1991 Idling Issues
Thread Starter
Racer
Joined: Aug 2021
Posts: 409
Likes: 82
From: NW Arkansas
1991 Idling Issues
A few seconds after starting my corvette the idle gets real rough and hunts around. It will also drop the idle, down enough that the interior lights will dim, but it usually is able to recover. Unplugging the MAP sensor will cause the car to idle steadily at the correct speed, but the sensor is brand new and the car doesn't rev smoothly when it's unplugged. The car starts idling rough before the engine is warm, maybe 30-40 seconds after the engine is started.
When I try to drive the car, revving the engine is really rough and it hesitates really bad. However, after maybe 30 seconds it smooths out and revs perfectly, although the idle is still rough. So if I wait a few seconds before driving the car, it'll drive perfectly but idles rough.
recently I cleaned the IAC, replaced the TPS, replaced fuel injectors, and the fuel pressure regulator.
The fuel rail is leaking a little bit where the return line connects to the fuel pressure regulator. I'm thinking about buying a moates cable to read sensor readouts, because I suspect a sensor might be the issue. I'm not sure though.
Any ideas?
Thanks!
(The car is a 1991 6-speed)
When I try to drive the car, revving the engine is really rough and it hesitates really bad. However, after maybe 30 seconds it smooths out and revs perfectly, although the idle is still rough. So if I wait a few seconds before driving the car, it'll drive perfectly but idles rough.
recently I cleaned the IAC, replaced the TPS, replaced fuel injectors, and the fuel pressure regulator.
The fuel rail is leaking a little bit where the return line connects to the fuel pressure regulator. I'm thinking about buying a moates cable to read sensor readouts, because I suspect a sensor might be the issue. I'm not sure though.
Any ideas?
Thanks!
(The car is a 1991 6-speed)
Le Mans Master
Joined: Feb 2000
Posts: 9,060
Likes: 214
From: Epping NH
Any codes being thrown? https://tech.corvettecentral.com/201...trouble-codes/
Always a chance you just got a bad new MAP.
Always a chance you just got a bad new MAP.
Drifting
Joined: Jan 2021
Posts: 1,312
Likes: 383
From: Northern California
Besides the obvious in repairing that O-ring leak quickly, I would ask; When you cleaned the IAC and replaced the TPS, did you follow the process to reset Minimum Idle Air and set the TPS position for .540 volts at closed throttle? Did you also verify that you have ~40psi fuel pressure with the vacuum line disconnected at idle? If not, there are several posts on this forum with the process for all of the above so let us know...
Instructor
Joined: Oct 2018
Posts: 225
Likes: 81
From: Vancouver, WA
I don't know if the procedure ifs different for a 90-91 as opposed to the 85-89's, so I am hesitant to post the procedure from my 85,88 FSM's.
Thread Starter
Racer
Joined: Aug 2021
Posts: 409
Likes: 82
From: NW Arkansas
Sorry for the late response.
There are no codes stored.
I reset the IAC when I replaced it, but I have not been able to replace the TPS adjustment yet. I wasn't able to figure out how to actually read the voltage of the sensor, but I bought a diagnostic cable from moates that I hope to be able to use with tunerpro rt to do that.
Fuel pressure was about 40psi and held with the ignition off, but I didn't check at idle because I tried that once and the gauge started leaking, and I had to get a new one. I think fuel pressure should be ok though because it has a new FPR, injectors, and fuel pump.
There are no codes stored.
I reset the IAC when I replaced it, but I have not been able to replace the TPS adjustment yet. I wasn't able to figure out how to actually read the voltage of the sensor, but I bought a diagnostic cable from moates that I hope to be able to use with tunerpro rt to do that.
Fuel pressure was about 40psi and held with the ignition off, but I didn't check at idle because I tried that once and the gauge started leaking, and I had to get a new one. I think fuel pressure should be ok though because it has a new FPR, injectors, and fuel pump.
Drifting
Joined: May 2020
Posts: 1,661
Likes: 701
From: Baton Rouge, Louisiana, metro area
Are you sure your new IAC is the right one? Did you compare the pintle shapes with your old one?
I found a loose connection on my TPS plug when my 91 was exhibiting similar symptoms.
I found a loose connection on my TPS plug when my 91 was exhibiting similar symptoms.
Thread Starter
Racer
Joined: Aug 2021
Posts: 409
Likes: 82
From: NW Arkansas
I'm pretty certain the new IAC is the right one. Comparing them they looked identical, and the symptoms didn't change at all after replacing it, so it seems like it had no effect on the problem.
Drifting
Joined: May 2020
Posts: 1,661
Likes: 701
From: Baton Rouge, Louisiana, metro area
Ok. Well…if you already checked the IAC, how about your vacuum lines? Hard to see a very small crack, especially if it back near the firewall.
The lines on my 91 were very brittle and had a few cracks. I replaced all my lines under the hood with new rubber vacuum tube. I ran new lines all the way back to the harness at the firewall behind the distributor. There is a little check valve near the distributor with a few lines plugged to it; I relocated that part to under my master cylinder.
The lines on my 91 were very brittle and had a few cracks. I replaced all my lines under the hood with new rubber vacuum tube. I ran new lines all the way back to the harness at the firewall behind the distributor. There is a little check valve near the distributor with a few lines plugged to it; I relocated that part to under my master cylinder.
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Thread Starter
Racer
Joined: Aug 2021
Posts: 409
Likes: 82
From: NW Arkansas
I haven't checked all the vacuum lines. I did spray throttle body cleaner around most of the vacuum lines attached to the intake to try and find any leaks, but didn't find any. I can do a more thorough search soon though.
There is a sort of box with large hoses coming from it next to the throttle body on the passenger side. It's in between the throttle body and A/C compressor. One of the hoses isn't connected to anything, although I don't believe it's causing a vacuum leak as I can't feel any vacuum being drawn from it while the engine is running. Do you know what this is for?
There is a sort of box with large hoses coming from it next to the throttle body on the passenger side. It's in between the throttle body and A/C compressor. One of the hoses isn't connected to anything, although I don't believe it's causing a vacuum leak as I can't feel any vacuum being drawn from it while the engine is running. Do you know what this is for?
Drifting
Joined: May 2020
Posts: 1,661
Likes: 701
From: Baton Rouge, Louisiana, metro area
I haven't checked all the vacuum lines. I did spray throttle body cleaner around most of the vacuum lines attached to the intake to try and find any leaks, but didn't find any. I can do a more thorough search soon though.
There is a sort of box with large hoses coming from it next to the throttle body on the passenger side. It's in between the throttle body and A/C compressor. One of the hoses isn't connected to anything, although I don't believe it's causing a vacuum leak as I can't feel any vacuum being drawn from it while the engine is running. Do you know what this is for?
There is a sort of box with large hoses coming from it next to the throttle body on the passenger side. It's in between the throttle body and A/C compressor. One of the hoses isn't connected to anything, although I don't believe it's causing a vacuum leak as I can't feel any vacuum being drawn from it while the engine is running. Do you know what this is for?
Do you have the Factory Service Manual? Solid gold, it is. Worth every penny.
Thread Starter
Racer
Joined: Aug 2021
Posts: 409
Likes: 82
From: NW Arkansas
Ah alright that's useful to know. I didn't have much luck figuring out what that is on my own. I do have a factory service manual, I got one from Rock Auto on a CD.
I'm up in Fayetteville. It's about an hour south of Missouri. I don't know if I would be able to meet anyone because my schedule is crazy. I only get a few hours at a time to actually work on the car, I use breaks in school and work and things to explore the forums and find out what I need to look at.
I'm up in Fayetteville. It's about an hour south of Missouri. I don't know if I would be able to meet anyone because my schedule is crazy. I only get a few hours at a time to actually work on the car, I use breaks in school and work and things to explore the forums and find out what I need to look at.
Burning Brakes
Joined: Aug 2001
Posts: 1,038
Likes: 15
From: Brick- Next to the land of Mojo NJ
Check TPS voltage. I put a new one and my 1991 and could not get voltage down sufficiently. Potentiometer must have been clocked wrong inside the case. Idled at like 1200 rpm because of this.
Drifting
Joined: May 2020
Posts: 1,661
Likes: 701
From: Baton Rouge, Louisiana, metro area
Thread Starter
Racer
Joined: Aug 2021
Posts: 409
Likes: 82
From: NW Arkansas
I apologize again for the late responses. My TPS voltage is about .88, which is high but is as low as I can get it to go and I don't believe that is causing the issue, as the car idles at the correct speed, it just becomes very rough after a few seconds.
If I give it low throttle (in neutral), the car will actually rev down and almost die. If I punch the throttle however, and rev it to 2k rpm or so, the throttle response is perfect and remains that way until I start the car again. Additionally, unplugging the MAP sensor completely fixes the idling issue, but the car will not rev with it unplugged.
The MAP voltage hovers right around 1 at idle, and when revving it drops to between .8 and sometimes .3, but usually goes back up after a few seconds.
Any ideas?
I just went outside and ran the car connected to TunerPro. The O2 sensor was reading 456.4, and never moved the whole time I ran the car (enough to warm to operating temperature. I kept it alive by pressing the gas slightly). The car was in open loop the entire time, even after the radiator fans came on. The software indicated that the 02 was not ready the whole time. This looks like the problem to me. Any thoughts on that?
If I give it low throttle (in neutral), the car will actually rev down and almost die. If I punch the throttle however, and rev it to 2k rpm or so, the throttle response is perfect and remains that way until I start the car again. Additionally, unplugging the MAP sensor completely fixes the idling issue, but the car will not rev with it unplugged.
The MAP voltage hovers right around 1 at idle, and when revving it drops to between .8 and sometimes .3, but usually goes back up after a few seconds.
Any ideas?
I just went outside and ran the car connected to TunerPro. The O2 sensor was reading 456.4, and never moved the whole time I ran the car (enough to warm to operating temperature. I kept it alive by pressing the gas slightly). The car was in open loop the entire time, even after the radiator fans came on. The software indicated that the 02 was not ready the whole time. This looks like the problem to me. Any thoughts on that?
Last edited by sheriffjim; May 2, 2022 at 10:10 PM.