Oil pumps

I have a 1997 GMC Sonoma Pickup with 115,00 miles and when it gets warm, the oil gauge will go down to 20 pounds of pressure or less. Does that mean I need to invest in a new oil pump, or simply add a thicker oil? And what is the thickest oil I can add to my truck?

What weight oil are you using now? Depending on where you live weatherwise, I would never use anything heavier than 10W30 in summer and 5W30 in the winter. In Southern california and Florida, 14 W40 would be OK duirning the warm season.

You probably need a new oil pump, so I would have a competent mechanic check things out and give you a good estimate.

I would start by having a mechanic use mechanical oil pressure guage on the vehicle and then go from there.

I was with the Dealer in 1997 and we were told 7psi (at temp and at idle) is just fine. Never saw the statement sucessfully challenged.

Seven PSI at idle sounds way too low in my opinion; no matter what the factory boys say.
Every engine I’ve seen that had that kind of pressure also had problems although they may have run fine.

Heck, my son’s 96 Chevy still carries 30 PSI at a warm idle and it has a shade over 250k miles on it.
Several years back he did have an episode where the gauge was showing about 10 PSI at a warm idle but a new oil pressure sender cured that.

The only engines I’ve seen in which 7 would be considered normal and acceptable would be old Harley Davidson motorcycle engines. Those are a different breed altogether due to the roller bearing lower ends and they only require a slight oil film to keep the rollers slicked up. Oil pressure is almost irrelevant on these.

Unless the oil pressure sensor or gauge is defective, I imagine your engne is just getting loose. However, 20 # of pressure is propably not too bad. My gauge has no numbers on it but it stays more than half way to “high” even with 129K miles on it.

It’s not too far off of the old “10 for every 1000”

Maybe so, and I’m familiar with that 10/1000 thing too. All I know is that if I have an engine with a legitimate 10 pounds of oil pressure at idle I’m not taking any long distance trips with it.

It’s not too far off of the old “10 for every 1000”

30% difference is HUGE.

So Mike 10psi at idle would work for you? It’s the old percentage ploy,when you say “It’s a 30% difference” that does sound big, but if you present it as “3psi off spec” (which 7 is spec) it doesn’t sound so bad.

Could be the pump, but I’d suspect worn bearings first. Like others have said, you need to confirm with a mechanical gauge. What’s the pressure while driving?

In regards to my son’s Camaro that I mentioned the manual states that car ('96 model, 3.8 engine) should have 6 PSI at idle and 60 PSI at 1800 RPM and to expect that kind of pressure swing is ludicrous in my opinion.
Warmed up at idle it carries 30 and 60 at elevated RPMs and that’s with 250k on the clock.

What’s the idle RPM?
We had a fella post here recently about low oil pressure at idle and his engine was only idling at 500 rpm.

I have a Sonoma and my gauge fluctuates also. I have been told it is common in the GMC line for the gauges to flux from 15 -20 up to 60. Mine does it and has never had a problem. Does not matter which oil I run.