1977 300D 123-617.912. 212K. Oil pressure is 45 lbs. at start. After engine warms it drops to 15. Rev engine 2500 rpm, oil pressure maintains at 30 lbs.

Do I need a new oil pump or could something be causing this variation?

Need some advice.

Thanks, Jfc697077

I would first verify with a known good gauge that those are the actual pressures -- guages have been know to fail!

If real, make SURE you have a good oil filter (WIX, Knecht, Mann, maybe Bosch), not a cheapo aftermarket. A plugged oil filter, or one where the internal seal has been dislodged, will give you low oil pressure.

When installing, make sure the filter goes straight down on the center post -- if you get it cocked and push, you will shove the internal seal off the seat and have low oil pressure.

It's also possible that the oil pressure relief spring is bad, since you are OK with cold start pressure and idle pressure, although I'd personally prefer more idle pressure hot.

I'm concerned about the low pressure above 1500 rpm, though -- should be 45 at any speed above fast idle. My brother's 75 300D runs 45 psi all the time except immediately after highway runs in hot weather.

If the relief valve and spring are OK and you have good oil (15W 50 Mobil 1 is what I use, or Delvac 1, etc), and you still have low high rpm pressure, it's time for a new oil pump. Shaft driven on the non-turbo models, can be replaced, I think, with the upper pan in place. If it's still low, the main bearings are shot.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Peter:

Thanks for the info. I purchased this MB last January. The engine was shot and I got a used one from the wrecking yard with 212K miles on it. A friend of mine and I pulled the pan off and changed the rod and main bearings & seals. Engine runs good but I'm not satisfied with the oil pressure. An aquaintence of mine has a 77 300D wagon and he said his oil pressure always runs at 45 lbs.

I wanted to make sure what you were saying about changing the oil pump with the engine in the car if it comes to that. I'm hoping I can indeed change out the oil pump without having to pull the motor.

I used a Fram oil filter, use Chevron Delo 400 15-40 desiel oil. I was told I could use 20-50 Castrol regular motor oil since I live in So. Cal.

I will check the pressure relief valve. But where do I find it? I'm new to MB mechanics and new to desiel mechanics and I do have a Haynes manual.

Thanks for your help.

Jim

That's a non-turbo engine, the oil pump is shaft driven of the IP drive and the oil pump is held in place by two bolts. Should be accessable after just removing the lower sheetmetal oil pan IIRC.

Ditch the Fram oil filter, they are junk. Get a Wix, Mann, or Knecht. Wix is usually available at better autoparts chain stores and is a excellent filter. More expensive, but you are getting what you pay for.

As a said before, have a care while inserting the oil filter in the canister, there is a seal half way up (between the full flow section at the bottom and the bypass filter at the top) that can get unseated, allowing oil to bleed past the seal back into the pan.

Unless you missed on the bearing sizes (crank ground to the first undersize, for instance, and you didn't know it) you should be fine there. Did you mic the crank? It could be worn badly enough to cause low oil pressure, but I suspect you would have seen that. MB uses enough babbit on those bearing you really have to work to damage the crank (lock the engine up from no oil, for instance).

Oil clearance on new bearings should be on the small side of 1-3 thousandths. More and you will have low oil pressure.

The relief valve is in the cansiter assembly, along with the various oil channels and the oil cooler thermostat.

Do use a known good mechanical gauge, it may just be a tired dash gauge!

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!