oil pressure

[MarcHogenboom]

Finally I got to the point where I could take the GMC for a testdrive and now it turns out I have way to low oilpressure. Cold it is 4 to 4.5 bar at idle and 7 at 1500 rpm, but as soon as the engine warms up it drops to about 0 at idle and 1,5 at 1500 rpms. The engine has 2 miles on it! I bought it in the US 2 yaers ago, so no chance of exchanging it under warenty. Besides, the shipping from the Netherlands to the US and back would cost as much as another engine.It's a '97 engine, unused. What can be the cause? is there some kind of presure regulater in the pump that could be at fault?

[MTTwister]

Marc - Here's hoping this is an electrical connection problem. Do you have access to, and can install, a mechnical gauge to get a True reading? Check connections to and around the OPS? Good Luck!

Edit - How does BAR translate into PSI? 1 bar = 15? 4 would then be 60#.

[MarcHogenboom]

I tried a machanical gauge and calibrated the electrical gauge. 1 Psi+0.070 bar.

[MarcHogenboom]

Oops,
should read: 1 Psi=0.070 bar

[JohnC]

Sounds like the oil pressure relief valve may be stuck open. A bit of debris could have been pumped into it and wedged it open. I've never had to work on one, but I recall it's in the same area as the oil filter adapter.

[BobND]

Oil pressure relief, I believe, is in the pump cover.

[MTTwister]

Somebody, check my math - 1psi = .07 bar

cold - 4 bar / .07 = 57.1428 psi idle
7 bar / .07 = 100 PSI 1500

Warm 1.5 bar / .07 = 21.4285 1500
0 bar / .07 = hmmmmm idle!

[JohnC]

This is the trend you'd expect if the relief valve is stuck. Can't relieve the pressure when the oil is cold and can't keep it up when the oil is hot.

Pump cover, huh? Maybe I'm remembering the bypass for the cooler...

[john8662]

The bypass is in the oil pump itself. There is an oil cooler bypass that is in the oil filter boss on the block, it's there for if the cooler becomes plugged etc. I had a low oil pressure situation happen to me, turned out to be the oil pump itself, but I didn't have as low of pressure as you, just lower than normal all-of-a-sudden. There is no need in replacing the valve in the oil filter mounting for the oil cooler, it won't cause low oil pressure. It's the bypass in the oil pump, or worse, a bad bearing somewhere.

[patrick m.]

low oil pressure in a new engine is most often miss-positioned pick-up screen.

either the screen is too high, too low (which stops flow), or .....fell off!


add another gallon, and check again, if pressure is up, screen fell off.

if not, drain oil and remove the pan, measure distance screen is from bottom of block and measure depth of pan. you should have 7mm to 11mm of clearance.

if clearance is fine, replace oil pump. dont mess with stuck pressure relief valve in pump, too risky. if it stuck once, it may stick again. Get a new one!

[MarcHogenboom]

Thanks guys, this means I have to disconect the block from the tranny to get the pan off. It is a front wheel drive TM 425, so there is no other way to get to the bolts than diconecting and shifting it sideways. Oh well, here goes another weekend. I sure hope you are right!