Backwards Oil Pressure!

[Raptor]

83 Wagoneer
AMC 360

Crazy as it sounds - Using a mechanical oil pressure gage - when I rev the engine up at idle, the oil pressure LOWERS instead of raises. Can anyone explain what causes that?

I removed the NAPA Gold oil filter as well as the oil pressure relief valve spring and poppet just to see if something was stuck or blown. I replaced the oil filter with a cheap FRAHM and replaced the same spring & poppet.

Started up the engine again, and BINGO - back to normal
Hot Idle = 20 psi
HOT 55 MPH = 50 psi
Cold = Much higher pressures as you would expect. Pressure increases along w/ RPM as you would expect it to.

Now - After approx three weeks - my Crazy Oil pressure problem is back again. This time I replaced the entire filter adapter & pressure releif poppit with a bulltear unit complete w/ mid-plate. Fired the engine up and No Joy! The "backwards" oil pressure situition still exists.

Any thoughts as to the root cause?

Jeff

[runnamuck]

low on oil?

[AMX69PHATTY]

Just a random thought, but are you SURE that the little PLASTIC BUTTON was installed in the BOTTOM of the OIL PICK-UP TUBE
before the Oil Pan was installed ? This little button keeps the oil pan from coming up against the pick-up and restricting oil flow.
It's any easy piece to forget. Usually put some silicone on it also to help make sure it stays put.
Just a guess.
:razz:

About Main Bearings, in my '69 service manual it says it is acceptable to use two different size main bearing halves, with the smaller I.D. bearing half on the bottom in the crank cap, althought the difference between the two halves should not excede .001". Many years ago, with the motor still in the car, I dropped the oil pan and installed new undersized crank bearings with a .001 size difference to restore low oil pressure.

[Goose]

Not to jinx anything here.. but on my 360 when it started doing this, It was the main bearings worn out.. on the throttle oil pressure down.. off the throttle pressure goes up.. and it was like 20 psi different..(SO pray it's the little button thingie..on the pickup screen)

[tarior]

I put roofing screws through the bottom of my oil pickup, they never come off.

[jeepsr4ever]

AMC quit using any kind of a button after 1974 I believe.

Tell us a little more about the parts in your motor

[Raptor]

What would you like to know? The motor is esentially stock from an 80' J Truck that was rebuilt approx 30,000 miles ago. It's one of those unknown rebuild jobs though. I have no records or receipts.

[tufcj]

I've also seen the same kind of pressure problems when the cam bearings went bad. Out of the 11 AMC V-8s I've rebuilt, 5 have had bad factory cam bearings. Seems like the cam bearings they installed at the factory weren't of the best quality.

If your engine was rebuilt without replacing the cam bearings, chances are that they are bad.

Bob
tufcj