My oil pressure light cam on at a traffic light yesterday. Not a good thing. The car is a '95 C2 cab with 82K miles. Let's try to figure out what happened.

I've added a quart of Mobil1 15W50 a couple of days ago as the car was a quart low. Everything was fine. Yesterday when the car was fully warm the pressure gauge goes low at the traffic light and the warning light comes on. This is about 20 minutes into an afternoon drive after getting off the highway.

Symptoms are strange. On a cold start the pressure seems normal, fairly high on the gauge (let's say between 4 and 5 mark). As the car warms up at idle the pressure gauge slowly drops towards 2. That takes 7-8 minutes or so before I shut it down to figure things out. The car is not fully warmed up at this point. If throttle is blipped, the pressure gauge goes up as usual.

So... The issue seems to clearly be related to oil temperature. I have the fan blowing constantly at full speed, so the oil never overheated, it always stays around "8PM" mark in normal driving. It's also clear that pressure is normal when cold (it's currently about 50F in Boston). The car does not seem to make any odd noises.

Any ideas? I'm hoping that it's something related to oil thermostat. I'm also thinking that adding a quart of oil is a red herring, a coincidence, but am I possibly wrong?

All help and thoughts would be appreciated. I'm very concerned.

 

Doing some searching I ran into this: https://rennlist.com/forums/6658531-post13.html

To quote the manual: Page 66

Quote:

"The oil pressure should be at least 3.5 bar at an engine speed of 5000 rpm. When the oil is hot, the red oil pressure warning light may light briefly at ldle without endangering the engine."

The pressure goes up and the light clears immediately as RPMs go up. But... it's not hot in Boston and the car has never done this before.

 

Hard to tell what the real issue is because your oil isn't all the way up to operating temp.
My pressure gauge goes down to 1 on a hot hot day, sitting in a traffic jam and idling for hours, with just stop and go driving. Temps will go to around 10pm but never higher. I'm thinking in your case that the thermostat was about to open or has opened and the oil is now going from the small to the big circulation. 15w50 isn't the super thin oil where the pressure drop would be more then with a 10w60 as an example.
I believe there is a spring and a piston that controls the oil pressure for the engine. Maybe the spring is loosing tension?
If you drive in the upper rev range and the oil pressure gauge still reads low then you know you have a problem but you didn't say it was low once you blip the throttle.
Take your scan tool, if you have one and run a log when your engine is cold all the way up to operating temp. You should be able to select oil pressure and temp in one log for easy comparison.
Good luck and keep us informed.
Ed

 

Ed, thanks.

Yesterday, on the drive home, when the oil light first came on, the car was at full operating temp (which is about 8PM since my fan is always on high). The moment I would start driving away from a traffic light, the pressure would go back up, the light would stay off. Stop and the pressure would drop again at idle and the warning light would come on.

On one had it's semi-normal for a car to do so. On the other, my first 993, nor this one ever exhibited that behavior. And it's pretty cold in Boston right now.

I do not have a scan tool. Is there a way to check things? Again, since I bypassed the relay (?) and run the oil cooler fan on high all the time is thermostat actually even doing anything?

 

Mike,

As an aside, you may want to rethink running the fan all the time, especially in your climate. It is beneficial to get the oil to 212F periodically to vaporize any moisture that has accumulated in the engine. If you don't do this, you may see what looks like mayonnaise on the inside of the oil filler cap.

 

Bobby, thanks. I have looked at the oil filler cap and it's very clean. I've been thinking about pros and cons of running high fan speed and for now decided to do so on this car. I might change that later on or make a manual switch.

 

You somehow need to verify what the actual pressure is. IIRC the rule of thumb is 10 psi per 1000 rpm (until the bypass opens). If a shop can install a temporary pressure gauge this would be the way to do it. Perhaps they can use the pressure sending unit port although I have absolutely no idea where it is on a 993.

 

The thermostat is still working to decide when the oil goes thru the cooler. Mine relay is bypassed as well, and my oil will get to temp and then fall once the thermostat opens. Even with the fan on high speed at all times, you'll get to operating temp, assuming you drive it long enough (a few miles).

What is the reading on your oil dipstick? That is a more reliable measurement than the gauge...

 

Dipstick reads about full right now.

But we are talking about pressure, not amount of oil. If it's just sitting there without being pumped...

 

Does the pressure gauge go up all the way up when you're above 4k rpm?
I don't know if there is a way to ohm out the pressure sender unit and compare it to a new one? That's what the ECU sees anyways. Or, mount a external gauge unit in a Y connection and compare the values. It's a bit of work but it would give you the answer if the sending unit is bad.
Is the idle speed where it should be? Maybe the idle is very low? Do you run the AC but the idle doesn't increase for the compressor? Other devices that pull lot's of amps?
I'm just fishing for stuff.

 

Anything like this?
I had an issue a few years back, turned out to be a faulty OP sensor.

 

Since oil pressure is mechanical I would suspects this is an o/p sender problem or bad connection at the sender.

 

Phil, it looks pretty much identical to your video. The question is, did yours go up at cold startup at idle and dropped as the car warmed up?

Ed, A/C is off now. When it turned on it works fine. Nothing pulls a lot of current and this problem just started yesterday out of the blue.

Chris, is there a quick and easy way to check OP sensor? Where is it located? It seems to be a $40 part. I'd replace it just to see if it's sensor's fault.

PS. Thanks, guys! Hope to get it fixed with your help.

 

You might as well replace it because getting to it is no 5 minute job.

I would have an oil pressure check done and after replacing the oil pressure sender, if pressure isn't what it is supposed to be, I might check the pressure relief valves.

# 40 - on top of your engine at about the same location as the oil filter...

 

FWITW, replaced OP sensors on two different generation 928's for friends this year .... similar symptoms, so rather than overdiagnose the issue, blew the $40 and installed the part .... problem(s) solved.