erratic oil pressure gauge

[krasuskyp]

My oil pressure guage has the DT's - it twitches between 3-4 often. The other day out of nowhere it twitched, then went down to 2, then twitched between 0.75 and 1.5 for awhile, with the light occassionally coming on.

After I cramped my hole from a long pucker, I suspected I have either a ground or sender issue possibly? Oil level and temps remain fine.

I shut it down, parked, came out hour later and it was fine for 2 days. It happened again yesterday, exact same scenario: on way home from work - meaning - after car had been sitting since going out @ lunch. AND, it corrected itself after parking, running into store, and coming out.

Anyone care to shed any light on this for me? Greatly appreciated.

[rsrmike]

I dunno man, that sounds scary. The gauge and the light work from two different sending units. Possible a loose ground in the dash??

I don't know how that would make the light flash though...

Hopefully someone will have a better answer, but I would look into a mechanical oil pressure gauge first-just for test purposes

[e170drvr]

Paul,
I had this issue on my 87. It was a sender issue, I simply cleaned the connection at the sender and it went away. Hope yours is as simple as that. Could be slightly loose as well.
Eric

[rsrmike]

I may have my wires crossed in my head.

Check the connector first, obviously.

As E mentioned, it may be loose, you can usually tighten the spade connector with pliers and push it on and off a few times.

I was thinking in my head when you unplugged it, it pegged upwards, but now I think that's temp gauges that do that?

Sorry man, hope it's just the connector

Good luck

[spuggy]

rsrmike is right, the idiot light sender (back of the motor) is entirely separate from the pressure gauge reading (sender at the front). I think the warning light comes on very low, like at 6-7 PSI or so.

I had the female spade on the pressure sender start to loosen up and not make a good contact, and the symptoms of that were that the oil pressure started to read uncharacteristically high as the RPMs rose and vibration degraded the connection (if you remove the pressure sender connector the gauge pegs high). Cleaned & crimped it tighter, and it went back to reading normal.

If the gauge and the light are both indicating low pressure at the same time, you either have an electrical problem that affects both, both senders going bad, or the oil pressure really is dropping low.

The oil pressure varies according to whether the internal thermostat is open or shut, the viscosity and temperature of the oil and the RPM; it dips lower when the motor is purging hot oil and collecting (hopefully) cooler oil from the tank. The cooler oil then will indicate a slightly higher pressure for the same RPM. Not that I often watch it at stop lights or anything

The oil pressure gauge is just a voltmeter - I find my oil pressure reads ~10 PSI lower when I switch the headlights on...

Give it a constant voltage somewhere between 0-12V (like a 9V PP3 battery for example), and watch it for 10 minutes. If it doesn't kick at all (and it shouldn't), there's nothing wrong with the gauge.

[krasuskyp]

Thanks for the feedback Mike / Eric / Spuggy!

Yes, QUITE spooky stuff - but - if it WAS a low oil pressure situation...

-Why'd it remedy itself and not remain constant?
-Why'd it occur both times under exact circumstances / timing?
-Wouldn't oil temp rise as motor chewed itself ()?
-Would truly low pressure result in such "electrical looking" flickering of gauge?

One thing I thought of is my dipstick resides in bottom of my tank (yeah I'm cool). When it first happened I worried exactly this, it 'may' block the oil feed - BUT - I posted to the forums and read 100000000000000 people have done this, some numerous times, and it's a non issue. So hopefully no correlation there?

Anywho, I'll give some of these ideas a whirl. Where's the sender located? Driver side near turbo I think?

Again, THANKS - this IS spooky sheeeeeeet! Pls. keep the ideas flowing. Hopefully the oil (pressure) will to!

[rsrmike]

The pressure sender is to the right of the fan, about a 1.5-1.75" cylinder w/ single spade connector on top...

Pressure switch is under induction at front of motor near flywheel, red, usually with large rubber boot protecting connector

[krasuskyp]

Tx Mike!

[e170drvr]

Paul,
Are your temps normal? It really sounds like a loose connection to me. If it is fickering rapidly. I dont think the oil pressure would do that. My car had the same exact symptoms that you describe, and I think I would focus on that connection first. I think this is a fairly common problem. I would guess that if the oil pump failed and the sender was good you would have no oil pressure, period. Hopefully others will chime in here
Eric
e

[krasuskyp]

Yes e - a rapid one-bar flicker. Temps were perfect.

I too am still guessing connection / sender, but tell ya what this is spooky Voodoo sheeeee-at!

I hope to get under there this weekend sometime - busy as heck - last weekend huge Audi event I throw, next weekend TR3 to Watkins Glen heh heh...

[e170drvr]

Paul,
I also seem to recall the gauge making kind of a clicking noise when it was doing it's dance. would also indicate a sensor issue.

[beepbeep]

Hmmm...it is not a sender issue if light came on and your indicator went from 4 to 3 bar. You have had some sort of oiling problem.

If sender was malfunctioning then it would either spike to max or go down to 0. Also, there is no way oil pressure light would flicker at the same time if sender was malfunctioning.

This summer, I was driving home from track in my SAAB when my oil pressure gauge started going down from 5 bar to 1-2 bar. At first I thought I had electric problem but elected to turn to nearest gas station. When I stopped and looked under the car there was big oil pool. Oil stick was dry. A nut holding the oil cooler pipe has worked itself out and car was spewing oil while I drove. Fortunately I could borrow some tools and tighten it. Engine holds 4L and when I refilled I got 3.5L into it.

Next time I drove the road I could see two oil marks extending over couple of miles.

So if both pressure gauge and oil light come on, there is something wrong with oiling. That's what those gauges are for.

Regards,

[krasuskyp]

I hear what you're saying Goran, thanks for the thoughts.

But I do not have any sign of oil drips. No smoke. Gauge reads normal since (still with flicker between 3-4bar). Light only came on when gauge flickered down to 1, flickering to 2, and only dimly.

So, the car healed then? That's what I'm saying - hard to be an oiling issue if it were so intermittent. And the very flickering gauge is not indicative of oil pressure activitiy.

I still haven't gotten under there to peek. Damn stuff to do.

[beepbeep]

Hmm...my 930 had oil switch on the back of the engine (prone to crack-induced leaking and hard to replace) and oil pressure sender on the front. There are two different wires going to those two sensors. A voltage drop to pressure transducer might have make gauge go down but oil pressure light is a dummy spring-loaded switch that shortens once pressure is below ~1 bar. There is no way to fool it with electricals...once it shorts the light goes on.

Maybe there was something in oil-pump feed that briefly blocked it and dislodged?

[e170drvr]

Yeah, Beep is right. I somehow missed the part about the idiot light going on also. those are two seperate circuits, and would not fail at the same time. Interesting, that temps are normal and no leakage, and the problem just went away.

[rsrmike]

I think you should have a look at your oil pressure relief valves.

You could check for scatches on the sides and top of the piston, a possible indication that something made one stick open.

Or possible the piston is damaged in some way?

I don't think this is real common on 911/930 w/ their fancy dry-sump system though

If it happens again, seriously consider a mechanical oil pressure gauge, even if it's temporary.

[beepbeep]

Now that Mike said that, I would also bet my money on sticky oil pressure relief valve. VDO transducers used on engine are usually quite reliable, so I would concentrate my efforts on finding the reason for oil pressure drop instead of chasing ghost in senders. Idiot switch is otherwise dead reliable and if it goes off at the same time as pressure gauge goes down, don't blame it on electrical error

[krasuskyp]

Thanks again Mike and Goran.

Mike, where are my oil pressure relief valves to check? Is this pretty straightforward?

Yous guys startin to spook me here 8-(. Crazy stuff.

If like you say "this isn't common" to these - WTF gives? Car's an '87 with ~ 40k and extremely clean.

Hoping for the best here - and if I truly was briefly "idiot light low" on pressure - that my 15min tiptoe home didn't muff nothing.

Weird.

[NathanUK]

The easy valve is next to the drain plug on the case. As you face the drain plug it's to the left.
There is another that doesn't look so easy to get to...

[rsrmike]

OK, sorry, I was way off on my relief valve explanation.

The lower valve is the main pressure valve that set the oil pressure.

The valve in the side is a saftey valve that serves as a backup.

I'm not sure where I got that other crap but I was rapidly typing a response while I was supposed to be loading the cutie in my avatar in the car to go shoe shopping...

In a 993, the main valve opens at 5.3 bar and the safety valve opens at 9 bar. ( I only have 993 factory manuals handy)

Your 87 most likey has similar but slightly lower specs FWIW, it's irrelavant in this case

If I were you this would be my plan of attack:

Get familiar with Oil analysis.

Order at least, a new lower pressure relief valve piston, spring, and updated shim and sleeve.
this will raise your oil pressure to 993 specs and can be helpful to your bottom end. It can make your cam feed oil lines leak as well, but they are tired already if it does.

Get under car and drain oil from tank and engine, collecting per analysis instructions

Jack up car and secure so you can check both pressure relief valves, ( if possible)
I'm quite sure you can check the main one on the bottom easily.

Be careful, they are spring loaded and tend to pop out when fully unscrewed, it's not bad, just be prepared for it

There have been several updates to this circuit as the engines evolved. It may be worthwhile to have the newer relief valve parts handy.

Pull the lower valve first as it's more likely to fail being a lower pressure

The piston should have smooth, polished places partially, or all the way around

It should not have any scratches or gouges.

Look at the top for any signs of something wedging it open at some point.

Try to look in the hole best you can for any debris or scratches (good luck w/ this one)

Reassemble with updated valve, complete oil change and save filter after draining

Send off oil, and cut open filter. Spread the filter media out and look for debris.

Just for kicks, have someone watch the oil pressure gauge w/ key on Engine OFF as you wiggle the wire.

Pull the connector a couple times and make sure it's tight, cross your fingers and hope for the best.

This is the best I can think of for now, I hope you missed my first version of this post all together.

Good luck