C4 Corvette Start and Idle issues
Thread Starter
Heel & Toe
Joined: Oct 2019
Posts: 20
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C4 Corvette Start and Idle issues
Hi guys! Im pretty new to the corvette community and im just starting to work on cars as a hobby while I am still in school. I recently started having problems with my car starting and sometimes it idles rough. Im going to list just a few of the problems that started occurring. Some might be unrelated but oh well.
-Car used to start right away. Now it has to turn over quite a few time to start up. Not consistent
-Smoke comes out the back. The longer the startup takes, the bigger the smoke. Only smokes when started
-TPS sensor light keeps coming on. Not sure why. Replaced it twice now. Sometimes stays off, sometimes turns on after the car starts rough.
-Idles rough. Kinda jerks a little. Not too bad
-When the engine is revved it sounds like an air hose is leaking?
I have swapped the fuel filter so I know that its clean.
BTW it is a 1987 C4 Corvette
Really hope you guys can help me out. Thanks!
-Car used to start right away. Now it has to turn over quite a few time to start up. Not consistent
-Smoke comes out the back. The longer the startup takes, the bigger the smoke. Only smokes when started
-TPS sensor light keeps coming on. Not sure why. Replaced it twice now. Sometimes stays off, sometimes turns on after the car starts rough.
-Idles rough. Kinda jerks a little. Not too bad
-When the engine is revved it sounds like an air hose is leaking?
I have swapped the fuel filter so I know that its clean.
BTW it is a 1987 C4 Corvette
Really hope you guys can help me out. Thanks!
Last edited by Aidan652; Oct 9, 2019 at 04:38 PM. Reason: Forgot Info
Team Owner
Joined: Aug 2004
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From: Park City Utah
Welcome to the forums. You'll learn a lot about cars, owning an older C4.
What color smoke?
You mean the Check Engine Light?
Have you checked for for vacuum leaks? Or is it possible the exhaust is plugged? How is the power once your on the road with it?
-Smoke comes out the back. The longer the startup takes, the bigger the smoke. Only smokes when started
-TPS sensor light keeps coming on. Not sure why. Replaced it twice now. Sometimes stays off, sometimes turns on after the car starts rough.
-Idles rough. Kinda jerks a little. Not too bad
-When the engine is revved it sounds like an air hose is leaking?
-TPS sensor light keeps coming on. Not sure why. Replaced it twice now. Sometimes stays off, sometimes turns on after the car starts rough.
-Idles rough. Kinda jerks a little. Not too bad
-When the engine is revved it sounds like an air hose is leaking?
You mean the Check Engine Light?
Have you checked for for vacuum leaks? Or is it possible the exhaust is plugged? How is the power once your on the road with it?
Thread Starter
Heel & Toe
Joined: Oct 2019
Posts: 20
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I believe that the color is white/gray. Smells like gas but only smokes we the car starts up. After 5 seconds it stops. Yes the check engine light comes on sometimes with the TPS but thats not the main problem. Havent been able to check for vacuum leaks. But I think i can hear one when it is revved quickly. I do not believe the exhaust is plugged. The power is great on the road. Idles a bit funny when at a stop light but always stays running. No performance issues when driving it on the road.
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If the Check engine light comes on and is TPS related this is a problem, if TPS is not correct how can the system control the fuel needed? You need to fix 1 issue at a time.
Last edited by FASTAZU; Oct 9, 2019 at 06:51 PM.
Team Owner
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From: Park City Utah
That you've replaced the TPS doesn't guarantee that you fixed it; there may be a harness/connector issue causing the actual problem. I'd look into that further.I'd check for vacuum leaks
Good performance is a strong indicator that the exhaust is NOT plugged. Good news there.
You could put a fuel pressure gauge on it and see...pressure...but also if there is leak down after it's shut off. It's possible a leaking injector could be flooding it, making it hard to start.
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Thread Starter
Heel & Toe
Joined: Oct 2019
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Likes: 1
That you've replaced the TPS doesn't guarantee that you fixed it; there may be a harness/connector issue causing the actual problem. I'd look into that further.I'd check for vacuum leaks
Good performance is a strong indicator that the exhaust is NOT plugged. Good news there.
You could put a fuel pressure gauge on it and see...pressure...but also if there is leak down after it's shut off. It's possible a leaking injector could be flooding it, making it hard to start.
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From: Hartford WI
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If you are determined to kep the car, get a scanner so we can see exactly what the ECM sees. Much easier for someone to read. Just because the output from the sensor is correct does not mean the ECM sees it that was because the wire might be broken. Ask me how I know.
Last edited by aklim; Oct 15, 2019 at 09:44 AM.
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Seems like you may have a leaking injector(s) since you say it starts better after it sits a day or so as opposed to jumping in after a few hours. That would account for the hard start/smoke. It's unburned fuel (causing the smoke) and fouling the plug(s) causing the hard start. When you put the fuel pressure gauge on, start the car. Check the pressure then shut down and see if it bleeds off pressure in a short period of time. That would indicate if you have leaking injectors. To search out a vacuum leak, get a can of brake cleaner and with the engine running (this is a quick and dirty test), spray the brake cleaner around where you think you hear the hissing. If the engine revs up/smooths out, you've found the general area of the leak. Good luck!
Team Owner
Joined: Jul 2004
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From: Hartford WI
Seems like you may have a leaking injector(s) since you say it starts better after it sits a day or so as opposed to jumping in after a few hours. That would account for the hard start/smoke. It's unburned fuel (causing the smoke) and fouling the plug(s) causing the hard start. When you put the fuel pressure gauge on, start the car. Check the pressure then shut down and see if it bleeds off pressure in a short period of time. That would indicate if you have leaking injectors. To search out a vacuum leak, get a can of brake cleaner and with the engine running (this is a quick and dirty test), spray the brake cleaner around where you think you hear the hissing. If the engine revs up/smooths out, you've found the general area of the leak. Good luck!
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So its an outlay but you need the Helm Manual. Helminc.com.
You need a fuel pressure gauge, it hooks up at the fuel rail. It should run steady at about 43-45psi, watch and note how long it takes for the pressure to drop when you turn the car off.
You need a scanner, Actron makes one that you can get at Autozone, for OBD-1 cars. You can buy the OBD-II and theres an adapter for OBD-1. This will tell you what the ECM sees as the TPS voltage. It should be 0.54 V at idle. IAC counts are another thing the scanner will tell you, if they are 100+, theres probably a problem there. If the idle is hunting or struggling, they'll be all over the place, most likely.
Look through all the hoses and see if you can find the vac leak. If you find one, unplug the battery and clear the code, and see if it runs right after a restart.
Lots of things to check, do one at a time.
You need a fuel pressure gauge, it hooks up at the fuel rail. It should run steady at about 43-45psi, watch and note how long it takes for the pressure to drop when you turn the car off.
You need a scanner, Actron makes one that you can get at Autozone, for OBD-1 cars. You can buy the OBD-II and theres an adapter for OBD-1. This will tell you what the ECM sees as the TPS voltage. It should be 0.54 V at idle. IAC counts are another thing the scanner will tell you, if they are 100+, theres probably a problem there. If the idle is hunting or struggling, they'll be all over the place, most likely.
Look through all the hoses and see if you can find the vac leak. If you find one, unplug the battery and clear the code, and see if it runs right after a restart.
Lots of things to check, do one at a time.
Last edited by vader86; Oct 15, 2019 at 11:05 AM.
Team Owner
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IAC counts are another thing the scanner will tell you, if they are 100+, theres probably a problem there. If the idle is hunting or struggling, they'll be all over the place, most likely.
Look through all the hoses and see if you can find the vac leak. If you find one, unplug the battery and clear the code, and see if it runs right after a restart.
Look through all the hoses and see if you can find the vac leak. If you find one, unplug the battery and clear the code, and see if it runs right after a restart.
I have always done it once the engine reaches closed loop. Never tried it with the engine cold. Know of any difference?
Last edited by aklim; Oct 15, 2019 at 11:19 AM.