High Oil Pressure

[Diseazd]

My 71 240Z (L24engine) runs 75 lbs. of oil pressure at 3000 rpm. It's not a defective gauge. What can cause high oil pressure in a Z motor? The engine has 2000 miles on it and runs perfect.

[djwarner]

There is a pressure relief valve in the oil pump itself. Also there is a clogged filter bypass where the filter mounts. Normal operation at 3000 rpm is for the pump to produce excess pressure and the relief valve to bleed this off.

[Diseazd]

What would you suggest.....a new oil pump?

[madkaw]

Will be watching this closely. I have been fighting high oil pressure since day one.
Nothing wrong with high oil pressure until it starts pushing oil past the rear main seals. I have a constant oil leak that I have pretty much given up on until I solve the high oil pressure issue.

My best guess at this time is that I have some kind of obstruction in my oil system somewhere that is driving up my pressure. I run a stock pump and have checked for obstructions as far as I can without dismantling the whole motor.


Isn't the filter bypass working if the pressure does not exceed 75?

[Diseazd]

I too run a stock oil pump.....I changed to a new oil filter (Niisan) after the first 200 miles as always when I build a new motor. The engine has 2000 miles, so if it were tolerance based, you'd think it would have been toast by now. Also, the crank was polished and passed as standard, so I used standard Clevite bearings. I'm very careful to keep things spotless when building my motors. I'm stumped unless the new oil pump is defective.

[madkaw]

There is a pressure relief valve in the oil pump itself. Also there is a clogged filter bypass where the filter mounts. Normal operation at 3000 rpm is for the pump to produce excess pressure and the relief valve to bleed this off.

What are you using for reference for this statement - FSM?

From all my reading the pressure of a hot engine usually runs approximately 10psi per 1000rpm. The relief valves are just for that- relief of excessive pressure above 70psi. Not to regulate pressure above 3000rpm.

[madkaw]

Some questions:

You run a PVC system?
What head and cam ? Is cam spray oiled or internal?
Did you enlarge the oil feed port on the block?

[djwarner]

First of all, having too much oil pressure is much less a problem than too little. That said, was the pump replaced, cleaned, or overhauled during the overhaul? Also, what oil viscosity are you using?

The engine manual states that the relief valve is not adjustable and the only maintenance checks are the free and operating heights of the spring. The plunger on the end of the spring could be hung up. The only other possibility is a partial clog in the relief channel that passes the excess oil back to the inlet side of the pump. In all cases everything is within the pump itself, so short of the pump being mis-assembled, the solution can be had by changing the pump.

I am assuming that you have independently verified the oil pressure and the oil filter has been recently changed.

From the engine manual, the relief spring's free and operating heights are 2.24" and 1.54" respectively. The oil pressure at idle is 14.2 to 17.1 psi at idle. And the relief valve should open at 54.0 to 59.7 psi. Relief valves typically open at around 2500 rpm. The pump is a constant displacement style pump ( that is the volume of oil pumped is constant for each revolution ). This means that as the pump spins faster, the greater volume of oil has to pass through the relief channel. As the flow rate increases, the pressure drop in the channel increases. So while the valve may open at 54 psi, the pressure will probably still increase somewhat at higher rpms.

Please note that the viscosity of the oil will affect how efficiently the excess oil flows through the relief passage. This is why a cold engine generates higher oil pressure. If you are using 20-50 weight oil, I would expect the oil pressure to be higher at the normal 180 degree operating temperature of the engine. ie 40 weight oil at 185 degrees has a viscosity of 19 Centipoises while 50 weight oil is 28 Centipoises.

So first I would check the oil pressure in a fully warmed engine at 2500 rpm, if it's 60 psi or less, stop worrying.

[Diseazd]

I did stop worrying and yesterday, I was trying to quiet what I thought was a squealing fan belt and the engine shut down. It is locked solid and will not budge.....my guess is that the bearings are welded to the crank.....trying to figure out what caused it. Check your filter and oil pump Madkaw.....high oil pressure is not a good thing......I think the squealing was bearings not fan belt.

[djwarner]

Wow, several posts came in while I was typing my last. Sorry to hear about your engine seizure. Sounds like a blocked oil channel deprived oil to the bearings. It's amazing that it took 2000 miles for this to show up.

[madkaw]

Wow- sorry to hear that. Bearing clearances weren't good?

I have about 14k miles on my engine now and still running strong. My issue must be different. I guess I will have to my engine apart to figure out why.

[Diseazd]

If bearing clearances were too tight, the crank would not have spun so effortlessly when the engine was assembled....standard crank and standard bearings....you wouldn't think too little, you'd guess too much if anything. I use 30 weight oil.......I know nothing about an oil pump, but suspect that.....an oil filter wouldn't be blocked that quickly, especially after changing the first one......I'm stumped.....I'll be pulling the engine when my "pressure" returns to normal....what a bitch!

[zKars]

My understanding of oil pressure is that is caused by the balance of two controlling factors. One is the internal relief valve in the oil pump, and the other is main and rod bearing clearance. The pump relief is a spring pressure controlled device that just allows internal bypass at a certain max pressure, but the bearing clearance and pump volume determine the overall pressure over the range of normal operating conditions.

I can't see one bad bearing causing high pressure. When the pump relief valve sticks, the pressure gauge becomes a tach, ie rpm determines pressure and it can quickly get WAY above 60 at as little as 3k. Doesn't sound like your case.

Time to take it apart and diagnose. If its one bearing seized and the other have correct clearance and no sign of scoring and starvation, likely just a blocked passage to that bearing and bad luck...

[Patcon]

I hate that. I have built a number of engines Z's and others. I agree the bearing clearances on a polished crank running standard bearings should be on the loose side not too tight. I am curious to see which bearings or whether it's all of them. Take some pictures of each bearing and keep them in order. I have had motors destroyed like this but from different circumstances. I have one motor I built that has excessive oil pressure that I have yet to identify. I tend to agree that you had a blockage or partial blockage somewhere, junk out of the filter, a piece of gasket or a loose piece of RTV something. If the bearings are evenly worn and cooked It would have to be a serious blockage to blank the whole crank. Remember that you may not be able to remove the bearings from the crank. I had one fail like that too, they were welded on. I really feel for the loss of time, parts and machine work.
Charles

[madkaw]

Did you plastigauge all bearings?

[Lazeum]

I'm sorry also to hear that. Was it on your L28 with L24 crank?
I'm posting also to follow up on your findings

Bleed valve from the oil pump should be able to maintain 60psi at 3000rpm but also at 6000rpm. Could too high pressure at 3000rpm be related to by-pass restriction since it should be capable to do the job also at twice the rpm?
I would suspect something is going on inside the pump. I would also check galleries inside the block between the sender and the pump.

Regarding crankshaft clearance, too much clearance would make the oil layer to fail withstanding high pressure resulting in metal contact => failure.
I don't know what are the limits, they might not be that small.
Bearing failure could also come from clogged galleries inside crankshaft.

[Patcon]

I also second Madkaw's question. I assume you assembled the bottom end did you plasti-gage or mic your clearances?
C

[Diseazd]

Lazeum.....No it was the L24 block and crank on the gold 71.....I didn't use plastigage, but every standard on standard I've ever plastigaged has been acceptable, plus the crank spun effortlessly with the mains torqued. Besides, the pressure increased later, if it had been a clearance issue, it should have started from the first start up, but you're correct, it could have been an out of tolerance shell, but I don't think so. I'm bummed and haven't pulled the engine yet, but will keep everyone informed. Luckily, I have the L28 motor with the L24 crank and rods almost finished....it didn't have anywhere to go......now it does.....could be the gods.

[Travel'n Man]

Sorry for the bad luck - but the good part is the new engine is going to rock. Let us know about the new configuration with the crank and rods.

[Lazeum]

You've got a golden back-up At least, there's some positive outcome to this failure.

[Diseazd]

This is the engine before installation .......sure was pretty.

[Diseazd]

Thanks everyone for your condolences in my time of need........you guys are the best!

[Captain Obvious]

Woof. That sucks... Hope you get to the bottom of it.

Hoping to help in that endeavor, I'm no hydraulics expert, but I think you can rule out filter bypass problems.

The purpose of the bypass on the filter is that as the filter gets dirty, the pressure required to force oil through it goes up. If the pressure differential between inlet and outlet of the filer gets too high (like in the event of a badly clogged filter), the bypass valve will be forced open and unfiltered oil will pass around the filter instead of through it. The engine will still get oil and the theory is that even unfiltered oil is better than no oil at all. There is a filter bypass valve built into the block, and most commercial filters have one built into the filter themselves.

All that said... The opening pressure differential across the filter required to open the filter bypass is only a couple psi. I didn't look it up, but probably about five psi or so. Remember that you're only concerned with the differential between inlet and outlet. The absolute pressures don't matter. The point is that a clogged filter should not result in significantly higher oil pressure. The pressure increase even from a completely blocked filter should only be the pressure required to force open the bypass.

Here's a snippet from the FSM that shows the lube scheme. You can see the pressure relief valve in the pump and the bypass valve on the filter:


As for the pressure relief valve getting plugged with something? I doubt it. It's built right into the pump and essentially short circuits the pump. If the pump pressure gets too high (and we're talking absolute here) it will force open a valve that connects the outlet of the pump back to the inlet. The passages are large and if you've got a goober in there big enough to clog them, then you've got other serious issues.

The valve might stick shut, but I'd be surprised if it got clogged shut.

My theory is that as your bearings started to weld and the clearances went away, you started bouncing off the pressure relief in the pump. Prior to that time, your pressure was limited by the bearings, but when the bearings went south, the pressure went up until the pressure relief took over.

Maybe you spun a bearing or five and blocked the oil supply holes to the crank as a result? Is that even possible?

My condolences...

[Jeff G 78]

OMG Guy, this is terrible! I'm so sorry to hear that your Safari Gold car is down.

The only thing I can think of is if there was a blockage in one of the oil galleys that occurred at the machine shop or shortly after startup. If a piece of flash got lodged in an oil passage, it could create the high pressure and starve the bearings. Hopefully, the damage is limited and has not ruined everything in the engine.

Have you drained the pan to see how much metal is in the oil?

Please let us know what happened.

[madkaw]

OMG Guy, this is terrible! I'm so sorry to hear that your Safari Gold car is down.

The only thing I can think of is if there was a blockage in one of the oil galleys that occurred at the machine shop or shortly after startup. If a piece of flash got lodged in an oil passage, it could create the high pressure and starve the bearings. Hopefully, the damage is limited and has not ruined everything in the engine.

Have you drained the pan to see how much metal is in the oil?

Please let us know what happened.

This is what I think is my issue. In fact, one oil pan gasket change I found the foil seal from a oil bottle in the bottom of my oil pan. If somehow a piece of that lodged itself somewhere, there is my partial blockage.

[Diseazd]

Thanks Jeff......I'm building two other engines at the time, so pulling that engine is on the back burner. I suspect oil pump, and will drop that this weekend. If the bearings are shot, I understand that when I drain the oil it'll smell like burnt toast. I go out of my way to keep an engine clean when I build them, and I never use RTV anywhere (I use form a gasket on the rear side seals and clean any excess on the inside), so that isn't a problem. I believe it's going to be oil pump related, but will have to wait and see. I've never heard of anyone having a failed oil pump....have you? Whatever it is, I hope that expensive E31 head is OK......we'll see, this is how you learn I guess......high oil pressure is a warning sign, just like low pressure.

[Blue]

Sad news. Strange problem. What does the oil filter contents look like? Open it up and have a look. I can only guess that something in the filter went down stream and blocked the main passage to the bearings.

[Captain Obvious]

I had a failed oil pump, but the failure mode was low pressure due to being all chewed up inside. Wasn't "positive" displacement anymore. Was sort of "fuzzy" displacement... I can't imagine a pump failure mode that would result in pressure too high other than a stuck pressure relief valve or incorrect spring pressure.

It wouldn't take a lot of movement on a main bearing to eclipse the oil feed hole and if that happens, you'll see a pressure increase. Not sure if one main would be enough to bounce off the relief or not.

So I'm still asking chicken or the egg?

Did the oil galley to the mains get plugged with something and starve the mains? In order to get to that galley, it would have to have been left in there upon assembly or it would have to make it through the oil filter (or be a part of the filter itself that let go).

Or did the mains grab the crank and due to too tight of an initial setup and eclipse the holes causing a chain reaction?

In any event, sorry it happened and awaiting the autopsy.

[djwarner]

Diseazd,

Could we have a follow-up. What did you find?

[Diseazd]

Sorry DJ.....I haven't pulled it apart yet. I had another new engine to drop in, so I haven't checked the bottom mains yet. I certainly will follow up and let everyone know what I find when I do. Guy