fluctuating oil pressure gauge

I just purchased a 2002 996 C4S with 30,000 miles. I have noticed that the oil pressure gauge fluctuates a little even when I am maintaining a steady speed/rpms...I know the rpm & oil pressure readings are linked so that they correspondingly raise and lower, however, I am not so sure the oil pressure should fluctuate at all when I am maintaining a steady rpm level. Anyone have a similar experience?

It should not fluctuate assuming warm engine at a constant RPM. Search for the oil pressure sender unit here, there are a lot of threads about cleaning it and/or replacing it to solve that problem. It could be other issues as well, but probably a good place to start.

Thanks. Do you have any thoughts on what else it could be if not that?

Baffles, oil pump spring, scavenge pump, blockage, instrument cluster problem, wiring, excessive wear, ummm..... Seriously start with the sender it seems to be most common for that problem. At the very least cleaning the contacts on it.

OK...thanks

mine is a bit jumpy too...it also was on the 944. I really only notice it when I'm holding the car at an elevated idle like when I am circulating coolant to help the engine cool. 944 spec racer lore says that it was a sign of a failing #2 rod bearing...but that was 944s.

I just had the sending unit replace on my 04 C2 for the 2nd time. I had the "failure indicated" warning and gauge indicated zero pressure for a few seconds. Happened 2x in a few miles. I decided that since it is a pretty important warning I would just have it replaced so that the gauge had meaning I could trust. When the dealer checked the car the diagnostics did show it as failed. They keep plenty in stock and as the mechanic said, there is a reason they have them on hand. Looked like it would be a simple DIY for those with the time and basic skills.

Same issue on my 02 c4s with 60k mi. Had the sender replaced last fall and problem corrected. It's back again. At a steady 3000 rpm it can fluctuate between 3.5 and close to 5 within a couple of seconds. Think I'll try a DIY fix this time.

Going to resurrect this thread since it is a couple years old to see if others have experienced and or determined cause of what I am seeing with my car. 2003 C2 51K on the odometer. History – since I purchased I would see pressure of 4.3 – 4.5 on startup and RPMs above 4K and a low of 1.1 at idle. Never liked the low idle number. I noticed that I would randomly get pressure fluctuations when holding RPMs steady in higher gears. Last week I changed the relief spring (read somewhere could be cause of jumpiness), and the sending unit. Now I am happy with pressure at cold start of 5 bar and warm idle after a long spirited drive of 1.4 bars. I am still seeing fluctuations, even more than before. If I hold the RPMs at 3800 I see a pressure of 4 bar then it starts to fluctuate to 3.4 – 4.2. The longer I hold the RPMs the more fluctuation I see. If I let of the gas, then raise RPMs again it steadily rises and will not come back till I hold the RPMs again. It is random and not always there. I do not feel it is mechanical is the behavior is a little different than before the pressure sender change, and not always there. Any thoughts guys or other things to check?

Oil pressure gauge operates from the crappy oil pressure sensor. They constantly go bad and produce irregular readings. Their life expectancy is roughly 10-20k miles. Replace your OP sensor and report back. OP sensor is located on the passenger side of the engine towards the front of the car.

hi, thanks for the response. i changed the sender along with the relief spring this last weekend. Still seeing issue, just slightly more pronounced with new sender.

I had a similar issue last year and brought it into my indy to get fixed. He told me that there is an oil pressure sensor AND oil pressure sender that can go bad. He replaced the sensor for me, and I haven't had any problems since. That might fix your issue.

so wait... i replaced the "sender" following the common DYI. there is a "sensor" too?

these are the items i replaced last weeked. I do not see a "sensor" listed on Pelican.

There's only one oil pressure sender/sensor, the one you replaced. Inside that sender/sensor, there's a pressure sensor inside, which drives the pressure gauge. There's also a pressure switch inside, which trips the oil pressure warning light. That's why the unit has two terminals, in addition to its metal casing which is grounded.

Thanks Ahsai. I'm stumped but don't feel it is mechanical as it is random in nature, and "different" than with the other gauge. Actually more "bouncy" now and all i did was replace the items above. thoughts on next steps?

I think it will help diagnose it further if you can hook up a mechanical oil pressure gauge at the sender port to verify the oil pressure to see whether it's truly fluctuating under the same condition. In that way, you can isolate whether it is a mechanical or electrical problem. As is, it's not clear to me the problem is not mechanical. Would be even better if you can rig up a tee connector so you can connect the sender and the gauge at the same time and compare their readings. http://www.holden.co.uk/displayProdu...?pcode=070.254 Another thing you could check is to wiggle the wires going to the sender lightly and see if the gauge reading fluctuates.

trick would be running that mechanical gauge all the way up front let me brainstorm that. thank you.

Can you reproduce the symptoms with the car being stationary? May not be a good idea to run the gauge in the cabin...unthinkable if it leaks...

leaving from work now. will try to recreate stationary on my way home.

Btw, it's not clear to me whether it's electrical or mechanical because usually when the sender fails, the pressure just drops to zero momentarily and bounces back up to normal. Or it just stays at zero for some time. In your case though, you said the reading fluctuates between some values so that's not as common afaik. I would also suspect more about mechanical if you can't reproduce the symptoms at stationary.

No symptoms on the way home in motion or stationary. Next time it occurs driving I will pull over and test. Then report back. If I remember right up hill under slight load seems to be when I notice it more. Third and fourth gear especially. Not sure if that means anything.

Here is a quick "blurry" video I captured this morning. 3000RPMs, 190 degress, volts 13.2. You Tube

I just replaced my oil pressure sending unit with OEM , the old one pegged up and stayed that way until oil was hot New one worked for 100 miles, now it's acting funny Just saying, it's a weak item

Thanks Zanzibar. My thought is to replace spring and sender back to the old set one at a time to see if I can isolate which one is causing this. I hate throwing money at things. If I go to my indy I can see it now. Drop the engine and change the pump. I'd rather purchase a second sender just to see if it acts differently than the two I have.