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Old 07-07-2005, 09:41 AM   #1
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Default GM 3800 oil pressure issue

My '90 Bonneville SSE has experienced a recent and dramatic drop in oil pressure. At first I suspected the gage, replaced the sender with no change. Mechanical gage tells me the dash gage is telling the truth. At idle, zero oil pressure, takes about 1500 RPMs to get the oil warning light to go off, at around 3500 RPMs I have about 25 psi.

I have taken the front cover off and cleaned & inspected all of the oil pump parts. All of the rotating parts check out within spec, no apparent wear. The relief valve moves freely and the spring seems stiff enough, although I don't have anything to compare it to (yet - I have an oil pump repair kit coming in).

I've had the car since 2000, had 104k well-maintained miles on it then, just over 196k now. I've used nothing but synthetic in it. Pressure was the "normal" 60 psi (or whatever the 3rd mark is on the dash gage), same as our '91 Bonneville, until this recent "event". I would think if the pressure was as low as it seems, the engine would have eaten itself up by now. But, it runs fine, no unusual noises, no external leaks. It's almost like a passage has opened up internally that is dumping the pressure, but still allowing flow to the vitals.

Any clues? The car is too good to throw away. I'd hate to have to get another engine for it, just did the transmission in January (it's first 6k miles of life was as a luxury rental in California - I suspect it may have been abused, the previous owner had just rebuilt the tranny before selling it to us).

Another item of interest: I would have thought if the pressure was as low as indicated, the fuel pump would have kicked off. I disconnected the sender/switch connector, started it up, the oil pressure gage pegged but the engine ran as if nothing was out of the ordinary. I think I have a fuel pump safety circuit issue, perhaps the start/timed relay is stuck closed (I didn't think to note if the pump continues running when the ignition is on and engine not started - the pump runs pretty quiet).

This is keeping me from getting the Camaro put back together.
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Old 07-08-2005, 08:44 AM   #2
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Default Re: GM 3800 oil pressure issue

I picked up an oil pump kit, compared the new parts to old, I can't see any difference. However, the housing depth compared to pump thickness would seem to be out of spec, so maybe that's it. Although, when assembled, they don't seem to be using a straight edge and feeler gage.

Guess I'll try to find another timing cover.
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'82 Berlinetta, 2000 LS1/4L60E. LS6 intake, shaved 317 heads, free-flowing air intake, long tube headers, 3600 stall, 3.73 posi. Best: 12.57 @ 109 at 5800' elevation; 11.82 @ 114 at HPT.
'57 Bel Air, my 1st car. In work: LQ4, L92 heads, LS3 intake, Erson cam; 4L80E, 4000 stall; 9".
'08 GXP grocery getter.
'95 Silverado 2500HD 6.5 TD. Just 335k miles so far.
'96 Vette Base Coupe. New to the herd.
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Old 12-10-2006, 05:40 PM   #3
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Ever get it fixed?
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Old 11-16-2009, 04:07 PM   #4
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Default I have a 3800 comaro

it will start warm up and then when it gets up to temp it will drop oil pressure i rebuilt the oil pump and put one new check valve but i did not do the one on the oil filter adapter will that let all pressure drop to zero..... and i have plastic gauge the main and rods they were all in sepecs the oil gallys are clean in the engine i just cant figure out whats wrong with it. the oil pan had ben dented i straight it out sice i had it out but i dont think thats what it was.. im lost
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Old 11-17-2009, 02:47 PM   #5
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Are you verifying that the pressure is dropping with a mechanical gauge?
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