300E Oil pressure reading at zero
 

I am concerned about the oil pressure reading on my 1993 300E 2.8, 146K miles. At startup, when cold, the pressure only reaches the 1.5 range, slowly falling to zero as the engine temperature reaches the 80 degree celsius mark it normally operates at. I have changed the oil and put in a Bosch filter, (which was also the brand of the old filter), but the problem persists. I do not hear valves or lifters rattling and I can see oil when I remove the oil filler cap, so I believe there is a decent flow of oil.

Is this symptomatic of a faulty oil sending unit? If so, how the heck do I replace it? I can see it with a flashlight just below the oil filter, but I don't see a way to get to it as it so close to the firewall area, nor do I see how it would be removed if i could get in there. My searches have revealed similar problems, but no one has described how to actually replce the sender, if that is the problem at all.

I'd appreciate any suggestions on what steps I might take to get to the source of this problem.

Hook up mechanical guage to confirm, or disprove reading. I wouldn't run it until this was done.

It's very likely that it's a bad sending unit. I would just throw in a new one and see what happens. I have never done a 104 sending unit but I would think access is from below - it certainly is on a V8.

Thanks for the input so far... I never considered attacking it from below, I will try that this weekend. As for testing with a mechanical gauge, I don't own one nor do I know where to hook it up. I'm guessing it would be where the sending unit is now, so I'd still have the same problem on how to remove what's there now, if my guess is correct. I'm learning here, so if you know I'm wrong, please steer me straight!

Could be bad sending unit, bad gauge, or bad ground to the gauge. But it is best to verify with a gauge of some sort. I hate to see you get a new sender only to realize it is a bad gauge or bad ground in the cluster. Mine dropped down once with a bad oil filter, but you said you changed that out already. Another time I knew I had fine pressure and it dropped. I tapped on top of the dash and it went right back to 3. I know I have a bad ground somewhere in my cluster cause my gas gauge goes to Full when it is not and I hit the dash and she goes back to whatever amount is actually in the tank, for a few minutes at least. Bounces all the time.

I just started having a similar problem with my '90 300e after changing the oil, but it sticks steady at 3 until it warms up and drops to near 0 when idling in gear and it never did that before. When I give it gas it jumps back up to 3. I plan to change the oil & filter again and, if it's not better, mechanically check the pressure. I'd appreciate any other thoughts. Thanks.

jmbig, instead of changing the oil and filter again replace the oil pressure sending unit.

That is normal and in spec according to MB anything over .7 Bar at idle is normal as long as it pegs 3 when you hit 2000 RPM. Did you use a lighter oil? Is it hot as hell there? When the temp is real hot and the engine equally hot, say near the 100 C mark, the oil thins considerably. You will get lower pressure when this happens but still be within spec. But as long as it is at 3 when you are above 2000 RPM then you have enough pressure and flow.