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I know the water temp gauge is just a binary indicator light, but what do we know about the oil pressure gauge? I ask because @Petza914 mentioned his hot idle oil pressure was never below 2 bars, while mine is 1.2 bars. I've tried to dig into this topic to get a bit more understanding.
What I think I know:
- If your oil pressure is truly low, you will get an indicator light.
- The manual says the pressure should be 3.5 bars at 5k rpm, but I dont believe states the idle pressure.
- If your pressure gauge reads anything except 0 with key is in the on position, the sensor is bad. I did just notice my gauge is slightly above zero when the key is on but its not consistent. I saw it at almost .5 bars when in the driveway at an angle, but between 0 and .2 bars when in the garage.
What I don't know:
- Im wondering how precise these gauges are. I saw a video of a guy plugging his 996 oil pressure sender from one car into 3 others. The variation between gauges was .75 bars. His gauge showed .5 bars but when he plugged that same sensor into another car the gauge read 1.25 bars.
- Is there a way to read true oil pressure with a Durametric or PIWIS, like you can with water temp?
- Can residual pressure make the oil pressure gauge fluctuate when the key is in the key is in the on position e.g. if I turn the car off, then put the key back in the on position, is it reading residual pressure? Or does this generally indicate a bad sensor.
I'll have to do these tests, like engine not running and key on. Last night, I drove the car to a chili cook-off at my Indy's about an hour away. With an oil temp of 200 degrees, I was at 5 bar driving down the highway at 80 mph and at 3.5 bar at idle. Maybe my gauge is off by a couple bar all the time.
That was an interesting test in the video. Did anyone else notice that the water temp gauge in the 996 was an a actual gauge that wasn't heavily buffered. Not sure why they changed that in the 997, but it's too bad.
That was an interesting test in the video. Did anyone else notice that the water temp gauge in the 996 was an a actual gauge that wasn't heavily buffered. Not sure why they changed that in the 997, but it's too bad.
Several years ago I think it was Edgy that posed an explanation. I seem to recall he said that there was acceptable variation in the water temperature reading (as per PAG) but a lot of people were bringing their cars in for warranty service because the gauge seemed to read high or low or inconsistently. So PAG, not wanting to pony out warranty claims from dealers for a problem that did not exist, simply programmed the water temperature gauge to sit at x degrees, despite any real variation. I guess the gauge does work once water temperature gets outside of the acceptable variation. Apologies if I mis-remembered any of that thread.
I checked the gauge in both cars this morning - key on and car not running. The gauge in my car reads 1.5 and in my wife's almost at 2. Hers actually swept up to 3.5, then came down to 1.5, then rose back up to the position you see in the photo and stayed there. Mine went straight to 1.5 and stayed there.
If I subtract 1.5 from my observations over the weekend, then my pressure numbers are pretty close to Bruce's.
Are we sure the gauge should read 0 with the key on and the car not running. Seems odd that both my 997s don't - one with 56k miles and the other with 103k miles. I get that with the engine off there can't be any oil pressure, so 0 is likely the right reading, but maybe it displays the last pressure before the engine was shut off or or something. This might be a technician diagnostic where they can see the last oil pressure when deciding to start a car for diagnosis.
Maybe others can post pics of what their gauge show with the key cycled and the engine not running for comparison.
Interesting results Pete. Thanks for checking into that. Can you cross wire the gauges between cars like that 996 video...? And is there any way to get live readings via PIWIS or a Durametric?
I will snap some photos of mine today. Everything I've read stated a non-zeroed gauge with the key in the on position indicates a bad sensor. But that could be an old Rennlist wives' tale.
The only reference in the owners manual: "With the engine warmed up and running at a speed of 5000 rpm, the engine oil pressure should be approx. 3.5 bar or higher."
I've seen nothing in the Workshop Manual related to oil pressure.
My take on this is that it's just an indication. As pressure varies with Rpm it's just an indication. When engine is cold it's always up on full,
When idling a warm engine it's most common around 1,5 from what I read. I actually saw almost 1,0 once when driving in queue on a super hot summer day.
So I just use it as an indication together with oil temp but I use oil temp more as an indicator of a warm engine than the oil pressure. I guess the reading also depends slightly what oil you use.
Interesting results Pete. Thanks for checking into that. Can you cross wire the gauges between cars like that 996 video...? And is there any way to get live readings via PIWIS or a Durametric?
I will snap some photos of mine today. Everything I've read stated a non-zeroed gauge with the key in the on position indicates a bad sensor. But that could be an old Rennlist wives' tale.
Lets find out.
Next time I have the Durametric cable out, maybe I can check values to see how they compare to what the gauge is showing. I'm probably not going to go to the trouble of cross-wiring the gauges in the 2 cars to see what that shows as I have other projects to do with limited time to do them - need to put the upgraded supercharger back into the 928 (it's just not the same down 120 HP), and need to add the paddle shifters to the Cayenne steering wheel.
02-03-2019, 03:22 PM
Oil Pressure Gauge: Accuracy vs Precision
