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77 p911s
 
Join Date: Jul 2014
Location: Australia
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High oil pressure 1977 2.7Lt 911

Very high oil pressure on cold start.
After about 10 minutes on idle, pressure lowers. Once on the road engine under load at around 3500rpm oil pressure 75-100. Driving the car gently (not to raise oil pressure ) 20 minutes from initial start up, oil pressure needle starts to move erratically.

Diagnosis procedural advice much appreciated.

P911 1977 2.7lt

Old 12-31-2019, 01:09 PM
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Sounds like a problem with the oil pressure release plunger or spring sticking.
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Old 12-31-2019, 05:21 PM
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It's a 914 ...
 
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Might be worth validating the pressure readings using a mechanical pressure gauge to rule out problems with your gauge or sender.
Old 12-31-2019, 07:40 PM
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Pressure sender.
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Old 12-31-2019, 08:06 PM
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My 71 had high oil pressure readings. It had the wrong sender for the gauge! They were exactly 2x what they would normally be.

It had a 0-5 bar sender should have had a 0-10. What was odd is the sender had the right date range and had been on the car a very long time... it’s possible the gauge was calibrated at some point and when they were rebuild a few years ago recalibrated.

Anyway... check your sender against a mechanical gauge as mentioned.
Old 01-01-2020, 07:25 AM
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^^^

I would agree with this, due to the eventual fluttering of the guage. The other suggestions are good, but this one should be the simplest to eliminate.

Rahl
Old 01-01-2020, 07:29 AM
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Vanilla - what is your history with this car? Had it worked normally, then became high? Or is it new to you.

If the gauge and sender are matched (the '77 should have come with a 10 bar gauge/150 PSI gauge, and sender to match - at least my Euro '77 did), what you describe is probably not sender related. Over time the sender's resistance wiring breaks, so the sender works normally up to a certain PSI, and then jumps full scale.

Sticky pressure regulating pistons are, I think, unusual. But might be the issue. It's the vertical one you would start with. You can remove its "nut" without losing a whole lot of oil, which is convenient. The spring will come out. Getting the piston out, if it doesn't fall out, with the engine in the car would be tricky - on an engine stand you can remove the idiot light sender and put a rod down that hole to tap the piston out. It seems unlikely the piston, being steel, would be damaged. Maybe a small brake hone could clean up the magnesium of the hole the piston rides in? The piston shouldn't stick.
Old 01-01-2020, 03:18 PM
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Related to this, my 7r block with 964 oil pump at full temp (190-210F) registers about 10psi or just below that at 1000rpm idle with a new sending unit and rebuilt gauge. Very different to this, my digital sending unit reports to the AEM Infinity running the show that the lowest idle reading I have is 14psi at 1000rpm, with various rpms also reading 20% lower than the actual digitally confirmed number.

Point being, the sending units and gauges can be all over the place. If truly concerned, confirm only via outside instrumentation or digitally as I arranged.
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Old 01-02-2020, 06:12 AM
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77 p911s
 
Join Date: Jul 2014
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Thanks all. As soon as I get time I'm going to do a manual pressure check. In the meantime however, I'm wondering if replacing the original distributor with a 123 electronic ignition might have something to do with the false readings?
Old 01-03-2020, 06:31 PM
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It's a 914 ...
 
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The distributor doesn't have anything to do with oil pressure, but I suppose it's conceivable that some wiring related to the oil pressure sender was disrupted during the install.
Old 01-03-2020, 08:56 PM
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77 p911s
 
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Location: Australia
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I went for a long hard drive yesterday and noticed both the oil pressure gauge and temperature gauge both moving erratically at higher revs. Considering changing back to the original ignition set up as Iíve never been happy with the performance from the 123 electronic ignition.

Old 01-06-2020, 12:26 PM
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