High Oil Pressure - 91 Grand Prix w/3.1L

[Terry K]

Suddenly found large pool of oil under my 91 Grand Prix SE w/3.1L. Found the oil pressure sender leaking. Replaced sender and notice new oil pressure was only 30 psi. Over next couple days, it read lower and lower. Replaced it with another new sender. Now reading higher oil pressure (+60 psi). Believing it fixed, I drove it normally. Over the next few days, I noticed the pressure again reading lower and lower. So I again replaced the sender and this time, also changed the filter and oil. Now reading +70 psi when cold. Once oil is hot, pressure is still at 70 psi any time RPMs are over idle…at idle, pressure drops to 35 psi (pressure moves with throttle changes when hot).

I suspect the oil pressure relief valve is bad/stuck. I’m trying to confirm where it is on my 3.1L engine. Online research points to various locations like under the oil filter, next to oil filter, or on the oil pump. Since removal of the oil pan is partially blocked, I’m hoping it’s not on the oil pump. And I don’t recall seeing it under the oil filter (but I wasn’t paying attention when I replaced it).

Anyone know definitively where the oil pressure relief valve is on a 3.1L?…or have suggestions to confirm it is a problem valve? I’m gonna hook up a pressure gauge to verify the high pressure. If it is high, I was wondering if applying high air pressure to sender port might free a stuck relief valve.

Edited March 14 by Terry K
added throttle movement note

[GnatGoSplat]

Might be worth confirming oil pressure problems with a mechanical oil pressure gauge to make sure it's not from bad oil pressure sensors.

I didn't find anything definite, but anything I could find mentions it's on the oil pump.

Is there any signs of sludging?

[White93z34]

At the best of times these oil pressure sending units are of questionable accuracy. But these gauges aren't really padded out at all meaning that they are fairly active as oil pressure changes. Which seems relatively similar to how yours is acting.

I'd be more suspect of junk new parts then any issues with your engine.

[Terry K]

On 3/23/2025 at 9:53 AM, GnatGoSplat said:

Might be worth confirming oil pressure problems with a mechanical oil pressure gauge to make sure it's not from bad oil pressure sensors.

I didn't find anything definite, but anything I could find mentions it's on the oil pump.

Is there any signs of sludging?

Engine is running smooth. Since I tried 3 different senders and the first two eventually dropped to a zero reading, I still wonder if it’s a bad sender. I know the odds of having three brand new ones being bad are slim, I know it can happen. I gotta get that mechanical gauge on before I move on.

[Black92GS]

Here’s the 3.1 L oil pump section from my 92 Regal FSM.  
 

It also lists the oil pressure spec for the 3.1 at 15psi at operating temp @ 1100 RPM. I’d assume that is a minimum spec, rather than being absolute. 
 

[Terry K]

On 3/28/2025 at 1:23 PM, Black92GS said:

Here’s the 3.1 L oil pump section from my 92 Regal FSM.  
 

It also lists the oil pressure spec for the 3.1 at 15psi at operating temp @ 1100 RPM. I’d assume that is a minimum spec, rather than being absolute. 
 

Yeah, it looks like the relief valve is on the oil pump. From what I remember about my car, there is no way to reach all the pan bolts without dropping the dropping the sub-frame. But I need to look again cause that doesn’t right…and I really hope I’m wrong.

Edited Sunday at 02:47 PM by Terry K