m103 oil pressure troubles
 

Car: 1990 300SE

Ever since we bought the car a few years ago, the oil pressure would sit at around 1-1.5 bars at idle and go straight to 3 when driving. But lately, when cruising in the city or accelerating, the maximum would be at 2.5 and the idle would go as 0.5 and sometimes slightly lower. We thought it would be the oil pressure sensor, but lately, it sometimes goes slightly below 0.5 at idle and the engine would make a slight noise. It rarely happens, but when it does, the only thing I can do is raise the rpms by about 100 and the oil pressure would go to 1 (and the noise would go away). It is not low on oil because that is always being checked every week. Although, we still do need to check the actual oil pressure with a gauge.

My dad thinks it might be the oil pump, and/or the rod bearings and main bearings. Any advice?

If we had to go through all that work, how much would the cost difference be estimated if we put another engine in, such as the M104?

The parts would cost about $460 if we were to replace the oil pump and main and rod bearings.

If you change engine types, you'll need to change electronics and wire harness also, ... exhaust is different (more than your estimate just for exhaust), better off scrapping the car and starting over than changing to a 104.

How many miles?

The bottom end on an M103 is BEEFY. Unless it has very high miles or has been run out of oil, very little is likely to be wrong down there.....


There might be more than one thing wrong leading you to a false conclusion. For example, you state that you don't think that it's the pressure sender because the engine has started making a noise at low idle. Noise at low idle could be something as simple as motor mounts. My 300SE sounded like hell at low idle until I replaced them. You could have a bad pressure sender and bad motor mounts.

Or, the oil pump could be bad......

You really need to do an oil pressure test to see if you are in fact suffering from low oil pressure.

PM me your email address and I will send you the MB test procedure for this.


-tp

The car has 170k miles right now. When we bought the car, the previous owner had some sporty low profile tires on the car, which made the car run at about 3800 rpms at 60. It could have possibly been run like that for awhile, but we have no clue since we bought it from a dealership.

Well, the motor mounts have been replaced in the last one to two years, but I'll check on them again. The noise sort of sounds like bad tappets (like those on our '90 420SEL), but not as loud...more of just a light ticking. It goes away when as soon as the car accelerates and the oil pressure goes up.

I guess the only thing we can do right now is do an oil pressure test.

Yeah, those tires were certainly no good for MPG. You were running about 800 RPM higher at 60 than you should be. This car should do almost exactly 3000 RPM at 60MPH. So, one bright spot is that you actually have less miles on it that what the odo says, providing the typical odo gear failure has either not happened yet or has been fixed.

As far as clicking/clunking sounds, go take a good look at your exhaust pipe for cylinders 1-3. This pipe likes to bottom out on either the idler arm or the cross member. This can cause intermittent tapping/clunking/chattering sounds.

You don't want to damage your motor, so you really do want to do a oil pressure test before you do much of anything else. If you are indeed getting low oil pressure, you will obviously want to fix that before addressing other issues.

What weight oil are you running?


-tp

maybe try changing the oil to a higher viscosity or simply change the oil. My 1990 300 was registering no oil pressure after warming up yet still ran, the dick that I bought it from had put synthetic in. I changed it to Castrol 20W-50 and the pressure is right on the money. I feel lucky that the damn thing doesn't leak through seals. I swear that these people that try to 'upgrade' routine maintenance via new products in fact wreck these things. Don't try to do this. Just put it back to what the factory said to do in the first place and hope for the best.

Cliff, I think you should do a little reading on oil, synthetic oil will not wreck anything, and is approved for use by Mercedes-Benz. Are you going to post this several times per day? Just trolling?

Either 10-40 or 10-30. Can't figure out which it was replaced last with...

Well, that's plenty thick to not be the issue.....

-tp

Cliffmac - If you had to put 20W-50 in there to get your pressure up, then something else is amiss. 20W-50 is thicker than what the motor requires, barring desert operation. Also, synthetic or not would make zero difference in your oil pressure. Viscosity would. You might need a new pressure sender.

-tp

Don't worry about high revs with that engine, they are made for it.

The only thing to worry about when revving a M103 high is how quickly the fuel tank is emptying! :eek:

Ain't that the truth. Doing a steady 75mph I'm lucky to get 16 or 17 mpg.