Erratic oil pressure readings

[pjo046]

My engine is freshly rebuilt. I changed all bearings, gaskets, piston rings on the engine, essensially every part that wears was changed. I also changed the oil pressure sender. The engine wiring harness has also been refurbished with new connectors, and new wires where needed.

My oil pressure readings are not as they should be. When the engine is cold, the needle is pegged at 5 bar. When the engine has reached operating temp the needle shows 1 bar at 750-800 rpm (idle), and it rises to 3 bar at 1800rpm. If I increase the RPM above 1800rpm, the needle jumps to 5 bar immediately. When cruising around 1600-2000 rpm the needle often jumps up and down very erratic between 1 bar and 5 bar.

I have used a multimeter between the wire contacts at the oil pressure sender and the connector in the engine bay right side (Which connects the engine harness to the fuse box and instrument panels). The resistance is very low indicating a good connection without shorts or open circuits.

The oil pressure readings were totally normal before the engine rebuild, and I haven't touched the wiring going from the engine bay connector to the fuse panel/instrument panels. Therefore I conclude that most likely the gauge (or wiring to the gauge) is not the culprit.

So, can there be any other explanation for this behaviour other than a "dead on arrival" oil pressure sender? The brand is FAE by the way.

I am thinking it must be the oil pressure sender, but would like to eliminate other possibilities before I go through the hassle of changing the sender as it is a very time consuming job. Anyone has any ways of determining if the sender is the problem? And what other things could cause this behaviour if the sender is not at fault?

[CHICKS]

How many miles were on the car before rebuild? Did you plastiguage or just use STD bearings?

Oil weight stay the same?

Do you have the old oil pressure sender to test?

It's more than likely the sender because it's a cheap FAE. I've heard a few people say they were junk out of the box. Probably best to go with OEM on this: Pelican Parts - European Automotive Parts and Accessories - Porsche • BMW • Mercedes • Volkswagen • Audi • Saab • Volvo • MINI

[944 Ecology]

A couple of questions for you:

1) One piece or three pieco OPRV?

2) Did you use the tool for aligning the oil cooler housing?

3) Did you save your old oil pressure sender?

[John_AZ]

If you have the 1 piece OPRV, did you change the "O" rings inside?

Check the wires to the oil pressure sender. They lay on the top of the water pump and block. The protective covering may be cracked and causing a short.

J_AZ

[pjo046]

All my wiring in the engine bay has new protective covering and new connectors, and some of it has brand new wires also. And I have checked for continuity and verified low resistance indicating good solid connections.

I have the old style OPRV. I did not have a proper centering tool at hand, however I used various other home made tools and tried and tested very many times untill I was certain that the housing was spot on and there was no binding whatsoever when checking.

Where is there an o-ring inside the 3piece/old style OPRV?

200 000 km (124 000 miles) on the car before rebuild. I used std size bearings, but I had a machine shop check the journal diameters on the crankshaft when they polished the journals. They were within spec. Furthermore, as I don't get too low readings on idle and at 1800rpm, I doubt there is too much clearance.

[pjo046]

oh, and I still have the old sending unit laying around. The reason I changed it was because I installed brand new parts all over, figuring the old stuff would fail sooner or later anyways.

The old sender was missing one of the connectors, and had the wire soldered directo to the connector post. That was another reason I wanted to change it.

[John_AZ]

Sorry, meant the 1 piece OPRV. Corrected post.

J_AZ

[pjo046]

Ah, that's good. I suddenly started to worry, as I couldn't remember using any o-ring on my OPRV. :-)

One quick thought I had regarding the issue beeing caused by a stuck OPRV:
It would seem plausible that the pressure would get too high at high rpms, because the valve didn't open as it should. But; Shouldn't the pressure rise be more gradual/linear? Now it is like once I reach the 1900-2000RPM threshold, the needle on the gauge bounces from where it is (Around 3 bar) and straight to top (5 bar). A valve suddenly closing or opening could cause such readings sudden differences in reading, but a valve stuck closed all the time shouldn't.

Or is my reasoning as to how the OPRV works wrong? I figure if stuck open, one would have dramatically low pressure readings at low RPM's. If stuck closed, it would be very high readings from about halfway up on the RPM register, but still a gradual increase as RPM's are increase. If stuck somewhere midway one would have lower than normal pressures at low RPM's and higher than normal readings at high RPM's.

And none of the scenarios fit how my gauge needle moves.

I know the signal given by the sender is measured as resistance. The higher resistance (ohms) measured, the higher the pressure reading on the gauge will be. As the needle acts as it does, it seems as once a certain resistance level is reached, the resistance from there on increases dramatically fast. Could a bad connection/short circuit/open circuit work like that? Or could a faulty sending unit behave like that?

[kdjones2000]

The FAE sender is a POS. Mine behaved in the same way.

Put your old VDO one back in and see if things are fixed.

[djnolan]

Check your crank pulley, if it is not fully seated the oil pressure won't build. I will look for a picture of mine.

[DannoXYZ]

I agree with the others, put the old sending unit back in.

Also confirm readings by replacing sending unit with mechanical pressure gauge. Get actual pressure readings at various temps X RPMs.