Pontiac Bonneville low oil pressure

Gary Schafer · 2007-01-05T03:52:24+00:00

1990 Pontiac Bonneville with 3800 engine.After driving several hours,pulled into traffic and wound the engine up a little to about 3000 rpmmaybe. Noticed the oil light on and no pressure on the gauge. Shut itdown checked the oil etc. Started it back up and didn't hear anyrattling or clanking. Revved it up to around 2000 rpm and the oillight would just go out and the gauge would just move. I had just hadthe pressure sender replaced before we left so I thought it wasprobably a bad sender again. Drove it home with the oil light on allthe way, about 100 miles in the sticks. Took it to a garage at homeand he put a regular gauge on it and found about 3 lbs at idle andabout 15 lbs at 2000 rpm. Normal pressure should be around 40 lbs.The 3 lbs. and 15 lbs. oil pressure was with the engine not fullywarmed up.Before this happened the pressure gauge would be right at mid scalewhen running on the highway and not drop at all at idle. After itwarmed up well the gauge would show a little below midway at highwayspeeds and drop slightly at idle.The sudden drop to 3 lbs happened all at once. The car has only 60thousand miles on it and the oil was just changed a few hundred milesago.Any thoughts where to start looking?Thanks!Gary

Jan 05, 2007 22 Replies

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William H. Bowen 19 years ago

GM SPECIFICALLY warns against using 10W-40 in most 3.8L/3800 V-6 engines.

I've used Pennzoil in GM, Ford and AMC engines for over 35 years and I've never experienced the problem you describe.

The more I think about this the more I'm convinced that the problem just may be the pressure relief valve hung open (or nearly so) by either a piece of debris or possibly the spring just gave up the ghost.

Before I dug into the engine any further I'd have the oil filter adaptor removed, pull out the PRV and the spring, then check the valve bore for any crap stuck in it. Also check the spring and the valve itself (a piece of debris stuck to the valve itself or a metal burr on the valve could make it hang in the bore).

If the pressure relief valve checks out OK, then you're either looking at a blocked pickup screen or another blockage on the suction side of the oil pump, a clearance problem on the pressure side (main, cam or rod bearings) that is leaking off all the oild pressure, or just a worn out oil pump (which is contained within the front cover).

Regards, Bill Bowen Sacramento, CA

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clare at snyder.on.ca 19 years ago

Do not use 10W40 for extended drain. I'll agree with that - but I won't go for extended drain on 5W20 or 5W30 either. I'm running 10W40 on my 3800 and will continue to do so.

Hey, I've used it before too - but never since. I'd heard of the problem before, but when it got ME I sat up and took notice. Was it just a fluke? Quite possibly - but once bitten, twice shy. Quaker State and Penzoil are both off my "recommended" list, as are the low end of ANY manufacturer's product line. I've been a mechanic since the late sixties and I've NEVER had an engine develop camshaft problems or bearing problems - including rocker shaft wear, following my recommendation, while those following factory recommendations were busy with engine overhauls. I don't like engine overhauls well enough to inflict them on myself or my customers. Also never had advanced timing chain wear - even on the dreaded 2.6 Mitsubishi Hemi. My recommendation? a wide range multigrade changed oftener than "necessary". Four changes a year, minimum. 3000 miles max per change, unless that 3000 goes on in less than 6 weeks.And the only 5w rated oil I would use would be 5W40 or better (so you can see I'm not using much)

I'm suspicious, given the history, that the OPR is gummy. It's a VERY low mileage car that was likely undermaintained by my recommendation. Very unlikely the spring has failed.

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