1991 Misfiring when warm
Thread Starter
Racer
Joined: Aug 2021
Posts: 409
Likes: 82
From: NW Arkansas
1991 Misfiring when warm
I have a 1991 corvette that is misfiring when it gets to operating temperature. (6spd manual). It will misfire all the time, even revving in neutral. I drove the car a lot yesterday (at operating temp.) and did not have any issues. Putting it under heavy load (floored it in 4th gear going 20mph) it smoothed it out but as the car sped up the misfire got worse. Idle is normal but revving in neutral causes roughness. The car is bucking very badly and driving it feels like the engine is ripping itself apart. Distributor recently replaced. Anyone have any ideas?
Thread Starter
Racer
Joined: Aug 2021
Posts: 409
Likes: 82
From: NW Arkansas
no... I didn't do that yet. I have a spark tester and it looks like they're working fine but I guess that's a good place to start. The ones on it aren't stock but I didn't put them on there and don't know how long they've been there. I'll go see about buying some new ones if you think that'll help.
Safety Car
Joined: May 2001
Posts: 4,759
Likes: 1,647
From: Fairfax Virginia
Hello Again Sheriffjim!
The next place to look might be the Ignition Control module under the rotor inside the distributor. That device is mounted on a heat sink and needs the thermal transfer material under the Module or else it will fail prematurely. These modules will start to fail when they get Hot.
I have seen Carbon tracking inside the Distributor cap that caused a regular miss. If you have a black light handy you can see the carbon tracks themselves if they are present
The other part that could do this is a would be the MAP or MAF sensor. When a MAP/MAF goes out the drive ability goes with it. Mine failed in my DD and it would miss under loads at first. On my C4 I have the MAF and they are notorious for having issues like you are describing. be sure to check BOTH relays that operate the MAF (IF you have the MAF).
Misfiring when Warm could also be an injector issue. Have you tried any Fuel Injector Cleaner to see if that helps?
Even a Spark Plug Wire can induce misses as they warm up and the resistance becomes an issue. I like to check the spark itself to be sure it is a HOT spark using a "gap widening test tool". I like to see the spark itself to know that it is working properly.
Hopefully others will pitch in their thoughts as well.
Merry Christmas to you and your family Sheriffjim!
The next place to look might be the Ignition Control module under the rotor inside the distributor. That device is mounted on a heat sink and needs the thermal transfer material under the Module or else it will fail prematurely. These modules will start to fail when they get Hot.
I have seen Carbon tracking inside the Distributor cap that caused a regular miss. If you have a black light handy you can see the carbon tracks themselves if they are present
The other part that could do this is a would be the MAP or MAF sensor. When a MAP/MAF goes out the drive ability goes with it. Mine failed in my DD and it would miss under loads at first. On my C4 I have the MAF and they are notorious for having issues like you are describing. be sure to check BOTH relays that operate the MAF (IF you have the MAF).
Misfiring when Warm could also be an injector issue. Have you tried any Fuel Injector Cleaner to see if that helps?
Even a Spark Plug Wire can induce misses as they warm up and the resistance becomes an issue. I like to check the spark itself to be sure it is a HOT spark using a "gap widening test tool". I like to see the spark itself to know that it is working properly.
Hopefully others will pitch in their thoughts as well.
Merry Christmas to you and your family Sheriffjim!
Drifting
Joined: Feb 2016
Posts: 1,384
Likes: 339
From: Simi Valley CA
A simple test I like to do is run the engine at night or in the complete dark with the hood open. You can see blue sparks if your plugs are arcing on anything. The 92 I recently bought looked like it had new plug wires. I did that test and they were arcing everywhere so they were cheap wires.
Thread Starter
Racer
Joined: Aug 2021
Posts: 409
Likes: 82
From: NW Arkansas
What changes when the car warms up? I was testing it and it wasn't misfiring, I drove it down the street to warm it up and it started bucking almost as soon as it reached operating temperature. So I would assume the car does something different once it warms up but I don't know what that might be. Any clues?
It also has a service engine light after the car started bucking, I'm going to go figure out what that is for
It also has a service engine light after the car started bucking, I'm going to go figure out what that is for
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Thread Starter
Racer
Joined: Aug 2021
Posts: 409
Likes: 82
From: NW Arkansas
Alright checking the codes I got H13 and H23. That indicates O2 and manifold air temperature sensors are faulty. What is the manifold air temperature sensor? can it cause misfires like this?
Instructor
Joined: Oct 2018
Posts: 225
Likes: 81
From: Vancouver, WA
You need to fix these codes first.
I attached a few pages from the 1991 Factory Service Manual on the IAT and O2 sensors.
O2 sensors do go bad and some people change them as a regular maintenance item.
Also note that you have a MAP (Manifold Air Pressure) system of measuring incoming air volume vs the MAF system used on 85-89 L98's. (assuming yours is stock).
I would do a full tune up (plugs, wires, cap, rotor, timing check) after you fix the Code 13 and Code 23 errors.
Thread Starter
Racer
Joined: Aug 2021
Posts: 409
Likes: 82
From: NW Arkansas
Bad IAT and O2 sensor can cause these issues.
You need to fix these codes first.
I attached a few pages from the 1991 Factory Service Manual on the IAT and O2 sensors.
O2 sensors do go bad and some people change them as a regular maintenance item.
Also note that you have a MAP (Manifold Air Pressure) system of measuring incoming air volume vs the MAF system used on 85-89 L98's. (assuming yours is stock).
I would do a full tune up (plugs, wires, cap, rotor, timing check) after you fix the Code 13 and Code 23 errors.
You need to fix these codes first.
I attached a few pages from the 1991 Factory Service Manual on the IAT and O2 sensors.
O2 sensors do go bad and some people change them as a regular maintenance item.
Also note that you have a MAP (Manifold Air Pressure) system of measuring incoming air volume vs the MAF system used on 85-89 L98's. (assuming yours is stock).
I would do a full tune up (plugs, wires, cap, rotor, timing check) after you fix the Code 13 and Code 23 errors.
Instructor
Joined: Oct 2018
Posts: 225
Likes: 81
From: Vancouver, WA
Bad IAT and O2 sensor can cause these issues.
You need to fix these codes first.
I attached a few pages from the 1991 Factory Service Manual on the IAT and O2 sensors.
O2 sensors do go bad and some people change them as a regular maintenance item.
Also note that you have a MAP (Manifold Air Pressure) system of measuring incoming air volume vs the MAF system used on 85-89 L98's. (assuming yours is stock).
I would do a full tune up (plugs, wires, cap, rotor, timing check) after you fix the Code 13 and Code 23 errors.
You need to fix these codes first.
I attached a few pages from the 1991 Factory Service Manual on the IAT and O2 sensors.
O2 sensors do go bad and some people change them as a regular maintenance item.
Also note that you have a MAP (Manifold Air Pressure) system of measuring incoming air volume vs the MAF system used on 85-89 L98's. (assuming yours is stock).
I would do a full tune up (plugs, wires, cap, rotor, timing check) after you fix the Code 13 and Code 23 errors.
Thread Starter
Racer
Joined: Aug 2021
Posts: 409
Likes: 82
From: NW Arkansas
I replaced both sensors. SES light is off but the problem is not fixed. I think it's running too rich and thats causing bad combustion, because it causes loud backfires and makes the exhaust smell like gasoline. Any ideas? I don't think it has to do with ignition because it seems like its running rich, and because it only happens after the car reaches operating temperature.
Burning Brakes
Joined: Nov 2017
Posts: 1,060
Likes: 80
From: Fort Mill, SC
I replaced both sensors. SES light is off but the problem is not fixed. I think it's running too rich and thats causing bad combustion, because it causes loud backfires and makes the exhaust smell like gasoline. Any ideas? I don't think it has to do with ignition because it seems like its running rich, and because it only happens after the car reaches operating temperature.
I dont know how long it takes the lt1 ECM to make the corrections once you fix something... could be it needs time to do so? Im being optimistic.
The only way I know of to avoid replacing parts is with an oscilloscope. You can pretty reliably diagnose things like your optispark without replacing it and then having done that incredibly long painful job winding up with the same problem and an inferior aftermarket distributor.
Sure, you will spend a little, and there is a learning curve but incredibly satisfying knowing you can diagnose without replacing parts and kneeling down to the good Lord before you turn the key. If you search I made a post on this where I sourced brand new for peanuts; ccp made unfortunately.
Beyond that, follow the service manual and make sure you have a diagnostic tool that goes deep into GM values.. a tech II built by the ccp unfortunately will do this.
Or; throw parts at it. at least you know they are new.. Im so done tossing parts at cars on a hunch.
Merry Christmas.
PS.. Sorry if this is not an lt1.. I forget if 91 is included. Same applies though for the most part..
Last edited by barchetta1; Dec 25, 2021 at 02:16 PM.
Thread Starter
Racer
Joined: Aug 2021
Posts: 409
Likes: 82
From: NW Arkansas
Alright I appreciate the advice. I won't rule it out, and I'll keep looking. My car does have an L98, so no optispark, however I will look into the ECM and distributor to see if I can find any faults there. I definitely don't just want to throw parts at it so I'm slowly but surely working through testing everything I can. I bought a factory service manual but its still in the mail, but I was able to find some charts that were useful.
I have a theory that the coolant temperature sensor might be faulty because a previous owner overfilled the coolant (completely filled the expansion tank), and the coolant level sensor went out less than a day before the issue started occurring, now displays low coolant light even though system is full. I honestly didn't realize the coolant was overfull until the light came on, and am in the process of getting what I need to flush and refill the system. From the charts I mentioned I noticed that the coolant temperature sensor was part of the ECM consideration for the air/fuel mixture. No SES light though... Either way I'm still testing, I won't rule out ignition though.
Thanks for the help and have a Merry Christmas!
I have a theory that the coolant temperature sensor might be faulty because a previous owner overfilled the coolant (completely filled the expansion tank), and the coolant level sensor went out less than a day before the issue started occurring, now displays low coolant light even though system is full. I honestly didn't realize the coolant was overfull until the light came on, and am in the process of getting what I need to flush and refill the system. From the charts I mentioned I noticed that the coolant temperature sensor was part of the ECM consideration for the air/fuel mixture. No SES light though... Either way I'm still testing, I won't rule out ignition though.
Thanks for the help and have a Merry Christmas!
Safety Car
Joined: Mar 2017
Posts: 4,202
Likes: 1,323
From: LeClaire Iowa
I don't think you can "overfill" the cooling system. YES, you can overfill the expansion tank but this will not affect the sensor. The cooling system is either full or not full. No way to compress the coolant to overfill it. Dan