1996 LT4 Running Terribly
Thread Starter
Instructor
Joined: Sep 2013
Posts: 100
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From: Livermore CA
1996 LT4 Running Terribly
Hi guys,
I have a friend who has a 1996 LT4. The car runs great most of the time, but there are times when the car wouldn't perform too well. It's a daily driver.
Every now and then, it would be driven, stopped at a stop light, then the engine would start running at 1200RPM, sometimes reaching 1600RPM on idle. After a while, it would have a hard time accelerating, and the check engine light and ABS light and other lights on the dash would come on.
After that, the car is turned off, started again, and it would go back to normal driving operation. Any thoughts on this? I'm not too familiar with C4's myself, so any help would be appreciated.
Thanks
-Arvin
I have a friend who has a 1996 LT4. The car runs great most of the time, but there are times when the car wouldn't perform too well. It's a daily driver.
Every now and then, it would be driven, stopped at a stop light, then the engine would start running at 1200RPM, sometimes reaching 1600RPM on idle. After a while, it would have a hard time accelerating, and the check engine light and ABS light and other lights on the dash would come on.
After that, the car is turned off, started again, and it would go back to normal driving operation. Any thoughts on this? I'm not too familiar with C4's myself, so any help would be appreciated.
Thanks
-Arvin
Thread Starter
Instructor
Joined: Sep 2013
Posts: 100
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From: Livermore CA
I don't know what the check engine light was for, problem is whenever it'd go off, the car would be miles away from home. Restarting the car takes off the lights, but I have yet to run a code reader on it. And the fact that sometimes it would run great as ever one way, then on the way home, then it would start revving at higher RPM. Any idea where the vacuum leak may be coming from?
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
Check the codes first. You will need a OBD2 code reader. A code might give you a hint.
The different idling speeds indicate a possible problem with the IAC motor. They can do those on and off problems like that. Vacuum leaks are usually there all the time.
Also just for fun, check out the EGR valve and make sure it's free to operate and not sticking.
The different idling speeds indicate a possible problem with the IAC motor. They can do those on and off problems like that. Vacuum leaks are usually there all the time.
Also just for fun, check out the EGR valve and make sure it's free to operate and not sticking.
Burning Brakes
Joined: Sep 2010
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Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
I won't ask in this thread what takes the place of the EGR and that system.
*** So now that I posted already, I have been updated the second time.
***
Last edited by pcolt94; Jan 8, 2014 at 10:13 AM. Reason: Another update
Thread Starter
Instructor
Joined: Sep 2013
Posts: 100
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From: Livermore CA
Hey guys,
Thanks for everyone's input. I'll be running a code reader on it. Btw, if the lights have turned off and the car is running perfect will the code reader still pick up something?
Thanks again.
Thanks for everyone's input. I'll be running a code reader on it. Btw, if the lights have turned off and the car is running perfect will the code reader still pick up something?
Thanks again.
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Burning Brakes
Joined: Sep 2010
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From the Grand Sport Registry...
NEW CAMSHAFT DESIGN...
The LT4's more aggressive camshaft profile compliments all the valvetrain upgrades. The cam has more lift, duration and valve overlap. Exhaust valve lift is .479" (12.17mm) (net is less .006" inch (0.15mm) lash), @ .050 (1.27mm) Lift, up from .459" (11.66mm) for the LT1, and the intake is .476" (12.09mm) (net is less .004 (0.10mm) inch lash), up from the LT1's .447" (11.35mm). Duration is also increased slightly on both exhaust and intake... 203 deg. Intake and 210 deg. Exhaust with 115 deg. lobe separation and 1 degree of retard. By increasing overlap significantly, it eliminated the need for an external exhaust gas recirculation (EGR) device
Thread Starter
Instructor
Joined: Sep 2013
Posts: 100
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From: Livermore CA
Hi guys,
Okay, I ran an OBD2 scanner/code-reader on it.. And to my surprise, it pulled up 0 codes. None whatsoever. This is very very very odd. Anyways, sometimes, the "ASR" light would come on. Whether or not that would affect performance, I'm not really sure. It hasn't ran roughly in two weeks' time, but there's still a chance it could do it. Perhaps the throttle sensor has something to do with it as well?
Okay, I ran an OBD2 scanner/code-reader on it.. And to my surprise, it pulled up 0 codes. None whatsoever. This is very very very odd. Anyways, sometimes, the "ASR" light would come on. Whether or not that would affect performance, I'm not really sure. It hasn't ran roughly in two weeks' time, but there's still a chance it could do it. Perhaps the throttle sensor has something to do with it as well?
Drifting
Joined: Aug 2006
Posts: 1,326
Likes: 18
From: Los Angeles California
Google "paperclip method" and try that set of steps out to get the ABS codes; they're stored on a different computer from the engine codes and wouldn't come up with an OBD reader. The OBD reader will only help you out with info when the check engine light is on right then.
Once you have more of a hint, you can begin the long, scientific process of diagnosis - be careful not to end it early and throw wrong parts at the problem. It's not always an intuitive answer, for example my LT4 has led me through a lot of obscure and weird problems before, some the car's fault and some mine, such as:
Once you have more of a hint, you can begin the long, scientific process of diagnosis - be careful not to end it early and throw wrong parts at the problem. It's not always an intuitive answer, for example my LT4 has led me through a lot of obscure and weird problems before, some the car's fault and some mine, such as:
- Overheating, due to the parts counter handing me a thermostat brand that doesn't quite fit and wouldn't open;
- Accessory belt too loose and worn out to charge the battery;
- Water in fuel tank from a storm, since the drainage valves above the cap were clogged with leaves; ran me out of gas
- Coolant temp sensor connector melted to the engine, came unplugged occasionally, making the car buck violently and stall;
- Optispark vent hose I re-routed was overstretched and dry-rotted open, vacuumed up engine crud and broke the distributor rotor, causing no start
- Knocking and check engine light flashing because a spark plug wire I re-routed fell against the exhaust;
- Fuel pump gave up all at once and for no reason;
- Optispark's sensor stopped outputting the signal that guides the whole engine computer, causing stalls and no start until I eventually found out with a multimeter;
- Catalytic converters gave up and got clogged, causing roughness and eventual no run;
- Ignition key resistor reader gave up and not making contact, causing no start;
- Interior light I left unscrewed during some work kept draining the whole battery because it was on a gravity switch
- Wrong interior lights coming on because I installed an interior screw through a live wire that had worked its way behind it, reversing current in the circuit
Last edited by LouisvilleLT4; Jan 21, 2014 at 04:01 PM.
Thread Starter
Instructor
Joined: Sep 2013
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From: Livermore CA
Wow, quite a list you've got there. Haha. I guess I should start looking for any work that's been done to the car. I do know that the clip that go on the TPS has been worked on. It was cut and re-wired with crimps. Could it be possible that the wires are not making contact at times due to the engine's vibrating nature? How could the TPS affect the intermittent rough idle and high RPM? At times, the car would suddenly jerk, and if the throttle is released and pressed on in a quick manner, the car would fix itself.
Drifting
Joined: Aug 2006
Posts: 1,326
Likes: 18
From: Los Angeles California
My bad, Google isn't pulling it up near the top of results anymore. Here is how you read ABS / Dash / Airbag / ride control codes:
http://forums.corvetteforum.com/c4-t...n-96-cars.html
http://forums.corvetteforum.com/c4-t...n-96-cars.html
Le Mans Master
Joined: Jan 2006
Posts: 5,621
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From: Orlando FL
As louisvilleLT4 had indicated the ABS codes are stored in module 9. But that is the brake computer and would be a real stretch for it to affect the problem as indicated in post 1.
I had suggested the IAC motor. But it's also a possibility the opti might be bad. At least on the 92-94 non-vented opti, the idle can wonder around. Not sure if the 95-96 opti can give the same effect if defective. Just saying.
But I still would either clean the IAC or replace it (I would personally do that) and see if it's fixed or the symptom changed. I would do this before I would ever replace the opti.
I had suggested the IAC motor. But it's also a possibility the opti might be bad. At least on the 92-94 non-vented opti, the idle can wonder around. Not sure if the 95-96 opti can give the same effect if defective. Just saying.
But I still would either clean the IAC or replace it (I would personally do that) and see if it's fixed or the symptom changed. I would do this before I would ever replace the opti.
Drifting
Joined: Aug 2006
Posts: 1,326
Likes: 18
From: Los Angeles California
Yes a stretch, but still the suspect TPS circuit would affect the ABS computer as that's one of the inputs to its decisions, and one of its outputs is also the throttle position (which could cause varying RPM weirdness). So there's a possible connection and it's worth checking what error code is there.
Lately my own ABS computer keeps lighting up, seemingly because of intermittently loose wiring to the accelerometer (g-force meter), another input to its decisions. The accelerometer is located near the car radio, which I have recently been taking out and messing with.
edit: Just thought of yet another thing and added it to my list of obscure diagnosis experiences up there
Lately my own ABS computer keeps lighting up, seemingly because of intermittently loose wiring to the accelerometer (g-force meter), another input to its decisions. The accelerometer is located near the car radio, which I have recently been taking out and messing with.
edit: Just thought of yet another thing and added it to my list of obscure diagnosis experiences up there
Last edited by LouisvilleLT4; Jan 21, 2014 at 04:02 PM.
Le Mans Master
Joined: Dec 2006
Posts: 5,139
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From: H-Town Texas
If the TPS looks like it was worked on, then definitely check that connection, it could have this effect.
Also, as mentioned, clean out the IAC.
Also check the map sensor on top of the intake.
It seems that there is something affecting the A/F ratio.
Being that it is intermittant, I would lean towards either a sensor, electrical connection, (TPS) or a sticking valve (IAC). The map sensor can accumulate debris causing a false reading as well.
Also clean the MAF sensor, and check the PCV valve.
Also, as mentioned, clean out the IAC.
Also check the map sensor on top of the intake.
It seems that there is something affecting the A/F ratio.
Being that it is intermittant, I would lean towards either a sensor, electrical connection, (TPS) or a sticking valve (IAC). The map sensor can accumulate debris causing a false reading as well.
Also clean the MAF sensor, and check the PCV valve.
Melting Slicks
Joined: Aug 2008
Posts: 2,082
Likes: 50
From: Palmdale CA
Wow, quite a list you've got there. Haha. I guess I should start looking for any work that's been done to the car. I do know that the clip that go on the TPS has been worked on. It was cut and re-wired with crimps. Could it be possible that the wires are not making contact at times due to the engine's vibrating nature? How could the TPS affect the intermittent rough idle and high RPM? At times, the car would suddenly jerk, and if the throttle is released and pressed on in a quick manner, the car would fix itself.