Oil pressure disaster

[Leadbut]

Just got my newly rebuilt 390 back in the old AMX. Fires up nice. Plenty of power.
Just when things are looking up. My oil Pressures drops off to about 5 psi after about 6 miles (190-200F).
Starts up fine about 55-65 psi cold.
Holds nice for a little while then just falls away.
Engine sounds great. Although I haven't driven it around at 5psi. I checked the gauge it operates fine. If i let it go long enough the idiot light comes on. Pulled the oil pump apart and there is some wear, but the clearances are in spec. I pulled out the Oil pressure bypass valve it seams OK, but it’s a very tight fit in the housing. The spring is new. I'm at my wits end I don't want to pull the motor again!

[BREK]

Does the plunger move freely without the spring in place? Have you tried changing filters?

[Leadbut]

With the spring out the plunger is tight.

I had to use a screw driver to get it out.
when Iput it back togeter the pressure of the spring is enough to reseat the plunger.

[jeepsr4ever]

Hmmm I wonder if your oil is draining back fast enough. Try adding 1.5 qts and see if it makes a difference.

[Leadbut]

I'll try that tonight.
During the rebuild I added the lifter valley bypass line. Could this be having an effect?

The pressure was low before the rebuild (not this low though).
Bearing clearances were still in spec when I disassembled the motor.

Could the oil pressure relief valve be so tight that it's getting stuck when the temp rises and the aluminium housing expands? Or does the passage the plunger is in actually get larger because the passage expands outward?

Does the plunger have a seat like a valve? Or does it just block the passage?

[jeepsr4ever]

When the temp raises the plunger actually becomes looser in the hole. I would be suspicious of either oil drainback or that valley line. Often valley lines hit the bottom of the intake and when you use a soft material (copper ect) you can easily tear the end off when installing the intake (if the bottom hits the fitting between the lifter banks)

[Leadbut]

That’s what I Thought, but one of the eggheads at work had a theory that the Stainless steel plunger would expand because it is in contact with the hot oil and that the aluminum housing with its greater mass and exposure to the cooling air... coefficient of expansion 23 vs. 17... blah, blah... spring pressure reduced by...internal differential pressure... blah...blah.... some story about his father-in-laws motorcycle.... blah...blah...ham and cheese sandwich for lunch...etc.etc.

I used braided hose for the valley by-pass. But it was a pain in the ass getting the intake on. I didn’t have to force anything but it took some screwing around.

Everything is new on this thing except the timing cover oil pump housing and the fuel pump.
If the oil was contaminated with fuel wouldn’t I smell it and see it in the oil?

[jeepsr4ever]

You may not see it but you would definately smell it. It is possible your valley line is leaking under pressure. This might mean a intake removal and inspection while priming the pump.

[tufcj]

3 things I can think of to check.

1. Do you have a newer oil pump cover without the filter bypass?
You might replace it with the newer model without the bypass.

2. Oil filter brand.
My CJ sees 10-15 PSI less pressure hot when using a Fram filter vs. a NAPA Gold (Wix).

3. Have you replaced the pressure relief spring?
I've seen the springs fatigue from years of heat. A new spring will sometimes restore pressure.

Any doubt on the relief plunger, pull it and clean it and the passage with generous amounts of carb cleaner.

Bob
tufcj

[Leadbut]

I have an old style cover with the oil filter bypass in it. I pulled it out and blocked it with a nut and bolt.
There is a FRAM filter on it now. When I started it up to breaking the cam I had a WIX filter on it. I changed the oil on late Sunday during half-time and I could only get a FRAM at the local shop.
I'll put another WIX on it or a NAPA gold then add 1.5 quarts of oil and see what happens.
If that doesn't do it I’ll start pulling things apart.

If the oil pan to oil pick up clearance is off would this make a difference only when the oil is hot? Wouldn’t it starve the pump all the time?