Navaros911

So my oil pressure readings were a little unreliable. The needle would sit at the bottom for 10 minutes after startup and then shoot up to the correct pressure. Next to this the readings were getting a little off from the normal numbers I expect.

So the oil pressure sender was replaced after testing the pressure with a resistance meter and revving the engine.

Now the needle always immediately shoots up, but the max pressure is lower than what I saw when I bought the car.

During idle she reads just over 1 bar.
During normal driving around 3k RPM the needle is just over 4 bar.
When I accelerate up to 4k RPM there is a minor increase, but virtually not noticeable.
When I redline her there is a maximum reading of 4.5bar.
During shifts pressure drops to 1 bar.

4 bar is reached at 2.3k RPM
2 bar reads at 1250 RPM
3 bar at 1600 RPM

Are these normal values?
6 months ago my car would see 5 bar readings every now and then, these days 4.5 is the most I will get to see.

Thanks inadvance,
M

crg53

I would say that looks very much like what my oil pressure show; I have never seen mine at 5 Bar, always 41/2 bar and at idle just around 2 bar with engine at operating temp.

altarchsa

Same as mine.

As an aside to all, I strongly suggest that you NEVER run the engine with the gauge showing no pressure, even if intermittent, just because you're assuming that there is a malfunction with the oil pressure gauge system.

On my '75 S, I thought my gauge was malfunctioning because it would read intermittently. I thought that I would surely hear some valve lash noise if in fact it was not getting oil. Then, on the way to work one morning, my engine seized. I had spun the bearings.
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I tore the engine down and shipped it off to a Porsche specialist to price line boring the magnesium case. The cost for the machine work and other R&R was more than the cost of a used engine. Luckily I found a guy with the same model car about 150 miles away who was upgrading his entire drive train and suspension from a later model car. I was able to buy his engine and trans for a reasonable price.

After I got the replacement engine in, I started it up and it ran beautifully.... for about ten minutes. Then all of a sudden, no oil pressure again.

Well, I checked the oil pressure gauge system and confirmed that it was functioning properly. I then deduced that the problem was outside the engine, since I was havng the same problem with a different engine. I pulled the oil tank-to-engine supply line (which is rubber on the '75) from the bottom of the oil tank and the oil gushed out for a second or two, then stopped. I stuck my finger up in tank and could feel an obstruction. I pulled the tank and found that the oil level sender float, which was made from two hemisperical pieces of copper soldered together, had come apart. Apparently, the oil had gotten so hot ('74-'77 911 owners will know what I mean) that it melted the solder. I soldered it back together,.....and wrapped it with some wire to assure that that would never happen again. It ran great for LOTS of miles.

Moral: If the gauge shows no oil pressure, stop the engine immediately, have systems checked until you find and resolve the problem.

Makmov

sounds awfully suspicious.

ian89C4

Possible issue with the intermediate shaft not properly spinning the cogs inside the oil pump?

I don't recall who posted the thread, but someone on here had an issue a while back with their intermediate shaft. IIRC some of the teeth had come off of the shaft and it was only intermittently spinning the oil pump. Symptoms sounded the same as the ones here.

altarchsa

I should have clarified my previous post by quoting the above as "Same as mine."

I'm betting on loose connection(s) of terminals at the oil pressure sending unit. It would sure be an easy first check.

If you wind up replacing the sender, be aware that some of the aftermarket senders have the terminals reversed. In other words, the small and large female portions of the spade connectors that are bolted to the sender are put on opposite of the original equipment. You can't just push the wires on reversed or it will just "peg out" the gauge. Find the labels stamped into the metal of the sender and make sure large/small terminals match the removed one.

ThomasC2

the most importent value is that the needle should be pegged at 5 bar above 3000 revs.Exact values below 3000 isn't an exact science except that you don't want to "zero" preassure.

Thomas

evoderby

As a rule of thumb 0.7Bar pressure is needed per 1000RPM, anything much above this level costs BHP....

Navaros911

Just to update and close the thread... the oil pressure sensor was replaced and now the pressure reading comes up about 5 to 10 seconds after start.

Case closed

Thanks for the advice guys!

FeralComprehension

And what's your reading when at temp and >3k rpm?

Steve Weiner-Rennsport Systems

FWIW,...oil pressure readings should be instantaneous when the engine is started.

JMHO, but some further detective work is needed for proper resolution.

Navaros911

Pretty much at max reading (5). Under most of my normal driving the needle is pegged to the top.

I rarely drive under 2.7k rpms except in bumper to bumper traffic.

Navaros911

Darn


I shot a video of the startup yesterday to count the time and I must say that I seriously exaggerbated... it takes around 1 second or less for the pressure needle to get up to normal reading (around 2.2 bar on a cold engine).

It is not instant though, there is a small delay. The pressure buildup is fine.

That being said very rarely (like one out of 20 starts) it will take about a little over 2 seconds to have pressure readings. Since everything checks out (normal buildup of pressure) I doubt there is something with the engine.

So maybe I'm lucky enough to receive some more advice from the experts here

KaiB

I'll take a WAG here and say the one second to the gauge reading is fine - it's an analog instrument after all.

In all my years, and all my vehicles I would consider one second "instantaneous".

Navaros911

I think I finally got to the bottom of this: the guage started behaving strange again. With the occasional 5 second dely.

So one day the delay must've been 30 seconds or so... I didn't even notice at first. So I very gently tap the instrument. Voila: problem sorted. Since then, every time it doesn't move, I tap the instrument and it instatnly indicates the correct pressure.

So I think there is something not right with the guage. Probalby a bad connection inside.

Now for the permanent fix I'll problably need to open it up.

Since I've never done that I'll search around a little on the forum and see what I can find.

If anybody has advice from the top of their head: let me know

Now it's time to wrap up here and head home in that beautifully freshly waxed 964.