BAD PCM? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 05-01-2004, 10:21 PM   #1
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Hi all....just went thorough 2 weeks of self-inflicted hell with my 97 bonnie. First of all...it started out as a rough idle...it set a check engine code for a general misfire.....ok.....so i went about and changed the plugs and wires.....alls smooth again...i turn out the light with my scan tool...about 8 months later...the rough idle returns.....and the check engine lite...this time its a code 1406 ...egr valve performance...i replace it with a gm original...and reset the lite....alls well for 2 months this time...i start getting a rough idle...check engine returns with the same code! Wait a minute! this was a new GM valve! is it defective? Or is there another problem? I figure it must be the pcm going bad...102,000 miles...i look at my local salvage yard..they have a used pcm for $75+tax...i go down there and pick it up...i forget to bring the GM tag off my computer[its in my haynes manual] LOL...he says to check the part number when i get home....i check it..and its the same..except for the calibration number...which is different for evry model...then i turn the used pcm over and find out its from a grand prix gt...i guess they cross-reference...i put this thing in...and hey! it runs smooth...except for one thing...the fuel gage doesnt work...and the check gages lite is on...i figure...hmmm must be the pcm then....this used one isnt calibrated for my bonnie...then i figure..hey this thing has a replaceable prom chip..so i take it back out and switch chips with my old one...i put the old pcm back in...it runs rough...the chip isnt causing the problem...so i take it back out...switch chips back...but wait! my original pcm has some corrosion on the bottom of the aluminum housing! this thing has been sitting in water....but i figure its ok because theres a gasket sealling the mating parts...then i start to think...what about condensation? hmmmm...i decide to open it up....i undo the screws..theyre slightly corroded too...i try to get it apart...oops have to take the chip out again...i slide the computer out of the case...i look inside...looks ok...albiet a bit dark.....lol...then i look closer...what is that thing n the bottom of the case? a switch? no...its a corroded peice of aluminum! this thing is vacuum=sealed...air and water tight...what is corrosion doing in here? Can anyone say condensation? Must be due to the vast temperature variations generated under the hood...this piece of aluminum was acting like a switch...shorting out the area of the pcm that was controlling the egr valve..and causing the check engine lite...and all my headaches!
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Old 05-01-2004, 10:25 PM   #2
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Good story, I'm glad you found the problem and it was a simple fix
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Old 05-01-2004, 11:27 PM   #3
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Don't envy you that one, good job though.
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Old 05-02-2004, 12:30 PM   #4
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Great job, I am very surprised the PCM had corrosion inside the box....those are usually sealed up VERY tightly.
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Old 05-02-2004, 12:34 PM   #5
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Great troubleshooting for sure. I bet you're glad your PCM isn't located where mine is (under the passenger side dash). It'* a real bear to get to, but then again, it'* not exposed to the elements, either.

Glad you got it figured out. GP Proms are not compatible with our cars. I can tell you that for sure.
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Old 05-02-2004, 01:16 PM   #6
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thanx guys...alls well...for now gotta love those bonnies!
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