LT1 Reoccuring Misfire
08-02-2010, 08:05 AM
#1
Drifting
Thread Starter
LT1 Reoccuring Misfire
I’ve been hunting down a misfire in my LT1 for a few months now.
I haven’t accepted that the problem is my opti just yet and this is why:
When the problem started I had a misfire at low rpm under high load. For example 1st gear, clutch fully engaged and under 2000 rpm – give it half throttle and it would misfire a few times, tach would be erratic, and then it would pickup and rev fine/powerfully. It also had an occasional miss in the higher rpm range, north of 5krpm – and this would be under full throttle.
I replaced ignition module with aftermarket unit (yes I did use the correct heat transfer compound) and the coil for an MSD. The problem disappeared for a while. Within 1500 miles it reemerged as a low rpm misfire – same low rpm misfire as described above. Above 2000 rpm there are no issues and it would rev powerfully all the way to the top.
After ohming out the MSD coil with a digital volt/ohm meter I determined it to be faulty as it had large variance of resistance between the tower and the different pin connections for the harness. So, I again replaced both coil and ICM. This time I used genuine GM parts pulled off a 93 LT1. I ohmed out the 93 coil and it checked out good with very close readings between tower and the different pinouts. With the ‘new’ coil and ICM the engine ran beautifully. No issues whatsoever. This moment of clarity lasted about a week. I took the car for a long leisurely drive yesterday to a hiking destination. On the return trip I noticed a misfire. I only noticed it in 1st gear – immediately after takeoff (normal driving) after the clutch is fully engaged and as I’m beginning to increase throttle it would stutter momentarily (less than a second) and then resume acceleration as normal – again only under 2000 rpm and probably closer to 1500. Could it be the coil again? The ICM? I don’t know but it sure is getting frustrating. Granted at this point everything was heat-soaked but I have to believe that GM designed these parts to run in a hot environment. I have the coil/icm mounting plate spaced off the cylinder head with a few washers to facilitate cooling. I do acknowledge that I threw 17 year old parts at a problem and kept my fingers crossed so it’s hard to say what the real problem may be.
The reason I don’t think it is the opti is because everytime I replaced coil and ICM the issue disappeared, at least for a while. If the problem was the opti I imagine replacing coil and ICM would not temporarily eliminate the issue. That said, my car is a 94 and to the best of my knowledge it has the original opti at 108k miles. I will be replacing it when I install a hotcam and upgrade to a vented unit (hopefully a project for this coming winter). The fact that my tach becomes slightly erratic during the misfire leads me to believe that this has to be an ignition issue and cannot be a fuel/injector issue – is that logic sound? Also as a side note, I checked the spark plug wires for damage and correct routing. I did not see any issues with the wires. They are AC Delco replacements with about 2k miles, same for the plugs.
I noticed the GM coil from the 93 seems to not rev as powerfully in the high rpm range as the MSD did. Could be in my head but when the MSD was working it seemed more powerful up top. Can anyone recommend a quality performance coil? After the MSD failed with under 2k miles on it I won’t be putting one of those back in. I’ve had issues with Accel coils in the past on other cars too. I’ve heard good things about Jacobs coils but I am unsure if they make one for the LT1 application. Any recommendations? American made would be nice! I know the coil isn’t necessarily my problem.
Thanks for reading.
I haven’t accepted that the problem is my opti just yet and this is why:
When the problem started I had a misfire at low rpm under high load. For example 1st gear, clutch fully engaged and under 2000 rpm – give it half throttle and it would misfire a few times, tach would be erratic, and then it would pickup and rev fine/powerfully. It also had an occasional miss in the higher rpm range, north of 5krpm – and this would be under full throttle.
I replaced ignition module with aftermarket unit (yes I did use the correct heat transfer compound) and the coil for an MSD. The problem disappeared for a while. Within 1500 miles it reemerged as a low rpm misfire – same low rpm misfire as described above. Above 2000 rpm there are no issues and it would rev powerfully all the way to the top.
After ohming out the MSD coil with a digital volt/ohm meter I determined it to be faulty as it had large variance of resistance between the tower and the different pin connections for the harness. So, I again replaced both coil and ICM. This time I used genuine GM parts pulled off a 93 LT1. I ohmed out the 93 coil and it checked out good with very close readings between tower and the different pinouts. With the ‘new’ coil and ICM the engine ran beautifully. No issues whatsoever. This moment of clarity lasted about a week. I took the car for a long leisurely drive yesterday to a hiking destination. On the return trip I noticed a misfire. I only noticed it in 1st gear – immediately after takeoff (normal driving) after the clutch is fully engaged and as I’m beginning to increase throttle it would stutter momentarily (less than a second) and then resume acceleration as normal – again only under 2000 rpm and probably closer to 1500. Could it be the coil again? The ICM? I don’t know but it sure is getting frustrating. Granted at this point everything was heat-soaked but I have to believe that GM designed these parts to run in a hot environment. I have the coil/icm mounting plate spaced off the cylinder head with a few washers to facilitate cooling. I do acknowledge that I threw 17 year old parts at a problem and kept my fingers crossed so it’s hard to say what the real problem may be.
The reason I don’t think it is the opti is because everytime I replaced coil and ICM the issue disappeared, at least for a while. If the problem was the opti I imagine replacing coil and ICM would not temporarily eliminate the issue. That said, my car is a 94 and to the best of my knowledge it has the original opti at 108k miles. I will be replacing it when I install a hotcam and upgrade to a vented unit (hopefully a project for this coming winter). The fact that my tach becomes slightly erratic during the misfire leads me to believe that this has to be an ignition issue and cannot be a fuel/injector issue – is that logic sound? Also as a side note, I checked the spark plug wires for damage and correct routing. I did not see any issues with the wires. They are AC Delco replacements with about 2k miles, same for the plugs.
I noticed the GM coil from the 93 seems to not rev as powerfully in the high rpm range as the MSD did. Could be in my head but when the MSD was working it seemed more powerful up top. Can anyone recommend a quality performance coil? After the MSD failed with under 2k miles on it I won’t be putting one of those back in. I’ve had issues with Accel coils in the past on other cars too. I’ve heard good things about Jacobs coils but I am unsure if they make one for the LT1 application. Any recommendations? American made would be nice! I know the coil isn’t necessarily my problem.
Thanks for reading.
08-02-2010, 08:56 AM
#2
Pro
My wife has a '94 Vette, and I am "tasked" with keeping it riunning.
We have a very similar problem that is driving me nuts. Occassionally between 1,600 to 2,500 RPM under light to moderate load the engine will miss. It's like a single cylinder misfire. In her case, I think it might be the injectors as they have about 175,000 miles on them.
Her car has an LT1 Hot Cam with a custom tune geared to drivability and not absolute paek power. I also changed the ignition system to Bob Bailiey's LTCC which uses LS1 coil packs (one for each spark plug).
A couple of thoughts about your car. I would be careful with the aftermarket high-enery coils that still use the opti-spark. For a while we had a Dyna-Tech optispark and they said not to use them. The reason is that the higher voltage casues more ozone wich eventually will (not if) take out the opti-spark, even a vented one (we converted her car to a vented opti-spark). The reason we went to the LTCC......really really good spark and no high voltage whatsoever through the opti.
In your case I would be lookiing at the grounds, bad connectors or possibly a broken (frayed) wire going to the ICM and coils. We have had a number of "issues" with corrded connectors.
We have a very similar problem that is driving me nuts. Occassionally between 1,600 to 2,500 RPM under light to moderate load the engine will miss. It's like a single cylinder misfire. In her case, I think it might be the injectors as they have about 175,000 miles on them.
Her car has an LT1 Hot Cam with a custom tune geared to drivability and not absolute paek power. I also changed the ignition system to Bob Bailiey's LTCC which uses LS1 coil packs (one for each spark plug).
A couple of thoughts about your car. I would be careful with the aftermarket high-enery coils that still use the opti-spark. For a while we had a Dyna-Tech optispark and they said not to use them. The reason is that the higher voltage casues more ozone wich eventually will (not if) take out the opti-spark, even a vented one (we converted her car to a vented opti-spark). The reason we went to the LTCC......really really good spark and no high voltage whatsoever through the opti.
In your case I would be lookiing at the grounds, bad connectors or possibly a broken (frayed) wire going to the ICM and coils. We have had a number of "issues" with corrded connectors.
08-02-2010, 10:01 AM
#3
Drifting
Thread Starter
Great suggestion. I will inspect the wire harnesses for corrosion on the terminals. May I ask how you cleaned yours? Or is it a matter of replacement? The coil and ICM harnesses are easy enough to access but I bet inspecting the opti harness will be difficult.
I had considered that the wiring could be faulty and was considering running a new ground and new hot wire (would have to be temporary as it would be always hot) to the ICM and coil as a test to see if it cured the problem.
I had considered that the wiring could be faulty and was considering running a new ground and new hot wire (would have to be temporary as it would be always hot) to the ICM and coil as a test to see if it cured the problem.
08-02-2010, 01:02 PM
#4
Pro
Her car is her only car and a daily driver so the work is done at a local shop. Also I work 350 miles from home so only home on weekends.
In some cases, the shop has been able to clean the terminals (take them out of the terminal block and clean the corrosion off). However, we have had to replace a numbers of the terminals too.
Electrical problems are the worst. A story............
Last summer she had half the dash lights and the lights on the door stay on all the time. All the other lights worked fine.
It was at the shop for DAYS. They tore the dash all apart to fiind out what was wrong. Turns out that the wiring harness in the door was resting on a screw (from the factory) and 15-16 years later it finally wore through the harness and insulation and shorten two wires. Fixed them and it has been fine since.
Good luck tracking down your gremlin.
In some cases, the shop has been able to clean the terminals (take them out of the terminal block and clean the corrosion off). However, we have had to replace a numbers of the terminals too.
Electrical problems are the worst. A story............
Last summer she had half the dash lights and the lights on the door stay on all the time. All the other lights worked fine.
It was at the shop for DAYS. They tore the dash all apart to fiind out what was wrong. Turns out that the wiring harness in the door was resting on a screw (from the factory) and 15-16 years later it finally wore through the harness and insulation and shorten two wires. Fixed them and it has been fine since.
Good luck tracking down your gremlin.
