300SE low oil pressure gauge

Today I took my 300SE on a Mercedes Club Pop up drive.   The Pop up drives are short notice mid-week drives.  I normally take the other cars to club events, but the 300SE had not been on a decent run for a while and the weather was terrible.   The drive was down through the Royal National park to Mt Kembla, Mt Kiera and then back.

The other cars on the drive were rather varied.   There was a Ferrari 458 Spider, an Audi SUV, a race prepped Toyota Yaris and a couple of other Mercedes including a C63, C250 and E400 Cabriolet.    It was quite amusing to be in a convoy in my 300SE following a C63 and the Ferrari.   I think the power difference was probably about 500hp.

It is also my first drive in the 300SE now on historic registration.   I now no longer have any cars on full registration, and will rotate the 60 days I have available on each car.   The roads were quite good even in the wet.   The view from Mt Kiera was also quite nice over North Wollongong.   The W126 does quite well for a softly sprung limo.

After leaving the meeting point I observed some strange behavior on my 300SE’s oil pressure gauge.    The normal behavior is the car idles above 1 bar when hot and pegs when accelerating.     All of a sudden, the reading was negligible at idle and rose very slowly on acceleration.    At this point I was not sure if I really had low oil pressure.

My first step was to stop and check the oil level in the car.   The level was on the low side, just above the low mark.   I added some extra oil and it made a very slight difference.   Even with the much lower gauge reading, the pressure was just inside the spec in the owners manual.   Based on that, I kept going on the drive.  The car was running really well and I was starting to think it might be a gauge problem rather than an engine problem.     Just in case I was easy on the engine and kept the revs below 3500.

When I got back I did an oil and filter change.   The car was due for one and this would eliminate contaminated oil. It’s been a year and about 5,000km.  The dipstick looked fine, but there was an outside chance the oil was contaminated with petrol or coolant.   As a tip, trying to do an oil change quickly just results in a lot of spilt oil.

After the oil change there was very little change to the oil pressure gauge reading.   Next was to run the engine without the oil cap.   I wanted to see that oil was getting to the top end of the engine.   I could see a steady stream over the cam lobe.   This made me think that I have good oil pressure.     If this is correct, then the issues is either in the sender unit, or the gauge.    On some further research, the sender units fail with the same symptoms I have.

The other possibility is that the oil pressure relief valve is stuck open or partially open and I really have low oil pressure.     The evidence seems to point to the sender unit.     I’ll be ordering one and replacing it as my next step.   The best troubleshooting step would have been to attach a mechanical oil pressure gauge but I don’t have one.

There is never a dull moment with old cars.