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!