Oil pressure gauge gone wonky...

[deniss]

After a recent oil change, my oil pressure gauge is being weird.

When I start the car in the morning, the gauge shows full pressure at idle. After I drive around for a while and the engine gets to its operating temperature, oil pressure at idle drops to zero - I can see it dropping as I coast to a stop. When I step on the gas, oil pressure builds back up. Above 1,200-1,500 RPM, the gauge shows full pressure with the engine at operating temperature.

I checked the obvious things -- oil isn't gushing from anywhere, and the dipstick still reads full.

I took other people's advice in related threads and replaced 2 o-rings on the tube that goes in the middle of the oil filter housing. After the car sat for 2 hours, I started it, and the oil gauge showed 2 bar at idle. I thought - GREAT! But then, after some driving, it went back to showing zero at idle.

I'm not noticing any unusual sounds from the engine since the oil change, but why this oil pressure gauge behavior began a day after the oil change is puzzling. I used the FRAM 2930 (made in Turkey) oil filter, as I have for years... A defective filter?? Coincidentally, the gauge started showing this behavior the same day I finished transmission fluid change, but I don't believe I caught the sending unit with anything.

Also, what's the torque for the 2 nuts on top of the oil filter housing? Could it affect this issue if I got them too snug? I torqued them to 25 ft-lb.

Anyone have a functional oil pressure sending unit laying around? Please send me a PM if you do

[cooljjay]

Doesn't this car have the electrical oil pressure gauge?

Did you use diesel rated 15w-40 motor oil?

I am personally not a fan of frame oil filters, and they had a lot of issues with them back in the day....I would suggest changing to Mann oil filters....I would say having items made in turkey....is just as bad as things made in China....

[Skippy]

The SD has and electrical gauge. More likely to go wonky with age than the mechanical kind. Probably a bad gauge or sending unit, but I would find out for sure before I used the car again.

[deniss]

I just went to look at the top end during idle.

With the oil cap removed and shining a flashlight inside, I can see thin, straight jets of oil being delivered onto the cam shaft from a round tube-like manifold near the top of the valve cover.

I looked a little more closely at the oil pressure sending unit, and it's sweating oil at the calibration screw, which is located next to the electrical connector. I overlooked that before, but the flashlight made it clear that it was glistening. Not dripping, but definitely sweating.

[Diesel911]

Sounds like you need to change the Sending Unit.

Also inside of that Tube you put the O-rings on Oil Filter Cap is a Spring loaded Valve. If something holds that Valve open the Oil Pressure will also drop at low rpms.

I don't know how to take it apart to get at that Valve. But, I was able to pull the Tube that is press fitted into the Cap out and used compressed Air to blow down that Tube and past the Valve.

[300D85]

Originally Posted by Skippy

The SD has and electrical gauge. More likely to go wonky with age than the mechanical kind. Probably a bad gauge or sending unit, but I would find out for sure before I used the car again.

+1

[deniss]

I don't have the option to park this thing -- it's my daily driver. Now, since oil pressure is up to 3 bar around 1,400 RPM, I would assume we're not at a critical stage here.

I'm going to pay a visit to a local MB dealer and get a new sending unit.

Does the oil need to be drained from the engine to replace the sending unit?

[ROLLGUY]

Originally Posted by deniss

Does the oil need to be drained from the engine to replace the sending unit?

No. You may get a teaspoon or so of oil dripping out, but not much more.

[Diesel911]

Originally Posted by deniss

I don't have the option to park this thing -- it's my daily driver. Now, since oil pressure is up to 3 bar around 1,400 RPM, I would assume we're not at a critical stage here.

I'm going to pay a visit to a local MB dealer and get a new sending unit.

Does the oil need to be drained from the engine to replace the sending unit?

You have already established that the Sending Unit is not working right. That means none of the reading including the 3 bar at 1,400 rpm is correct.
However, you have been driving with it like that. If you actually had zero Oil pressure at idle something would have already been ruined.

[deniss]

I decided to order the sending unit made by Meyle - it was $35 and hopefully will work ok. I think Meyle is a pretty good brand.

Also, I decided to attempt to remove the oil filter housing and replace the housing-crankcase gasket and the housing-cover gasket. The oily "mud" around the bottom of the filter housing is getting out of control.

Will I need to drain engine oil in order to remove oil filter housing or not?

[Skippy]

Originally Posted by deniss

Will I need to drain engine oil in order to remove oil filter housing or not?

Short answer: No.

Longer answer: Pull the lid from the filter housing and let the oil from the housing drain into the pan for a few minutes before removing the filter and then the housing. You will get a little drippage from what oil remains on the inside of the housing. At this point you could re-use the old filter, but there's no good reason for not putting a new one in. Of course, this would also be an excellent time to change the oil as well, but not absolutely necessary.

[deniss]

Thanks for the information.

For better or worse, I just did an oil/filter change 4 days ago, but I may do that again shortly after that job is completed (given I succeed!).

[deniss]

I replaced my original oil pressure sending unit, which turned out to be leaky, with the new Meyle unit, and the problem seems to be solved. At operating temp, the engine idles at around 2 bar, and as soon as some throttle is applied, it tops out at 3 bar.

It was too windy today to work on the oil filter housing seals. Instead, I thoroughly cleaned oil filter housing, especially near the seals and around the oil cooler line connections. This area accumulated so much dirt and grime, it was almost like unearthing a fossil. With the area cleaned, I will keep an eye on it to see where the seepage is coming from before I embark on replacing filter housing-crankcase seal.

Also, I purchased the seals from Mercedes. The housing-crankcase seal has that waffle pattern that's shown in another thread on this subject - it is NOT like any other aftermarket seal out there, so get this one from the dealer. The housing seal for the oil pressure cover that I got from the dealer is the Reinz seal, so you can get that elsewhere for cheaper.

P.S. When you remove the oil pressure sending unit from oil filter housing, more than a teaspoon of oil comes out... I'd say more like 2-3 tablespoons, so have a few shop rags or paper towels ready to sop that up.