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Old 01-14-2010, 10:08 AM   #1
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Default Bad PCM symptoms??

I'm suspecting my PCM may be bad...my car's running like crap...as in misfiring on like 4 cylinders but not throwing any misfire codes...we checked the coils...changed plugs and wires...put in a different maf...haven't done O2's yet

I was just wondering what kind of things happen if a PCM goes bad? Any of this sound familiar?
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Old 01-14-2010, 10:50 PM   #2
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That is not consistent with any type of PCM failure that I have seen before. Hard misfires with no codes can come from intermittent shorts to ground that do not blow fuses. Also, one misfiring cylinder will 'look' lean to your O2 sensor and drive that bank of the engine into extreme over fueling trying to correct what it sees as being lean. If it is run like this, plug fouling and additional misfires from over fueling can follow.
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Old 01-15-2010, 08:11 AM   #3
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Bad LS based PCMs are not a common occurrence. Misdiagnosis of them is far more common.

I would look elsewhere for the cause of the misfires. Just because the computer does not recognize them does not mean it's bad.
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Old 01-15-2010, 11:19 AM   #4
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Quote:
Originally Posted by Frost View Post
That is not consistent with any type of PCM failure that I have seen before. Hard misfires with no codes can come from intermittent shorts to ground that do not blow fuses. Also, one misfiring cylinder will 'look' lean to your O2 sensor and drive that bank of the engine into extreme over fueling trying to correct what it sees as being lean. If it is run like this, plug fouling and additional misfires from over fueling can follow.
So where should I look for these grounds that would cause these misfires? And would bad front o2's cause this to be even worse?
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Old 01-16-2010, 02:22 PM   #5
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bump...taking it to the mechanic monday if I can't get this figured out
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Old 01-22-2010, 09:44 PM   #6
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Wasn't a pcm issue...I had 1! valvespring going bad that eventually broke causing all my problems...changed the spring and now the thing runs like a top
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Old 02-15-2012, 08:30 AM   #7
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Quote:
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That is not consistent with any type of PCM failure that I have seen before.
I know this is an old thread, but what are the typical symptoms of PCM failure?

One day I drove my T/A home (stock PCM), waited 30 minutes and then it wouldn't start (crank, no start) when I went to leave. I went around making sure the grounds were tight, checking fuel pressure, and then one time after not changing anything, it starts. Started fine the next dozen times, moving it around in the driveway. A few days later, confidence rebuilt, it left me stranded on the side of the road a few miles later. It just died and cranked but wouldn't start again. Trailered it home and sure enough, fire up right away getting it off the trailer and ran fine. There goes any diagnosis.

Last night, it starts fine and I was poking around under the hood. "Feels" like an electrical fuel issue. I gave the PCM and surrounding wires a wiggle and it died, wouldn't restart. YES! Moved the PCM/wires, starts. Unwrapped the wired from the loom to the PCM and didn't see any chaffing or exposed wire. More wiggling and I couldn't get it to die again. Got dark, gave up.

I'm 90% it's going to be a broken wire or chaffed wire I've not yet discovered. HOWEVER, there are some other slightly odd things that happen and want to rule out the PCM. For one, after a minute or two sitting at a stoplight or whatever, the fuel gauage will drop to zero for a few moments, sometimes until I start moving. And then, when it started on the trailer, I noticed the tach read zero RPM's for five seconds or so when it first started.
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Old 02-15-2012, 08:30 AM
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