Low Oil Pressure M103
 

Hello folks, this is the first time I've posted anything here and I'm looking forward to your help.

Here's my situation: I've owned Mercedes for over 40 years, and 3 years ago I purchased a 1988 300CE M103. The car has been fantastic, up until this last winter. We had a long winter with lots of snow, cold weather, and the car sat for about 3 months. When I tried to start it, it would NOT start. It would turn over but not fire. I thought it might have been bad gas.

I replaced the spark plugs, distributor cap, and rotor. The car immediately started but idles very rough. For the first time ever the oil pressure did not go to a full 3. It was close but not like it was before. I drove it out on the highway and when I came back in town, the rough idling improved. However the oil pressure still has not come back to where it used to be.

The car has 175,000 miles on it but I feel has been well-maintained. My question is, what would cause the oil pressure to suddenly not be as strong as it was prior to this winter? Any help or insight would be greatly appreciated. I use a full synthetic oil 5w40.

(The car starts right up but does hesitate and runs a little rough when taking off. Once on the roads it runs strong with plenty of power but something is not quite right.It is as if the car needs to burp to get back to normal. The oil pressure used to bang straight to 3 but now slowly rises to just below 3.)

Thanks.

Welcome to the forum.

My guess is the rough running is unrelated to the oil pressure reading. There probably isn't an oil pressure problem just a reading error. Replace the oil pressure sending unit located on the back of the oil filter housing. Take off and replace the oil filter to access the sender.

Mine did something similar at 95K, it now has over 265K.

Good luck and please complete/close the post!!!

x2 on the oil pressure sender. Had this same issue on my SDL. Was reading 3 and ~2 at idle, then suddenly wouldn't go above 2 anymore and would "flick" the needle at idle.

Replacing the sender solved the problem with little effort.

Ken, check the condition of the ignition wires. It may be time to replace them ... recommend Beru wires. Also, it's possible a new coil would improve the rough idle. Hopefully, you used the spark plugs specified in the Owner's Manual, and gapped them correctly.

Thanks all for the great input. I will put in a new coil and a new sending unit. I have new plugs on order (the CORRECT ones); I will gap them correctly and we'll see if this corrects the problem.

I appreciate all the help very much and I'm sure I will be asking for help in the future.

And you're right ... a Cadillac doesn't stretch. But I've seen 'em burn to the ground!

If you ordered the correct Bosch plugs, they'll be pre-gapped from the factory. Make sure to use some good copper-based anti-seize compound on them, aluminum heads are unforgiving of dry plugs.

Check the cap and rotor. M103 are pretty hard on those parts. Basically, I'd start with a tuneup and progress from there.

Oil pressure sender is a common failure, and your symptoms match it exactly. Easy fix, and pretty cheap too.

Ken, reading your original post I see that you are using synthetic 5W40 oil. Original fill for that vintage MB vehicle was 15W40. Not sure how cold it gets where you live, but I run Mobil 1 15W50 in our cars of that vintage. The newer engines have tighter tolerances and use 0W40 oil, but I think a higher viscosity oil is a more sensible choice for the 124 cars ... especially engines with some wear. That vintage engine with lower viscosity oil will be more likely to burn oil, in my opinion.

175,000 miles? 5W-40 synthetic? History of difficulty starting that may have diluted lubricating oil with gasoline? Change the oil to 15W-40. Synthetic or non is your choice, but I'd go with non.

I've run 10W-40 in my M103 for the 20 yrs. I've owned it. Apparently working just find.

My MB has been inactive until just recently. About a month ago I filled the gas tank up to the top for the first time in awhile. The fuel gauge went up about 3/4 of the way. A short while later it kicked itself up to full.

Maybe you sender needs replacing. Maybe the elect. connections simply need cleaning. Inactivity breeds resistance in elect. circuits.