C5 intermitent engine miss
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1st Gear
Joined: Jan 2019
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C5 intermitent engine miss
I have a 2001 with a miss in cylinder #7. It will miss every day for a month or so, then run beautifully for a day or so. I have replaced plugs, wires, coils, coil wires, and injector, with no change. Is there something else, or is the ECM the only thing left?
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Joined: Sep 2013
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From: Cape Coral, Florida
Welcome to the Forum !!
An intermittent misfire like this can sometimes be pretty difficult to diagnose. Did you pinpoint it down to #7 with a scan tool ??….if so does your scan tool show misfire counters and are they counting up when you detect the daily misfire ??...you pretty much changed all the parts that can possibly cause a misfire but a compression test should still be performed even for an intermittent condition like yours...if you don't have a compression gauge you can do a "clear flood crank" (gas pedal to the floor...hold it there...and crank the engine over)...the engine will only crank over and listen to how the engine sounds while it's cranking...even and consistent. You can pick up a compression tester kit at Harbor Freight and they are not to expensive or maybe one of the chain stores can loan you one...while the engine is missing pull the plug wire off the #7 coil pack and see if the spark jumps across...to see if the "electrical" portion of the fuel injectors are OK maybe you can also borrow a "noid light" kit too...while #7 is misfiring see if the light flashes consistently indicating a good injector pulse (see enclosed video)…. if the scan tool is yours what do your long term fuel trims look like ??….if it's either a fuel or ignition misfire your fuel trims will be elevated...if it's ignition related they might be around 6-8% or so...an fuel related misfire will be higher...over 10%...there is a lot of excess oxygen in the cylinder without fuel in it !!...don't even think about a bad PCM yet !!….Good Luck !!
An intermittent misfire like this can sometimes be pretty difficult to diagnose. Did you pinpoint it down to #7 with a scan tool ??….if so does your scan tool show misfire counters and are they counting up when you detect the daily misfire ??...you pretty much changed all the parts that can possibly cause a misfire but a compression test should still be performed even for an intermittent condition like yours...if you don't have a compression gauge you can do a "clear flood crank" (gas pedal to the floor...hold it there...and crank the engine over)...the engine will only crank over and listen to how the engine sounds while it's cranking...even and consistent. You can pick up a compression tester kit at Harbor Freight and they are not to expensive or maybe one of the chain stores can loan you one...while the engine is missing pull the plug wire off the #7 coil pack and see if the spark jumps across...to see if the "electrical" portion of the fuel injectors are OK maybe you can also borrow a "noid light" kit too...while #7 is misfiring see if the light flashes consistently indicating a good injector pulse (see enclosed video)…. if the scan tool is yours what do your long term fuel trims look like ??….if it's either a fuel or ignition misfire your fuel trims will be elevated...if it's ignition related they might be around 6-8% or so...an fuel related misfire will be higher...over 10%...there is a lot of excess oxygen in the cylinder without fuel in it !!...don't even think about a bad PCM yet !!….Good Luck !!
Le Mans Master
Joined: Aug 2005
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From: CA.
St. Jude Donor '13-'14-'15
You need to 1st diagnose if the misfire a REAL misfire or a FALSE misfire---A real misfire you should be able to hear or feel the misfire But often after certain mods you will get a false misfire---- If you are getting a false misfire ths is commom with certain mods--However they can be tuned out---let us know
