Oil press gauge pegs up at max pressure

[DutchmanPhotos]

Hey all,

We recently installed a 3.2 in my 1972 911T...

I installed a new 1 - 5 bar pressure sensor.

My gauge is a 0 - 140 psi from the 1972 car.... You'd think it would read half the pressure on the gauge, but when I start it up and drive off it pegs all the way up to 140 psi... As soon as I let the throttle go and go to idle, it reads about 20 - 25 psi when the engine is warm.

If I install a 3.2 gauge (1 to 5 bar) it also pegs to max pressure.....

All other readings like oil quantity, oil temp, etc, are normal, and the engine does not leak excessivly..... Oil quantity stayed the same for the last 1000 kilomters..

Anyone know what this could be.....? Wiring problem maybe..?

How can I check this...?

Thanks!!

[timmy2]

To check wiring is easy. Remove wire to gauge and turn on ignition, gauge should read max, touch wire to ground, gauge should read zero. (Or reverse to this)
I would suggest using the calibrated '72 sender. Each sender is matched to the gauge it is paired with. The resistance on the gauge is calibrated for a corresponding value in the gauge for the reading. Correct sender = correct reading.

[Roger 911]

I just installed a 3.2 in my '72 as well. Big fun! I installed the appropriate sender for a '72. Works fine. If the needle pegs as soon as you turn on the ignition, that is a wiring fault. If it pegs after start up, that is a most likely compatibility issue.

One thing I did notice is that the oil pressure seems to be consistently higher with the 3.2 than it was with the 2.4. I had/have cam line restrictors in both and used 20W50 in both. The 3.2 shows about 25 psi at idle after warm up. Then goes up to around 60 psi with just about any increase in revs. The 2.4 would hover close to 0 at idle after warm up, then increase more progressively with revs. Both had comparable psi at redline (I think around 100-110).

Good luck.
Roger

[Walt Fricke]

Dutchman

Your 5 bar sender will show 10 bar (~140 psi) if you have 5 bar of pressure (~70 psi ~) and a 10 bar gauge. The gauges are identical inside, they just have different faces/scales. It is the senders which have different resistance relative to pressure.

All such systems will peg if there is a break in the sender wire - it is the way they are designed.

The break can also be inside the sender. Depending on where it is, either it will peg immediately, or it will peg at some pressure when the internal rheostat slider passes the break in the coil winding.

And all will read zero if the sender wire is grounded.

Except for one thing, I'd think your problem is the 5 bar sender. Cold oil is going to have higher pressure, so when you get some RPMs into the engine, you might well be producing 75 psi pressure. Dropping to idle lowers that to something more normal.

If the needle position (forget the markings) is the same with both gauges, this may be all that is going on here. You don't say what the idle indicated pressure is with the 5 bar gauge.

Me, I'd have bought a 10 bar sender, which should avoid all this.

My suspicion about the change from 10 to 5 bar is that owners fretted more than necessary about low hot idle pressure. So Porsche fixed that with a gauge which had the needle higher, and in a more reassuring position. Nothing of note changed in the oiling system from the late 2.7s through the 3.2s (not counting better front oil cooling).

[DutchmanPhotos]

Hey guys,

Thanks for the advice so far....

We had the 2.7 sender on there, but it pegs way up when I turn the ignition on...

The car had a generator failure though, could that have messed up the sender...?

Generator is fixed already.

[Roger 911]

I know when my sender wire was loose, the gauge would peg when it wasn't making contact. I don't recall the technical reason for this, but that was the behavior.

Walt,
Were the oil pumps the same on 2.4s and 3.2s? I thought there was a change somewhere along the line. The low rpm oil pressure is so much better on the 3.2 than the 2.4. I thought part of this might be due to an improved pump (with the rest being a newer aluminum case versus a tired mag case).

Thanks.
Roger