marlinspike

As a reference, the normal oil pressures displaed on the dash are ~2 bar at warm idle and 3 bar (the max) when driving.

Earlier this week, I was driving along and suddenly the oil pressure dropped to about 0.4 bar. I pulled over, looked at the oil and it looked fine. I checked the connections for the gauge and they were fine. Started it back up - proper oil pressure.

Drove about a mile before the oil pressure dropped to 0.4 bar again. Pulled over, called for a tow.

Took it to a Mercedes shop, which said it tested to have fine pressure and was just a bad wire from the sender.

Took it home yesterda. Drove prob 25 miles and all was fine. Drive it today about 2 miles down the road the pressure dropped to 2 bar, so I shut it off and coasted to a stop. After a few minutes, I started it again. It drove with proper oil pressure for about 0.5 mile, at which point it went down to 2 bar, and ~1.5 bar when I slid it in neutral before shutting it off. I pulled over again, waited a few minutes, and it got me the rest of the 1.5 miles home without a problem.

Any ideas? For what it's worth, the oil pressure sender failed about a year and a half ago, so the current one is fairly new. I remember when the last one failed, it was pegged at 3 any time there was electricity going to it. Also, I remember that if you took the cable off a working sender it would peg it to 3.

Oh, and FWIW, oil pump on this car is chain driven.

yanko 15

i would check the sender

any possibility of the gauge needing calibration?

marlinspike

There's no calibrating these gauges that I know of. They either work or don't work.

Is there any way I can actually test the sender other than just swapping it?

I'm going to change my clothes and pull out the oil filter to see if it has maybe collapsed.

I'll note that I don't hear any strange noises or anything like that.

marlinspike

Well i took out the old oil filter, it pretty much looked fine except for a couple tiny pieces of glue had come up and been trapped by the filter, as had a strand of filter fiber - i doubt that was enough to do anything, but i changed the filter out anyway and drove 11 miles without incident

Then again, I drove a bunch yesterday without issue too.

Fabrik8

This is a tough one to troubleshoot unless you can test the sensor while the oil pressure is reading low (without damaging the engine). Basically you can't really test the sensor when the sensor isn't showing a low pressure reading. If it's working normally, you'll get a normal reading when you test the sensor obviously. If it's showing low pressure, you can either test the sensor or use another pressure gauge/sensor to verify the actual pressure against what you're seeing. Luckily it's an analog pressure sensor so it should be pretty easy to test.

The fact that the pressure problem is not consistent would point to something that's probably not a catastrophic mechanical failure (as opposed to a false reading from the sensor). If it was a spun bearing, collapsed filter, or a pump impeller failure, you wouldn't build proper pressure at all anymore. So unless you have something like a pickup tube seal failure (or a dislodged tube, etc) or the impeller is cavitating, any pressure problems should be fairly consistent (high or low...) over some range of load/RPM/etc...

marlinspike

Well, the shop I took it to that said it was a frayed wire did say they drove the car around with their gauge on it first (I guess so they knew they wouldn't damage it as they ran it), and I did notice it had ~40 miles more on it than when I dropped it off, so they must've driven it a ton looking for the pressure to drop.

But let's say it shows low pressure. Is there anything I can do to test the sender that doesn't involve having my own gauge?

I'm not really worried about wasting $60 on a sender when I should have bought an instrument cluster gauge. What I am worried about is replacing the sender when I'm actually getting low pressure. Is there anything that could actually cause such intermittent pressure issues?

Fabrik8

If it is showing low pressure, you can check the voltage reading with a multimeter, which will show if the sensor is sending the proper voltage or not. If it is (sending the proper voltage) the problem is likely in the gauge or the wiring. If it isn't, the sensor is bad or you actually do have a pressure problem. If the sensor is consistently showing a low reading, you can rig something up with an air compressor and a regulator so you can mimic the same pressure with air instead of oil.

Just like everything else, it's really hard to troubleshoot an intermittent failure. It's fine to get another $60 sensor, but if the problem is actually in the gauge then you haven't solved anything.



If it is having fairly consistent problems (like every time you drive it) then I would say put an aftermarket gauge on it (along with the stock setup) to verify what you're actually seeing. Then at least you can see whether the problem is electrical or mechanical. The mechanical problem is really the important part, because that's what actually causes engine damage.

marlinspike

I doubt it tells you anything, but if you're interested, this is the oil circuit on this car (except I don't have the oil cooler).

marlinspike

Oh, and it's hard to see, but here's the pump. Kinda a big pickup. I'm not really seeing what seal could be an issue.

Fabrik8

I was talking in general terms. A lot of engines have seals or O-rings to seal the pickup tube to something else like the pump or a galley in the block. If yours doesn't, that's OK.

I can count the number of Mercedes V8s that I've working on using one hand.. with the fingers chopped off (that means zero...).