Oil pressure drop

[89Rustbucket]

Ok I have a question for a buddy of mine. Kinda stumped me. Its a 90 Chevy 1500. When started Oil pressure is fine, but once is heats up oil pressure drops and the engine starts to bog down a bit. He replaced the Oil pump thinking that would fix it and it still happens. Full oil. Doesn't leak or loose oil. Just drops when it gets to about running temp. Any ideas?

 

[apache]

Generally speaking when an engine and its oil is cold and thick it produces higher pressures.After it warms and things expand and loosen up and the oil thins and it drops much of the pressure its engine time.
Id make damn sure with a couple checks with a quality pressure gauge plugged in where the sender is/was to eliminate a gauge issue.

 

[89Rustbucket]

Well i mean severe drop. Like it sits just above normal till it gets warm then drops to about 20 and bogs down. And it bogs down ONLY when the pressure gets that low. we ran a different gauge about 30 min ago and it still says the same. And yeah i know about oil thick and thin thats why this is kinda confusing me. Cuz it used to run like a champ for yrs Now it just bogs down when it gets warmed up

 

[Babaganoosh]

My engine as well does something similar. When I first start it, it runs 40 when it's completely warm idling gets 20. When it's warm and the wheels are moving it's back to 40. I just put it off as it's a high mileage motor and frankly I'm not going to fix it just curious as to why.

 

[TerryD]

The engine is worn out. That's all there is to it. As for the reason its running poorly when the pressure drops, the only thing I can think of is that its possible the lifters are also worn as badly as the rest of the bearings (causing pressure drop) and they are dropping when they get warm, not opening the valves far enough.

As far as oil pumps go, they are RARELY the problem. My 7000 RPM Camaro motor is running the oil pump that came new in the block in 1976. I pulled it apart and checked the tolerances, and it was well with in spec. The thrust bearings in that block were worn so badly the crank was starting to hit the webbing on the main journals, but the pump was fine.

When it comes to Chevy/GMC motors, its best to use a stock volume oil pump and focus on getting the oil to return to the pan. I opened up all the galley drains (except over the cam, I plugged them) with a die grinder and installed a windage tray/splash guard in the camaro motor.

 

[dyeager535]

Is the oil pressure drop related to engine RPM?

If engine RPM goes low enough, pressure takes a nose-dive.

 

[YZEATER]

how many miles are on the engine?

change the oil pressure sender.

 

[rcamacho]

Idle oil pressure is really not an issue unless it is below 10/20lbs. You generally need 10PSI per 1K RPM. For a high mileage engine, you can crutch clearances a bit by going to a higher viscosity oil within reason (30wt or 20w50) and appropriate to the season. Additionally, you can R&R the oil pump and increase the high pressure relief by extending the spring a bit (careful). You want less than 100psi otherwise you will blow filters. On that subject, make sure you are running a decent WIX/napa filter. Fram are generally junk and can collapse causing pressure loss.

 

[Jason4x4]

Plugged exhaust?

 

[onetonbb74]

x2 on the plugged exhaust, a crappy oil pressure engine still gets oil and shouldn't make the motor bog down per say sounds like a plugged cat. My buddies been running his motor w/ 10psi for 5or so years, ive never seen it above 15psi on a cold day and ive seen it at 0 when running all warmed up and it still makes power.

 

[89Rustbucket]

Well thank you all for the information. I will take it all into consideration tomorrow. It just seems so damn weird to me. Maybe ill pull the cat off first and see how well it does. After that i will check the sender. If those don't work maybe ill have to tell him to pick up a new engine LMAO. ohhwell. Glad we got an extra one.

 

[4X4HIGH]

Did this happen all of a sudden or has it been this way for some time, the oil pressure issue that is? Oil pressure is determined by the clearance of the crank and bearings and the cam and bearings. If this engine hasd many miles on it then i would say the tolerances are greater than they should be which in turn means less oil pressure. Since he replaced the pump and still has the same problem you can rule out the pressure relief spring in the pump as being the culprit. The last thing would be a clogged oil filter in which the oil is bypassing through the small bypass valve instaed of flowing properly through the filter. Wix is the best filter you can get by design.

 

[89Rustbucket]

Yeah it happened suddenly. It was a cold morning when he started it and then when he got to the highway to drive to work it started bogging down. He had a full tank of gas. He drove it back from work and same problem. Been like that ever since. He just did an oil change about 3 weeks ago before snow started flying. It started about 1.5 weeks ago.