it is a possibility of the opti. I don't jump on opti either, but just my opinion from what I feel here.
That bucking or roughness up a expressway speed when high gear is used and where the engine is at low RPM and under load. That just seems to be a key factor. You better believe before I did my first opti I did hours of troubleshooting and looked at everything.
People tend to tip toe around these days before they say opti. Many other troubleshooting steps have already been indicated which is correct to do. Not that many symptoms stated in the initial post but my thinking was that it is a possibility of the opti. I don't jump on opti either, but just my opinion from what I feel here.
That bucking or roughness up a expressway speed when high gear is used and where the engine is at low RPM and under load. That just seems to be a key factor. You better believe before I did my first opti I did hours of troubleshooting and looked at everything.
Le Mans Master
Joined: Feb 2000
Posts: 9,060
Likes: 214
From: Epping NH
Something like that should be setting ECM codes, pull them to see if you are getting any:
http://www.corvettephotographs.com/c4vettes/codes.htm
http://www.corvettephotographs.com/c4vettes/codes.htm
Le Mans Master
Joined: Jan 2006
Posts: 5,620
Likes: 206
From: Orlando FL
Something like that should be setting ECM codes, pull them to see if you are getting any:
http://www.corvettephotographs.com/c4vettes/codes.htm
http://www.corvettephotographs.com/c4vettes/codes.htm
If you lose one of the pulses from the optical sensor within the parameters of setting a code, then a code will be set. But many times the cap is bad and that does not set a code.
Drifting
Joined: Aug 2009
Posts: 1,971
Likes: 243
This doesn't sound ASR related to me. Instead, I would start looking for a simple ignition issue. How do the plugs look? Age/condition of ignition leads? And my personal favorite - failing ICM.
My car had this exact problem but it was only once the engine warmed up and the ICM was heat soaked. OP, does your engine suffer this symptom when cold or only up to temp?
My car had this exact problem but it was only once the engine warmed up and the ICM was heat soaked. OP, does your engine suffer this symptom when cold or only up to temp?