I've often wondered...what reduces engine vacuum and also causes the oil pressure to increase when the AC is on or for several minutes after the engine is restarted?

I'm even more curious now than before because the oil pressure on my '92 at warm idle has recently dropped a little, but the oil pressure isn't always lower at warm idle than it used to be.
For example, the oil pressure might be 32 PSI at warm idle (with the AC off), and after I shut off the engine and return and restart the engine several minutes later, even though the engine temp is the same or higher than it was when I shut the engine off, and the RPMs at idle are the same as they were before I shut off the engine, the oil pressure may now be 40 PSI. What could explain this difference in oil pressure?

Thanks.

Thats not a very big change. It could be just from the engine oil cooling down a little and becoming a little more viscous. It may be from a pressure sensor starting to go out. It may be from old, dirty oil and partially plugged oil filter. Are you overdue for an oil & filter change?

Thanks, but the oil didn't have time to cool down, as I said, the engine temp was the same or higher than it was when I shut off the engine.
I changed the oil (10W30 as usual) and filter (Delco as usual) again recently just to make sure that the difference wasn't due to oil that somehow changed viscocity or an oil filter that somehow changed the oil pressure, but the oil pressure at hot idle is reading the same as before, lower than it used to.
I doubt that the oil pressure sensor is faulty, it doesn't jump around or wander up and down or anything unusual.

Well you may have a bearing or bearings that are starting to wear. Possibly pump wear? If you really want to get to the bottom of it, you will need to test the pressure under the same conditions with a mechanical pressure guage and compare the results with your current readings to start with.

It's possible that an engine problem is beginning, but again, because the oil pressure isn't always lower at idle, I'm wondering if a sensor, the ECM, or another external component could cause this behavior.

Bump...what causes engine vacuum to change when the AC is on or when the engine is restarted?

A/C adds load to the engine, the more load an engine has applied to it, the less vacuum it will produce. The reason is because the IAC will open a lttle when the A/C is on to give the engine more air in order to keep the RPMs from falling. The more air an engine has given to it, the less vacuum it will produce.

So when the engine gets more air, how does this cause the oil pressure to increase while the RPMs remain constant?
I'm wondering if maybe my IAC isn't working properly, or maybe even the PCV that I recently replaced could cause the recent change in oil pressure at idle.

The oil pressure gauge on these cars are flaky, before you panic confirm the reading with a mechanical gauge first.

Im thinking why the oil pressure gauge goes up is because there is a bigger load on the electrical system (besides the engine spinning faster to power the compressor). The gauge reports its reading by how much resistance is induced, so less power = more pressure. On my car when I turn on my headlights the pressure goes up by about 5 PSI, yet the engine RPM is constant.

This is why you should confirm oil readings with a mechanical gauge first, the one in the dash is just a guideline since on a lot of cars they are not accurate.

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1993 Bonneville SE "Old Ironsides" 244,xxx - Junked.
1996 Bonneville SSEi "Double Trouble" 164,xxx - Stock
1997 Bonneville SSEi 40th "Purple Spaceship" 204,xxx - Aeroforce Interceptor, B&M 70264 Trans Cooler, Intense FWI, High-Flow Feline, non-Bose head unit w/ 5 band, Alpine 6x9's, 3.5 MPS, SD Headers, rear disc brake conversion. Future: 1.9 Rockers
1997 Buick Park Avenue "Blue Floaty Thing" 124,000 - Daily Driver/Winter Beater

I've wondered the same thing because mine acts the same way. Even cruising at highway speeds, the oil pressure will come up just a bit when the compressor is running.

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WHITE WHINE - 1992 SSE Supercharged 236.26 ci (.040 Over) 15.090 at 90.2 MPH on old engine w/ slipping trans & melted O2 sensor - Gen 3 M62 and matching TB, Gen 2 Pully, Zillamotorsports Ported LIM, YT 1.72 Roller Rockers, SII FPR & Injectors, Hypertech Thermomaster chip w/ 160 Thermo, TransGo Shift Kit, Infinity/Pioneer Speakers & a 10" Alpine Type R Sub, all the watts, 140 amp Alternator, Ricepipe CAI w/ heatshield, Pilot Angel Eye Foglights, Clear Corners, '02 17" Chrome Bent 5's, Magnaflow F-Body Muffler and Hi-flo Cat, Ceramic Coated Ported Exhaust Manifolds, Fan Override, Monroe Reflex struts, red calipers
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