excessive oil pressure

[madkaw]

Guys I need help here figuring this out. I have been fighting an oil leak in the rear main seal area which I have posted here. I have done everything including a speed-sleeve on the crank where the rear seal rides.

I have always thought my oil pressure was too high on this engine, but discounted it because of the stock gauge.

Today I installed a new oil pump(standard pressure and volume) and installed a mechanical gauge inside the car. No change.

The oil pressure reaches 70psi before I get to 3000rpm. At cruise it will stay right at 70psi-cold or warm it doesn't matter. If I understand the pressure relief valve and how it works, 70 is the magic number as far as max pressure before the valve bypasses.

When I look at the FSM, it shows what looks like two relief valves, one in the pump and one by the filter-is that correct?

I am at the point that I think I have a partial blockage somewhere in the system that is driving the pressure up-but where? And how will I find it?

I have tried to blow pressurized air thru the oil galleries between the filter and pump and they seem clear.

I really want to figure this out before the car leaves the garage again.

Shoot me some ideas guys-please!!!!!!!!!!!!!!!!!!!!!!!!!!!

[zKars]

Suspect the pres. regulator in the oil pump. Take it apart and clean/inspect.

[beermanpete]

Your description indicates proper operation. The oil pressure rises with engine speed to the regulation point and then stays there as the engine speed continues to increase.

What is the oil pressure at idle? It should be about 15 psi or so when the oil is warm.

The relief valve in the filter is a filter bypass not a pressure regulator. It allows the oil to go around the filter if the pressure drop across the filter is too high. Normally this will only occur if you race the engine at start up (cold, thick oil) or if the filter gets restricted by too much dirt (change oil and filter more often).

[Zed Head]

Seems to me that that seal area is not supposed to get pressurized. Your oil pressure is good, the oil just doesn't have the proper return path to the crankcase. Looking at various pictures of the bearing cap and bearing, for whatever reason the gap between the bearing and the seal must be filling with oil faster than it can drain out. I would focus on that aspect of the situation, rather than oil pressure.

But I've not had an engine apart so I'm just going off what would make sense if I designed an engine. Maybe Nissan did something different.

[zKars]

Clearly its not the pump if the switch made no change. Sorry for jumping to conclusions.

Very suspicious that it stays at perfect 70.

[Zed Head]

More on the oil behind the seal - I am fairly sure (not certain though) that there is a drain hole or slot at the bottom of the main bearing cap between the seal and the bearing. There's a gap between the seal and the bearing for sure, so no reason for oil pressure buildup. I also have faint memories of shining a flashlight in there while changing a seal and realizing that I could see in to the crankcase.

I don't see any reason for oil pressure to have anything to do with the leaking seal. The oil should be fed in to the bearing, run out the sides and down in to the crankcase.

Of course, I could be way off.

Edit - there might be a way for pressure to build up if the drain hole (if it exists) was clogged. Maybe that's the issue.

Edited August 2, 2012 by Zed Head

[SteveJ]

What if the PCV is stuck shut? Couldn't that raise the pressure from the buildup of combustion gasses?

[madkaw]

Thanks for the ideas guys.

The PCV has been checked.

I might just have to live wuth a leaky engine. I cannot see myself tearing into the engine for the rear main seal again. I've replaced everything including side seals and oil pan gasket.

The constant 70psi seems weird!

[madkaw]

Old thread-current problem. I have my engine out to look into my head problems and thought I would check my oil pressure on a stand without the engine running. I drove the oil pump with a large drill and monitored oil pressure with a new gauge. With the drill running full speed-2500rpm, oil pressure runs pretty constant 75 psi. I figure that 2500 rpm on a drill duplicates about 5000rpm engine speed since the oil shaft normally runs half engine speed. Even with the drill set at 1250rpm(should be 2500rpm engine speed), oil pressure is 75psi. I don't think that's right.

II am going to check all oil passages the best I can without dismantling the short block, but that might be next.

Rear main seal leaks at the seal ID. I tried turning the crank by hand while applying oil pressure and I can see the oil wanting to seep at the crank. Looking at the seal carefully I can see that it is in slightly cocked on the 6-12 oclock position, so I know that's an issue.

[Captain Obvious]

I don't know what oil pressure would be expected at what RPM and temperature, but I guess I wouldn't be surprised to get 75 psi at 2500 RPM on a stone cold engine in un-worn good condition with thick oil. You're clearly bumping against the pressure relief valve built into the pump, but that's not necessarily a problem.

As for the leaking rear main seal, I don't think there should be any link between the oil pressure and the leak at the rear main. There's no way that seal should ever be seeing pressurized oil. The back side of that seal has gotta be open to the pan somehow. Been so long since I've been in there personally that I don't remember how, but it's just gotta be.

The cocked rear main could certainly be an issue... Not sure how much misalignment your typical garter spring seal can tolerate.

You said that you turned the crank while applying oil pressure and you could see it trying to seep. Does it do the same seeping when you turn the crank and there is NOT oil pressure?

[madkaw]

Your right Captain, a cold motor or oil would drive up pressures, but my engine runs that high all the time--- hot or cold. I did this without the engine running and a NEW gauge to make sure this wasn't a false reading.

The front of my engine was coated with a fine mist of oil dust from the front seal.

The rear seal does need to be redone.

[djwarner]

Steve, normal, warm oil with a standard oil pump you should see 14.2 to 17.1 psig at idle. At 2800 rpm you should be generating enough pressure to open the pressure relief valve in the pump. At that time the pressure should be 54.0 to 59.7 psig.

Nismo does sell a high pressure regulator spring for this pump. I don't know what the specs are for that pump, but the stock spring should have a free length of 2.24". Fully compressed it should measure 1.54"

The pressure relief valve (regulator) simply returns excess oil back to the inlet side of the pump. Our oil pumps are a constant displacement type. That is it outputs the same number of CCs with each revolution and generating whatever pressure needed to make that happen. Once the pressure is sufficient, the piston in the relief valve moves back, opening a port back to the inlet side of the pump. Once cracked open, the relief valve will trade off additional oil pressure for additional volume of oil bypassed until the volume of oil put out to the engine and through the relief valve equals the CC capacity of the pump. - or until the piston in the relief valve bottoms out.

The piston bottoming out is exactly what happens with a cold engine starts with thick oil.

At this point it would probably be worthwhile measuring the relief spring free and compressed to verify it is to spec. Also - the exploded parts drawing shows there are three washers in the nut-cap that are used to adjust the set pressure for the regulator. Removing one two or three washers will reduce the regulated pressure.