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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 05-08-2014, 09:17 PM   #8
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Bump, alien did you solve this problem? I'm trying to rule out a pcm failure in my cts-v.

Last edited by Lbar; 05-09-2014 at 12:10 AM..
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Old 05-10-2014, 09:35 AM   #9
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The only non-physically damaged bad GM PCM I've experienced was in my 02 WS6. I had an intermittent cylinder 5 misfire that kept getting worse. All the usual troubleshooting including re-flashing the PCM didn't resolve my problem. As a last variable I threw a spare PCM in the car and the problem cleared. I moved my suspected bad PCM to another car and the problem followed even after flashing for that combination.

The true test for a bad PCM would be swap with a known good one (if you have that ability) and see if the problem clears. Even better if you have access to two vehicles with the same style PCM and swap between them like I did and see if the problem follows.
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Old 05-11-2014, 01:40 AM   #10
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Thank you for replying gofast. I have 05 CTS-V and I don't know anyone who has one. The PCM is right under the splash guard, behind the driver side fog light. A quick swap if I can find someone willing. I will ask around.

Here is my issue(s) so far. It began with bad camshaft position sensor codes. A P0371, Timing Reference High Resolution Signal A Too Many Pulses and a P0372 Timing Reference High Resolution Signal A Too Few Pulses code. I had a occasional hard start with backfiring into the intake and a little later a random bucking after warmed up with a certain amount of throttle lightly accelerating in lower gears.

I replaced the cam sensor with a new Delco part. I did this at my friends shop. The first start up with the new sensor was a hard start just like before. My tuner friend said I did not need a crank relearn. On the first start up I had a new problem, a P0118 Engine Coolant Temperature Circuit High Input and one of the cam sensor codes returned, P0342. He hooked his HP Tuners up and when he cleared the codes my temp sensor would come back on for 15 seconds before throwing both codes again and dropping my temp gauge to cold and kicking my radiator fans on. He said that one of my o2 sensors was showing rich without being out of spec. However, the occasional bucking seems to have stopped.

His guess was a bad PCM because there was too much wonky stuff going on. But since he only has experience with 1 bad PCM in his career and the symptoms seemed much different to me I'm not going to run out and get one until I've exhausted all other possibilities. I've gone through most of the grounds so far, they are fine. I got a new coolant sensor and plugged it in (didn't install it). It did not fix my temp sensor issue. Then it rained for the rest of the day so I'm postponed until tomorrow.

My thoughts are that this new camshaft position sensor is somehow faulty and causing problems. It is just too coincidental that I have issues with the coolant temp sensor wiring right after I installed the new cam sensor. I can't tell what wires they have in common but the two seem to be apart of the same feeder of wires. Tomorrow I plan to check the PCM ground and shake the wires attached at little bit. I'll check the other grounds underneath the car. If that yields nothing then I will put the old camshaft position sensor in and see if my temp gauge comes back on. If I regain my temp sensor then I will get another cam shaft sensor.
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Old 05-11-2014, 01:40 AM
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