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).
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