Random Misfire
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Joined: Apr 2004
Posts: 52
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From: Terrell TX
Random Misfire
Got code P0300 the other day when I scanned the other day and I have been trying to diagnose the problem on my own but I couldn't find much information when searching.
I first noticed this after I had a Vig2800 installed. Current mods are muffler elims, no cats, and O2 sims on the rear.
The Problem: I can start the car when cold and let it run up to a point where is seems to change modes suddenly and starts to idle rough and the misfire count goes up. The O2 sensors seem to be reading properly, I cannot see any arching on the ignition side of things, and the fuel injectors are in proper working order. I tried to force the computer to not go into closed loop mode by disconnecting the oxygen sensors, and the idle seems fine with hardly any misfires, however once I shift into gear it does the same thing as before where the idle is erratic and rough and this continues even after I shift out of gear. The car seems to run fine while accelerating and is also fine while cruising at a constant speed.
If anyone has any recommendations on what I should check next, any help is appreciated.
-Shane
I first noticed this after I had a Vig2800 installed. Current mods are muffler elims, no cats, and O2 sims on the rear.
The Problem: I can start the car when cold and let it run up to a point where is seems to change modes suddenly and starts to idle rough and the misfire count goes up. The O2 sensors seem to be reading properly, I cannot see any arching on the ignition side of things, and the fuel injectors are in proper working order. I tried to force the computer to not go into closed loop mode by disconnecting the oxygen sensors, and the idle seems fine with hardly any misfires, however once I shift into gear it does the same thing as before where the idle is erratic and rough and this continues even after I shift out of gear. The car seems to run fine while accelerating and is also fine while cruising at a constant speed.
If anyone has any recommendations on what I should check next, any help is appreciated.
-Shane
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Joined: Dec 2001
Posts: 51
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From: Nashville TN
I'll tell you exactly what this is, because I had the same problem when I put my vigilante 2800 in.
its the reduced 'flywheel' mass of the smaller tq converter. It can't mask crankshaft vibrations as well as the stock converter. The crank position sensor is picking this up and the PCM thinks its a missfire. That code is going to cause the PCM to unlock the converter in 4th gear too, which is a MAJOR annoyance.
I had to wait until a later version upgrade of LT1 edit came out so I could change the threshold for the missfire test from the stock 375rpm+ to 3000+ rpms.
Aftermarket cams do this too, and at the time I was running a Comp cc304 210/220, but never got the missfire with the stock converter.
its the reduced 'flywheel' mass of the smaller tq converter. It can't mask crankshaft vibrations as well as the stock converter. The crank position sensor is picking this up and the PCM thinks its a missfire. That code is going to cause the PCM to unlock the converter in 4th gear too, which is a MAJOR annoyance.
I had to wait until a later version upgrade of LT1 edit came out so I could change the threshold for the missfire test from the stock 375rpm+ to 3000+ rpms.
Aftermarket cams do this too, and at the time I was running a Comp cc304 210/220, but never got the missfire with the stock converter.
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Joined: Apr 2004
Posts: 52
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From: Terrell TX
Interesting, do you think that the new version of TunerCat will be able to fix this? I don't want to get a mail tune that costs the same ammount if I can just do it myself, and I really wouldn't get LT1 edit now that TunerCat's software will be released soon... hopefully...
Thanks for the help!
Thanks for the help!
