Misfiring issues
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Joined: Feb 2004
Posts: 174
From: London, England
Misfiring issues (update)
A misfire problem has occured on a friends 1994 LT1 Auto Corvette.
The car had no issues until it was parked up in a garage for about 1 month recently. The car started up with a bit of misfiring but this soon cleared and it drove fine, as the engine warmed up it started to misfire again. It got to the point where putting the car into drive and trying to pull away caused it to cut out. Acceleration was very bad and caused a lot of backfiring and misfiring there is also a lot of knocking/pinking.
After the engine has been left to sit for an hour or so it would be very difficult to start. The car starts fine from cold with no issues.
To try and rectify the problem we brought a new set of spark plugs, the old ones appeared to be in good condition, this improved the condition to the extent that it now no longer cuts out and is generally better although it still misfires and knocks apon acceleration/revving.
We did the following to try and further correct the problem but to no success:
Replaced HT leads and Coil lead
Checked voltage to ICM/Coil
Systematically removed each plug lead individually and then ran the engine to see if we could isolate a problem cylinder, all responded the same, I guess this also means the fueling is adequate?
Checked the spark at each plug lead
Checked the voltage at the TPS
Checked for vacuum leaks
Checked EGR operation
We don't have access to a scanner, plus we have the obd1 pcm with the obd2 connecter harness, although the engine light is not on.
Any other suggestions or ideas would be welcome, thanks!
The car had no issues until it was parked up in a garage for about 1 month recently. The car started up with a bit of misfiring but this soon cleared and it drove fine, as the engine warmed up it started to misfire again. It got to the point where putting the car into drive and trying to pull away caused it to cut out. Acceleration was very bad and caused a lot of backfiring and misfiring there is also a lot of knocking/pinking.
After the engine has been left to sit for an hour or so it would be very difficult to start. The car starts fine from cold with no issues.
To try and rectify the problem we brought a new set of spark plugs, the old ones appeared to be in good condition, this improved the condition to the extent that it now no longer cuts out and is generally better although it still misfires and knocks apon acceleration/revving.
We did the following to try and further correct the problem but to no success:
Replaced HT leads and Coil lead
Checked voltage to ICM/Coil
Systematically removed each plug lead individually and then ran the engine to see if we could isolate a problem cylinder, all responded the same, I guess this also means the fueling is adequate?
Checked the spark at each plug lead
Checked the voltage at the TPS
Checked for vacuum leaks
Checked EGR operation
We don't have access to a scanner, plus we have the obd1 pcm with the obd2 connecter harness, although the engine light is not on.
Any other suggestions or ideas would be welcome, thanks!
Last edited by CamaroUK; Apr 4, 2005 at 04:06 PM. Reason: updated
Thread Starter
Registered User
Joined: Feb 2004
Posts: 174
From: London, England
Re: Misfiring issues
***UPDATE***
We just found out that when the misfiring occurs there is arcing to the block from where the coil lead plugs into the opti.
This all occured before we touched anything, we since replaced the coil lead as stated in my original post and still its arcing, so we put the old lead back in and it appears to have corrected the problem to an extent which has had us confused.
The misfiring is almost completely gone, the engine revs freely but now the engine is still knocking/pinking under load/acceleration.
Why would the spark arc to the engine and not go into the opti? I am guessing the knocking is a result of a lack of spark to some of the cylinders but why?
We just found out that when the misfiring occurs there is arcing to the block from where the coil lead plugs into the opti.
This all occured before we touched anything, we since replaced the coil lead as stated in my original post and still its arcing, so we put the old lead back in and it appears to have corrected the problem to an extent which has had us confused.
The misfiring is almost completely gone, the engine revs freely but now the engine is still knocking/pinking under load/acceleration.
Why would the spark arc to the engine and not go into the opti? I am guessing the knocking is a result of a lack of spark to some of the cylinders but why?
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Joined: May 2003
Posts: 2,275
Re: Misfiring issues
This will sound dumb but remember current flows in the path of least resistance. Theory would say that the path from the coil to the opti has more resistance than from the coil thru air to the block. Some where from the coil all the way to the plug, the resistance has increased. I believe even though you don't see the arcing, it's still there.
Why don't pull the cap off the opti and have a look inside? If it's been on for while, you will be surprised.
Why don't pull the cap off the opti and have a look inside? If it's been on for while, you will be surprised.
Thread Starter
Registered User
Joined: Feb 2004
Posts: 174
From: London, England
Re: Misfiring issues
***Update***
Ok, it seems that the arcing problem has been corrected by modifying the connector of the opti end of the coil lead, at least we can no longer see any sparks around the area.
Now the misfiring was apparently cured and the arcing also, but the car detonated badly apon acceleration once the engine has reached its normal operating temperature.
We assumed the opti was refusing to deliver spark to the plugs, and therefore guessed that the non sparking cylinders are resulting in detonation under the compression.
As a long shot to deter detonation we tried 98 RON petrol instead of the regular 95. The problem is now apparently cured, and the car accelerates without missing a beat.
I have to ask how can this be? If the detonation was caused by lack of spark at the plugs, which was remedied by higher octane fuel, we should now have a misfire but we don't. Whats going on?
I would bet putting 95 RON back in there would allow the detonation to return.
Any thoughts?
Ok, it seems that the arcing problem has been corrected by modifying the connector of the opti end of the coil lead, at least we can no longer see any sparks around the area.
Now the misfiring was apparently cured and the arcing also, but the car detonated badly apon acceleration once the engine has reached its normal operating temperature.
We assumed the opti was refusing to deliver spark to the plugs, and therefore guessed that the non sparking cylinders are resulting in detonation under the compression.
As a long shot to deter detonation we tried 98 RON petrol instead of the regular 95. The problem is now apparently cured, and the car accelerates without missing a beat.
I have to ask how can this be? If the detonation was caused by lack of spark at the plugs, which was remedied by higher octane fuel, we should now have a misfire but we don't. Whats going on?
I would bet putting 95 RON back in there would allow the detonation to return.
Any thoughts?
Registered User
Joined: May 2003
Posts: 2,275
Re: Misfiring issues
The problem you had is "preignition". Where the fuel burns before the spark ignites. Low octane fuel burns too fast. In certain conditions, before it should. I think 95 is a little too low for our vehicles. Apparently you have no spark problem. One thing that will definately help is using Seafoam to clean out any carbon build up within the cylinder heads. Just don't use it with the cats connected.
Also, a jump or 2 in heat range cooler in plug choices could help you go back to lower octane.
Also, a jump or 2 in heat range cooler in plug choices could help you go back to lower octane.
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