What to look for... Rough running '93 GM 3.8L


Old 01-01-08, 05:41 PM
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Join Date: Jan 2008
Posts: 2
What to look for... Rough running '93 GM 3.8L

Hi, first time poster here... I recently got a 1993 Pontiac Bonneville from my Mother (NON-Supercharged). It ran fine when I got it, but over the course of a month or two has started to run less smooth. The other week I did a cylinder cut out test and found that not one cylinder was mis-firing at idle. I then pulled plugs and checked their condition: all were clean and dry with pretty square electrodes and slightly large gaps. I regapped them and put them back in. During the course of removal, I broke one spark plug wire (I was careful, but it was 'welded' on). I replaced the wires with a new set of AC Delco wires from a local parts supplier. The car ran good and I assumed that it must have been a wire shorting out under a load.

Then the car started running less smoothly again. I again checked each cylinder (pulling the wires at the coil packs) and determined that all coils were firing and all cylinders had about the same RPM drop. I checked the fuel pressure and it is about 35-40PSI at idle. I did check the air filter and found it to be in good condition. This car was maintained pretty well. I checked a few intake manifold bolts and found some of them slightly loose. Last week when I replaced the wires, I had checked for codes and found no codes.

So... Where to look next? Is there any one item in particular that goes bad on these cars? Any help would be greatly appreciated.
-Thanks, Ken
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Old 01-01-08, 08:16 PM
Join Date: Dec 2005
Location: Arkansas
Posts: 97
Does this model have the vaccuum multi-splice right above the throttle body? Some of the earlier model 3.8 engines had several vaccuum lines all coming out of one main vacuum feed "splice" (for lack of a better word) The splice itself is plastic or rubber I think and I have seen many of them crack on the underneath side causing a vaccuum leak and drivability issues. The vaccuum lines themselves were plastic as well and were bad to get hard and brittle and crack causing a vaccuum leak as well.

If it has that splice that I am talking about with SEVERAL vaccuum lines coming out of it then you should take that off and look it over really well as well as the lines going to/from it. If it doesn't have that piece I would still check it over good for vaccuum leaks.
Old 01-02-08, 09:15 PM
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Join Date: Jan 2008
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Cool, Thanks for the suggestion. I will check it out A.S.A.P.
Old 01-03-08, 07:29 AM
the_tow_guy's Avatar
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Join Date: Feb 2001
Location: SW Fla USA
Posts: 11,589
Any chance that it is loosing coolant? Some of the 90's GM's had intake plenum problems causing coolant leaks.

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