[gto4evr]

Hello again....this car's starting to bug me now. got the starting issue sorted out on that other thread (thank you again!) and the other issue I thought was just a bad gauge apparently isn't.

background again:
69 GTO

400 4spd, basically stock rebuild in 2007

This year when I pulled the car out of storage, I noticed my oil pressure is very high out of no where. I've got an aftermarket oil pressure gauge on the car since I didn't trust idiot lights to the fresh build and ended up just leaving it under the dash. From the beginning of the motor build in 2007 up til last year, I'd get 40 psi at a cold start idle, once the car was all warmed up, it floated around 20 psi at idle and 60 under a load.

this year when I pulled it out of storage, other that the starting issue (now solved) it ran great, but would hit 60 psi cold start up, 80 under a load, then after completely warmed up it doesn't drop below 40 psi. I figured the aftermarket gauge was just dying so I just replaced it with a new one, and.....same results.

cold start just now, sitting at 60psi idle and hits 80 when revved up.

after fully warming the engine up it idles at 40 and revs at 60. engine temps are always fine at a consistent 195 just like it's been for the past 14 years. changed oil and bled the line to the gauge, no difference.

So any ideas on the sudden change to idle pressure? I searched the forum but couldn't find any similar threads.

[Bill Hanlon]

Buy a relatively cheap mechanical gauge. Using a T, temporarily install it into the same hole with the 2007 gauge. You can just leave the new gauge under the hood. Do they read the same?

[padgett]

Thoughts #1: relief valve stuck shut

#2 " aftermarket oil pressure gauge" - electric or mechanical ? If mechanical is probably correct. OHOH GM electric senders are notorious for going bad, GM has issued TSBs on senders before and I used to just keep a spare just for when not if.

[steve25]

Well high pressure in and of itself is not a bad thing as long as it does not blow out the filter, but you should keep an eye on the dizzy gear if it’s a stock one .

[gto4evr]

both aftermarket gauges are mechanical, with the hose full of oil coming up to the gauge tapped from the housing where the standard idiot light sender tapped in..

first one from 07 was a summit 0-80 gauge
second one is a sun tune 0-100 from around the same time (I had it on the car before the restoration when I went to a double gauge setup with temp and oil instead of just the single oil)

what concerns me was the sudden change. 14 years of 20 psi and now this year 40 psi.

why would a bypass stuck shut cause this? wouldn't it also exceed the old top out of 60 psi under a load once warm? and why would it even try to be bypassing at idle? I'd think a stuck bypass wouldn't have any affect at all at idle if it was the case unless I'm missing something in my thought process here.

everything reacts the same under a load with it topping at 60, the issue is it doesn't return to 20 like it used to, stays at 40 no matter how hot. something's gotta be causing the sudden difference.

also, there's no blockage or air in the line. I took the line off the gauge while running and the oil pushes through no problem. bled it both with the first and second gauges.

[tom s]

Did I miss somewhere what weight oil is being used?Tom

[Schurkey]

Originally Posted by gto4evr

it floated around 20 psi at idle and 60 under a load...

...80 under a load.

Originally Posted by gto4evr

the old top out of 60 psi under a load once warm?
...everything reacts the same under a load

There's nothing in the engine oil system that measures load. An automatic transmission might vary line pressure based on engine vacuum or throttle position, but there's nothing like that in the engine oil system.

Originally Posted by tom s

Did I miss somewhere what weight oil is being used?

Important.

[68WarDog]

Originally Posted by tom s

Did I miss somewhere what weight oil is being used?Tom

This^^^

[gto4evr]

sorry, I hate leaving details out!

10W30 oil. synthetic/blend after the oil change, had regular 10w30 in it before the oil change test. there was no oil change between the 2020 20psi idle and the 2021 40 psi idle.

it's had regular 10w30 since day one. has 5000 miles on it since the rebuild in 07

[padgett]

Aprapo of nothing I remember Herb Adams saying a Pontiac engine did not need any oil pressure at idle, just some flow.

[gto4evr]

Originally Posted by Schurkey

There's nothing in the engine oil system that measures load. An automatic transmission might vary line pressure based on engine vacuum or throttle position, but there's nothing like that in the engine oil system.


Important.

I should have been clearer. by load I mean the oil pressure at high rpms.

in the past, I'd get 20 psi at idle and 60 at high rpm with a hot motor. cold engine was basically +20 to both numbers and hot, it would be 20/60. Now I'm getting 40/60 with hot conditions and 40/80 with cold condition.

so is it possible my memory's shot and somehow I did do an oil change at some point last year and used the synthetic blend, and that's what's caused the oil pressure change and I just didn't notice it? does blended synthetic oil increase psi? I just can't see that, every time I do an oil change I write down the mileage and date on the filter box flap, and jam it behind the air line pipe over the door on my garage and I have every oil change for each car I own as a history of flaps. The flap previous to this change was dated 2017 and 900 miles less than the change I just did.

another piece of info if it matters, filters are the napa gold 1258

[padgett]

Old numbers sound right. Are two reliefs in the oiling system, one for a clogged oil filter bypass and the other is a pressure relief valve near the oil pump. This sound most like a relief valve not opening or only partly opening.

[Schurkey]

Originally Posted by padgett

This sound most like a relief valve not opening or only partly opening.

I don't think a stuck pressure relief valve would make any difference at idle.

[Formulajones]

Originally Posted by gto4evr

Hello again....this car's starting to bug me now. got the starting issue sorted out on that other thread (thank you again!) and the other issue I thought was just a bad gauge apparently isn't.

background again:
69 GTO

400 4spd, basically stock rebuild in 2007

This year when I pulled the car out of storage, I noticed my oil pressure is very high out of no where. I've got an aftermarket oil pressure gauge on the car since I didn't trust idiot lights to the fresh build and ended up just leaving it under the dash. From the beginning of the motor build in 2007 up til last year, I'd get 40 psi at a cold start idle, once the car was all warmed up, it floated around 20 psi at idle and 60 under a load.

this year when I pulled it out of storage, other that the starting issue (now solved) it ran great, but would hit 60 psi cold start up, 80 under a load, then after completely warmed up it doesn't drop below 40 psi. I figured the aftermarket gauge was just dying so I just replaced it with a new one, and.....same results.

cold start just now, sitting at 60psi idle and hits 80 when revved up.

after fully warming the engine up it idles at 40 and revs at 60. engine temps are always fine at a consistent 195 just like it's been for the past 14 years. changed oil and bled the line to the gauge, no difference.

So any ideas on the sudden change to idle pressure? I searched the forum but couldn't find any similar threads.

It could simply be that the 1st gauge was inaccurate, maybe corrected itself during a long hibernation????, and the new gauge you're now using is okay as well.

I wouldn't worry about it. 40 at idle and 60 at higher rpm when warm is pretty common and probably closer to what is really going on. I have a few engines here just like that and they've been happy for years and thousands of miles, Pontiacs and Chevrolets.

[PontiacJim1959]

And this is why gauges are bad. If the engine is not "clattering" or "knocking," the oil pressure is fine. If the coolant isn't boiling over/out the radiator, engine temps are good.

Idiot lights do have a use and you don't constantly look over at them and get into a tizzy the minute there appears to be a fluctuation in the needle on the gauge, and then you stay awake at night worrying about your car. This is the cause of 90% of the ED amongst car & motorcycle enthusiasts - as our cars have become more important than a piece of...............

[Goatracer1]

I agree with jim1959. I removed the inaccurate name brand gauges in the car I recently bought. Antifreeze does not boil until over 260 degrees and oil pressure doesn’t have to be over 20 pounds at idle. You know the lights work as soon as you turn on the key. The lights also will draw your attention and your wife/girl friend can’t say she didn’t see them

[gto4evr]

took the car out yesterday, hot as hell out there, I think it was around 100 out. anyways, first 5 minutes of the drive the oil pressures pushing upwards of 85 psi now. car was really running hot too. stuck in traffic temp gauge came up to 230. after about 10 minutes of driving, it settled back down on the oil pressure to the 40 idle and was 65 at higher rpm. not sure if the overheats related or just because of the extreme temps outside. when I finally broke out of traffic to get some airflow through the radiator, the temp came back down to 220 but wouldn't drop below.

I just can't accept the car was 20 psi for 14 years and now suddenly 40. i've been driving these pontiacs for 35 years and 10-20 psi at idle's always been the norm on any car I ran a gauge on. something has to have changed.

[Formulajones]

Quite honestly, any car that only has 10 lbs. at idle I view as having an issue. Worn or excessive bearing clearances are usually the cause.

People in here complain about noisy lifters all the time but think 10 psi of oil pressure at idle is normal.

Out of a dozen cars in the stable, they all idle at 35-40 lbs. of pressure, and cruise 60-70 lbs.

Your 40 lbs. at idle is not a problem. Running 230 degrees is though. Personally I'd be more concerned with trying to get those engine temps under control and forget about the oil pressure.

[dataway]

I agree with the suggestion that the original gauge was wrong .... first startup of the year, cold oil tweaked the mechanical gauge to read what it should have been reading for years.

Two readings agree, one does not. I'll put my money on the two that agree.

Don't assume the first gauge was correct just because it was in the car a long time. Could have been malfunctioning for years and the recent first cold start of the year freed up some sticky linkage.

What pressure pump are you using? Those numbers look normal to me, about exactly what mine runs with an 80 psi pump.

[padgett]

At various times since 1970 Pontiac has used 40 psi, 60 psi, and 80 (SD) psi reliefs. Always figured any engine that can exceed the mid (40 psi) line at 2000 rpm hot is OK.

Factors that can affect idle psi are oil brand, age, and rating (I use Mobil 1 10w-30 in American cars), oil temperature (oil takes longer to warm up than coolant), type of oil punp (gear or georotor), RPM, and of course engine condition.

That said a built 400 with an 041 cam will idle faster than a 400-2bbl with an 067.

In general newer cars are designed for higher pressures (my '11 CTS runs 25psi at a 550 rpm hot idle.