oil pressure problem

[jomama]

i have good oil pressure at start up once the motor warms up oil pressure drops instantly from 50 psi to below 10 psi ?i bought the car with a spun bearing. motor was almost new i found crank and con rod bad had repaired all plus all new bearings in motor. could this be a bad tstat?88 930 turbo motor

[Flieger]

What rpm are you citing the pressure reading at? What is "instant" drop of oil pressure when warm? Oil gets to 185 F and the pressure gauge suddenly drops to 10 in a tenth of a second?

Pressure fluctuates with rpm and temperature. You should have about 15 psi per 1000 rpm when warm (180 F +). Only measure the pressure above 2000 rpm, it is more accurate.

When cold at idle (1200 or so for me) I see 60 psi. If I rev it, it can easily go 80 or more. That's why I take it easy for a good number of miles.

[jomama]

thanks for the reply.at 2000 rpm cold is 50 to 60 psi just as it warms up it drops to 10 psi with almost no movement in pressure reading.i also have a 78 930 with 100.000 miles on it .the oil psi operates as you descibe the 88 has may be a 5 psi at 3000 rpm i have already pulled the motor to check for any mistakes every thing was in place .i re sealed motor installed new oil psi check valve springs correctly.i put motor back in .i am not shure how to check oil thermostat or oil cooler tstat.

[Flieger]

Thermostat should not do this. Whether open or closed, pressure is OK. When mine open to allow oil to the front center oil cooler, there is no sudden drop in pressure. The thermostat opens slowly and progressively.

Have you checked to make sure the oil pressure sender is not faulty? Could be a bad ground for the gauge.

I have heard of oil pumps craking, also.

Sorry I cannot help. I am curious to hear others suggestions.

[edbaus]

I have experienced an oil pump cracking. This will not happen on a 78 turbo (or not likely) it is the scavange side that cracks, so the engine just fills up with oil.

Buy a cheap mechanical gauge and get a reading that way. Make sure that there is not an issue with the sender or the gauge itself.

The next thing to look at is there are two spring relief valves. both are mounted with flat blade screw driver heads. Very large. About an inch in diameter. One comes from the side of the motor and the other is underneath.

Pull those out and make sure there is nothing wrong with them. Stuck or some junk right there.

The next thing to do would be to take off your oil filter and cut in up and see if you have metal and take the drain plug out of the motor and see if there is metal there too.

Ed

[jomama]

thanks for the reply.this is in my 88 930s i checked the pump thru no wear or cracks on visual.the check valve spring housing checked .new springs installed.i have only run the motor for 20 min at a time .changed the oil twice due to installing all new bearings last oil change showed very little metallic showing in oil almost clear. first thing i installed was a mech oil psi gauge in line with factory sender.what is considered to be to low oil psi for porsche motor?i don't want to spin a bearing.

[edbaus]

I had a 2.7L race motor that would not read pressure on the gauge at idle. It ran with about 10 psi per 1000 rpm. It ran for 3 years like that till I tore it down for rebuild. No issues.

I would say that if you are not getting 20 psi at 2000rpm, then that is too low.

If you are not getting that, then it sounds like you have an issue with the build.

Did you plastigage the bearings when you installed them to check for fit. Did the mains get turned to and you have standard size bearings in them. That would defiantly leak oil and drop pressure.

Ed

[sww914]

What kind of oil are you using?

[jomama]

i am using 20/50 .i bought the car with a spun bearing.all i can figure is there is a faulty part in the sys failing when at operating temp.oil temp at 190

[DW SD]

Can you be certain the pressure sender is not failing?

long shot but.....see if the pressure relief valve is sticking in the bore at temperature. Maybe the shape change of the bore from temperature causes the piston to stick, releasing the oil pressure.

Doug

[jomama]

looks like i need to pull the motor .at start up with mech gauge i am getting 80 psi reading next to the oil tstat. the reading at the cam tower 60 psi . when oil temp hits 180 readings change to 20 and 10 psi i do have a10psi per 1000 rpm but it just seems to low for a new motor .i will let you know what i find ..thanks for all your help

[richardlw]

Check your oil pump. 20W-50 will pump almost with anything as is it real thick when cold. As it heats and thins (usually lower VI than most other multigrades) and can run around the pump gears.
I'm not specifically familiar with the pump on your car, but on most, a thick gasket on rebuild will leave too much clearance and the oil will circulate around the gears instead of going through the engine. You want it just think enough that the gears don't bind. On my Vair that is an extremely thin piece of paper, although 2 pieces came in the rebuild kit for the engine just in case I needed more.