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98 Malibu 3.1 PO300 code after new PCM install

439 Views 6 Replies 4 Participants Last post by  mechfishy
I recently installed a new pcm and went for a short test drive and the car ran great like a new Malibu. Then the next day I was driving to work and I got a CEL light. I keep my scanner in the glove box at all times in this vehicle so I read the code and it was a P300 (multiple or random misfire) but I didn't notice difference in the engine so I cleared the code. while driving home the light came back on and it was a P300 code again. I never had this code before I installed the new PCM and I can't figure out what may be causing this problem. The car idles good and all of the live data seems to check out fine as well. I found a vacuum leak at the EVAP purge solenoid so I fixed that but still getting the same P300 code. Any suggestions on what could be causing this....bad program on the new PCM????
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The subject line should read PCM not "ppm".....I should preview before hitting "post thread" :rolleyes:
The subject line should read PCM not "ppm".....I should preview before hitting "post thread" :rolleyes:
Thread title was edited. While I was there, I also capitalized Malibu and added an O to the DTC for ya. Pretty painless, eh? :)
There was a member here by the name of RalphP who put a a lot of miles on a 3.1 malibu.
He was very familiar with the car and he attributed many problems to the following.
His words follow.

When mine was surging, it was the PCV tubing grommet into the upper intake, the one below the throttle cable bracket.
I'd be sure to check that also, to make sure that hasn't gone gummy with age and allowing air into the intake behind the MAF.
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Thanks for the edit and thanks for the MAF info, I will look into that next. Just a quick update, I installed the old PCM in the car and no more P0300 code (so far) after 50+ miles but I’m still getting a P1665 Evap code after I have changed both the Evap solenoid and the purge solenoid. It’s always something with this car it seems like. We bought it new in 98 and has over 265K miles on her so I use it as a daily driver because it usually will get around 26 mpg when nothing is going wrong with her.
You certainly can't sneeze at 265,ooo miles on an car! Hopefully you'll find the little gremlin and kick it to the curb.
Does this help?


I'm feeling lazy tonight. So I decided that instead of putting a lot of thought into this I would search for something that had gotten through that part.
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