Chasing intermittent misfire
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Joined: Mar 2005
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From: Melbourne Fl.
Chasing intermittent misfire
I've been chasing an intermittent engine misfire for about a year in my 06 LS2. It has 16000 miles on it. The car is not driven every day and sometimes it sits for a few weeks before it is used again. Usually the misfire goes away for about a month or more then reappears for a short time. It's not going away now and I'm trying to figure out the cause. The misfire is evident during acceleration and causes the check engine light to flash then set a P0300 code (random misfire). It can also be detected in the idle exhaust. So far I've:
1. Had good spark on all cylinders as indicated using one of those spark detectors that is placed between the plug and wire.
2. I checked the fuel pressure and found that it does not leak down when the pump is turned off.
3. I looked for vacuum leaks in the hoses around the top of the engine.
Next I am considering cleaning the fuel injectors with a canister type fuel injector cleaner that connects to the fuel rail.
Is there anything else I should be looking at?
Warren
1. Had good spark on all cylinders as indicated using one of those spark detectors that is placed between the plug and wire.
2. I checked the fuel pressure and found that it does not leak down when the pump is turned off.
3. I looked for vacuum leaks in the hoses around the top of the engine.
Next I am considering cleaning the fuel injectors with a canister type fuel injector cleaner that connects to the fuel rail.
Is there anything else I should be looking at?
Warren
Last edited by Warren Seale; Jul 28, 2019 at 11:22 AM.
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Joined: Sep 2013
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From: Cape Coral, Florida
Warren,
Do you have a scan tool to determine exactly what cylinder is possibly misfiring ??...if not you can do a “cylinder drop test” and possibly locate the misfiring cylinder (video enclosed if you don’t know how to do it). If you have located the misfiring cylinder and without any specialized equipment such as a lab scope it’s comes down to “swap tronics”...swap a plug, wire, coil pack, and then injector and see if the problem occurs on the other cylinder...if it moves you’ve found the problem. It may also be mechanical so a compression test may be in order...you can do a “clear flood crank” where you hold the gas pedal to the floor and keep it there while cranking the engine...the engine won’t start (injectors will not fire) so listen to hear any variations in cranking sound !!
Do you have a scan tool to determine exactly what cylinder is possibly misfiring ??...if not you can do a “cylinder drop test” and possibly locate the misfiring cylinder (video enclosed if you don’t know how to do it). If you have located the misfiring cylinder and without any specialized equipment such as a lab scope it’s comes down to “swap tronics”...swap a plug, wire, coil pack, and then injector and see if the problem occurs on the other cylinder...if it moves you’ve found the problem. It may also be mechanical so a compression test may be in order...you can do a “clear flood crank” where you hold the gas pedal to the floor and keep it there while cranking the engine...the engine won’t start (injectors will not fire) so listen to hear any variations in cranking sound !!
Le Mans Master
Joined: Aug 2007
Posts: 8,735
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From: WV
Make sure all plug wires are all the way up inside the coils, these sometimes come loose..
One time when I removed the valve cover shields I pulled one of the injector wires loose , this caused a miss ...Check the injector wires under the shields.. Find someone with a "Tech 2" scanner, these are excellent for pointing you in the right direction , or sometimes pointing directly to the cause..WW
One time when I removed the valve cover shields I pulled one of the injector wires loose , this caused a miss ...Check the injector wires under the shields.. Find someone with a "Tech 2" scanner, these are excellent for pointing you in the right direction , or sometimes pointing directly to the cause..WW
Last edited by WW7; Jul 28, 2019 at 02:42 PM.
Thread Starter
Pro
Joined: Mar 2005
Posts: 714
Likes: 62
From: Melbourne Fl.
Warren,
Do you have a scan tool to determine exactly what cylinder is possibly misfiring ??...if not you can do a “cylinder drop test” and possibly locate the misfiring cylinder (video enclosed if you don’t know how to do it). If you have located the misfiring cylinder and without any specialized equipment such as a lab scope it’s comes down to “swap tronics”...swap a plug, wire, coil pack, and then injector and see if the problem occurs on the other cylinder...if it moves you’ve found the problem. It may also be mechanical so a compression test may be in order...you can do a “clear flood crank” where you hold the gas pedal to the floor and keep it there while cranking the engine...the engine won’t start (injectors will not fire) so listen to hear any variations in cranking sound !!
https://youtu.be/5ewJIBS9L98
Do you have a scan tool to determine exactly what cylinder is possibly misfiring ??...if not you can do a “cylinder drop test” and possibly locate the misfiring cylinder (video enclosed if you don’t know how to do it). If you have located the misfiring cylinder and without any specialized equipment such as a lab scope it’s comes down to “swap tronics”...swap a plug, wire, coil pack, and then injector and see if the problem occurs on the other cylinder...if it moves you’ve found the problem. It may also be mechanical so a compression test may be in order...you can do a “clear flood crank” where you hold the gas pedal to the floor and keep it there while cranking the engine...the engine won’t start (injectors will not fire) so listen to hear any variations in cranking sound !!
https://youtu.be/5ewJIBS9L98
The computer only gave me a P0300 code so it wasn't telling me which cylinder was the problem. After reading the shop manual a couple of times my gut feeling told me it was ignition rather than fuel. So this is exactly what I did even before I read your recommendation. However, I did not use the spark size test nor rpm test since the computer does compensate for rpm change when the cylinder wire is removed. I solely went by the sound and feel of the engine. I found the affected cylinder when noticing no difference in sound with the plug wire removed or attached. The plug wire had the correct resistance so I changed the spark plug and that fixed it. The spark plug had no noticeable sign of damage but it was wet with fuel. It is hard to believe that the plugs are an issue with only 16000 miles on them. I'm just going to replace them all.
Warren
Last edited by Warren Seale; Jul 28, 2019 at 04:40 PM.
