Gradually falling oil pressure

[clark800]

I'm having a problem with the oil pressure on a recently rebuilt late model Atomic 4 engine, about 3 hours since the rebuild by a very experienced mechanic. I'm using SAE 30W oil and the oil pressure gauge, oil pressure sender, and wiring are all new. The pressure starts out high at 75PSI cold, but over time the pressure gradually falls to 11PSI after 1 hour at 1600RPM (this is where I ended my test).

I've read that problems with the oil pressure relief valve can cause symptoms like:
- a decrease in oil pressure when adjusting the valve to increase oil pressure
- oil pressure decreasing as RPM increases
but this doesn't sound like the problem I'm having.

I'm going to try to find a mechanical pressure gauge to verify the readings, but I'm not too hopeful that this is just a faulty gauge because of the pattern of the readings.

I saw a part in the catalog that says:
"When the output from an oil pump drops off below a certain point, oil pressure will decay over several hours of operation even though all bearing clearances are normal. When this condition occurs, the problem is seldom excessive wear on the oil pump gears themselves, but wear within the housing of the rear main bearing cap in which the gears rotate (including the four holes in the housing which serve as bearings for the rotating shafts of the gears)."

This sounds like it could be the problem I'm having; is this something that is likely to be missed in a rebuild? Is there anything else I should consider? Could this be due to a faulty spring in the oil pressure regulator?

I've attached the pressure, temperature, and RPM data from a 1 hour test run where it started in neutral for 10 min, then shifted to forward and I increased the throttle a bit at 15min.

[Mark Millbauer]

Clark800,

Definately get a mechanical gauge to confirm. It will probably still show the gradual decline in pressure but the numbers will be more accurate.

How did the oil pressure behave before the rebuild?

Were the crank journals correctly measured, machined as necessary,and the bearings replaced?

How about the cam bearings which are often overlooked during A4 rebuilds? Along with crank and rod bearings, cam bearing clearances play a big part in oil pressure, particularly as the oil heats up and thins.

Switching to 40W or 20-50 will help. As long as your maintaining 10lbs per 1000 RPM, you'll be OK.

I'm particularly curious about your dilemma as i had the same exact issue once.

[joe_db]

Is that water or oil temperature on your graph?
Falling pressure would normally be caused by one of two basic causes:
1. Something wrong with the pressure control valve.
2. Oil too thin for the bearing clearances.

Number 2 can be caused by worn bearings or oil being too thin. Oil can be too thin because you put the wrong kind in or because it is too hot.
You might start with different oil - I have very good results with Shell Rotella 15w-40. If nothing else, the results might be educational. Before I even did that I would take the pressure valve out and make sure all is well.

[lat 64]

I agree with Joe.

A multi-vis oil may be in-order.

A "very experienced mechanic" still has to make judgement calls on the wear tolerances of parts, and some he measured may be on the high range of acceptable clearance.
That, combined with poor quality oil, low volume of oil, and high temp(of oil) may all be contributing to low pressure in their own little way.

Cheers,
Russ

[sastanley]

I have a pretty worn out engine, and burn a little oil thru the #4 cylinder...I've run thru almost every different type of oil we've ever discussed here, and I am presently running Shell Rotella synthetic 5w-40. I get pressures a little higher than I'd like when cold (60+), but it settles down to 35-40 when warmed up for the duration of the trip. I do have the Indigo oil filter and included pressure regulator remotely mounted, and have bottomed out the stock regulator in the block.

All the things mentioned above sound like good details to talk to the mechanic about to help confirm or deny your findings.

[joe_db]

I agre with Joe.

A multi-vis oil may be in-order.

A "very experienced mechanic" still has to make judgement calls on the wear tolerances of parts, and some he measured may be on the high range of acceptable clearance.
That, combined with poor quality oil, low volume of oil, and high temp(of oil) may all be contributing to low pressure in their own little way.

Cheers,
Russ

+1
Don't forget - hot 15W40 is *thicker* than straight 30 weight when hot

[Administrator]

Other than a change in the viscosity of the oil, or loss of oil, what would explain the change over time?

Bill

[joe_db]

Other than a change in the viscosity of the oil, or loss of oil, what would explain the change over time?

Bill

Some part of the engine increases clearance over time, which is backwards to how it usually works AFAIK.

[Administrator]

Just curious... Mechanical fuel pump?

Bill

[JOHN COOKSON]

I'd do a couple of things.
Confirm the OP readings with a mechanical gauge as mentioned.
See if you can change the OP with the adjustment device when the readings get squirrely. If you can't influence the OP with the adjustment then the ball may be off center on the spring or the seat may be not correct. MMI used to sell a tool to dress the seat. I don't see it in the online catalog. Maybe I'm not remembering correctly......

TRUE GRIT

[Tim]

I had a similar problem after rebuilding my engine. I could set the oil pressure before the engine warmed up, but then it would drop like a rock once the engine warmed up. I could then adjust it back up, but the gauge would peg high when starting cold. I purchased the cone style pressure valve from MMI, this helped a little but pressure was still too high when cold and too low when warm. The problem was most likely damage to the valve seat for the oil regulating valve. I installed the indigo oil filter/pressure regulator and now the oil pressure stays at 40 psi under all conditions.

[Peter]

MMI used to sell a tool to dress the seat. I don't see it in the online catalog. Maybe I'm not remembering correctly......

TRUE GRIT

Product No. - TOOL_05_172

at http://www.moyermarine.com/cgi-bin/s...rebuilder.html

Peter

[clark800]

Thanks so much for all the advice! I'm on vacation now, so I won't be able to test anything for a while, but I'm looking forward to trying these recommendations.

To answer some of the questions:
- I bought this engine after the rebuild, and I haven't heard from the mechanic recently, so I'm not sure what it was like before the rebuild.
- The temperature graph is for the water temperature. I thought about this ambiguity right after I posted
- The engine has a mechanical fuel pump. Does this have an influence on oil pressure?

The Indigo kit sounds like an interesting option, but I'm a little confused about how it works. It says it connects to the oil sender port, so does that mean the stock oil pressure regulator stays in place and works in parallel? Wouldn't that mean installing the kit could only lower the pressure by providing an additional relief point? Or can it be installed in place of the stock regulator?

[Tim]

The original oil pressure regular gets plugged when installing the Indigo oil filter. The oil pressure is then easily adjusted by a pressure regulator on top of the oil filter housing.

[Administrator]

From the "Long Shot" Department:

You have a mechanical fuel pump. When the diaphragms in these pumps begin to fail, they leak fuel (by design) into the engine oil. If this were happening, the presence of the gasoline would surely kill the viscosity of the oil. The longer the engine runs under these conditions, the worse the effect.

Two quick checks:

1. Sniff the oil dipstick for the presence of gasoline.
2. Check the oil level for anything suspicious. In severe cases, the level might actually rise with the addition of the fuel.

Other, wiser members will amplify or correct as needed, please.

Bill (Admin and Forum Executive Vice-President in charge of long shots)