Wonky oil gauge or??

[andedav]

When i got the car i immediately put new oil and filter in. When it starts the oil gauge goes immediately to 5. When i switch off it stays at 5. When i turn the ignition back on but not start the car, it drops back to 0. So hopefully i have a wonky gauge or the engine is under some pressure, advice??

[MPDano]

Check the wire going to the Pressure Sensor next to your Oil Filter. Previous owners seem to always put the wires backwards. It wont hurt to switch them around. Clean them well while your in there.

[JK McDonald]

Hey Andedav, You didn’t mention the year of your car, if it sat up for an extended period of time prior to your purchase (windows left down, damp interior, electrical issues, etc...) or how long you have owned it. Is this something new or can we assume that you noticed your oil pressure gauge was stuck at full scale after the initial start up ?

Both of my 928’s have always indicated a full scale oil pressure reading while running and all the gauges return to zero when the key is turned off. Many Mercedes also display’s a maxed out oil pressure gauge at all temps with the engine on. You may just have a slight drag from a little corrosive friction on the needle pivot points within the oil pressure gauge. With the key turned off – try using your palm to lightly bump against the instrument pod to see if the needle will fall back to zero.

A defective sending unit would not hold the gauge at full scale with the key off so you may find it is just a defective gauge. It is not difficult to replace if this turns out to be necessary. On the other hand - just operating the car through a number of run - shut down cycles may free up the gauge.

Let us know what you find there is lots of good advice on this site–
Good Luck Michael

[andedav]

She is a 1978 which i bought sight unseen( bad move). By the look of her sat some time. In need of much TLC. BTW, what should the gauge read running normally??

[MarkRobinson]

5bar at startup, once warm, 2-2.5 bar at idle, but goes to 5 when the engine is rev'd.

Probably a good idea to unhook the sender from the harness (the small vertical cansiter next to the oil filter that has two small wiring connections nut'd on). You can unscrew it with a large adjustable wrench: take it off slowly, ensure you get all the parts & note carefully how they all go back in. Clean it all out thoroughly (it's probably gonna be gummed up a bit with oil sludge), and then replace it.

If the oddness continues, do some research on cleaning all the harness grounding points: under the dash (both sides), engine compartment, by the headlights, etc. Also, cleaning up the 3 multi-pin connectors behind the dash pod helps eliminate troublesome electrical oddities.

Mark

[JK McDonald]

Hey Andedav, Other owners may chime in with some different readings but all of the older 928's that I've worked on had their oil pressure gauges full scale any time the engines were running.

There may be two theories for this operational design -
1) This was intended for the oil gauge on high end German cars of this era.
2) It was a sales ploy to sell more of these cars to women - they all feel more secure with a vehicle when a gauge shows full.....

There is an old story about a wife who destroyed the engine in her husbands Porsche. She told him that she was just trying to get home and only one gauge was reading low. Although there was no oil pressure, it had lots of temp, fuel and RPM. I thought everything would balance out.

One other comment (just from my experience) - automotive gauges are not known for absolute accuracy and are typically considered relative instruments. This simply means that you compare their normal reading on a daily basis and if something suddenly looks wonky - it may be time for some troubleshooting.

Good Luck, Michael

[andedav]

Thanks all, will check it, just had the entire interior ripped out and new/refurbished going in. I'll check this out and send pics. Tires look puny on this thing.
Thanks.

[JK McDonald]

Hey Mark, That’s a nice stable of Porsche thorough-breds you have there.....Of course you’re right about the oil pressure that should be observed with a reliable sending unit, clean wiring contacts and an accurate gauge.

The majority of 928’s that I’ve helped breath new life back into still had the original 30 year old oil pressure monitoring components in place. Along with some wiring and gauge issues, an additional problem is sometimes found when these old mechanical, variable resistance pressure sending units have internally fill with coagulated oil. This can deteriorate it’s ability to accurately respond to or register the oil pressure.

Andedav, just for some peace of mind on your new purchase, you might temporarily plumbed in a short flexible line mechanical gauge to get some real numbers. Make sure you are not laying directly under the sending unit when it's removed - they actually can hold quite a bit of oil - don't ask....

Michael

[MarkRobinson]

Thanks for the compliment!

What I did on my race car: $160 Speedhut Oil pressure gauge: 0-100psi, has warning lights for "too low AND too high" oil pressure (adjustable LED warning light illumination too).

Sender was a snap: removed old sending unit: it's metric threads are just over that of the 1/8" NPT for the new SPeedhut sender, so just cleaned up & JB welded in-place: remove the old sending canister completely. Gives me a good indication on where my '86 100k mile re-sealed engine stands:

Cold engine startup: 100+psi.
1m later, 90-95psi.
As engine warms up, goes down to 25-30psi (warm oil, not hot, but warm cheap oil as I'm breaking in the engine).
Goes right back up to about 100psi when on-throttle.

Mark