1991 300D 125K
I drove to Canada this weekend (total 900+ miles).
While driving towards Canada (added 1 bottle diesel fuel additive), I noticed that my oil pressure was not all the way up to level 3.. It was a little bit below .. It would sometimes go up to 3, but then it would slowly come down to 2.7 or so. I was going 80MPH at 3000 rpm.
On the way back, (after filling up the tank, but no additive), I noticed it to be up to 3 all the time..

Could the additive cause that, and .. what is the true meaning of the oil pressure gauge anyway ?

Perhaps it was the change in altitude?

what exactly does this gauge measure ? is it pressure in the oil pan ? or the cylinders ?

As far as what the gauge measures, oil gauges measure pressure downstream of the pump. (I.E. oil ascending unit if I'm correct) One possibility is that the diesel additive caused the engine to run warmer than usual and the increased temperature dropped the oil visocisity. To test this you'd need to run a tank doing you normal driving with and without the additive. Was it still in the "Ok" area? Have you changed your oil lately? We're you in higher altitudes or warmer climates? Thinner air will make the engine run warmer as well as the climb to get there.

Frank.

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'82 300SD

You better look into this quickly! The OP gauge should NEVER drop below 3 bar at anything over 1,500rpm or so! It may be the sending unit or the gauge itself. If the engine locks up suddenly, you'll know it was something else.

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Regards,
Aaron

Ok , that was a scary posting from Aaron.
The problem did not happen on the way back, and since then. What exactly should I have it looked at ? Any ideas Aaron ?

-andreas

I have used a standard screw-in engine compression guage to check oil pressure and rule out a defective guage or sending unit. Just use whatever adapter you need to connect it to the place where the sending unit goes.

Well, if it didn't happen again, I would think that maybe the gauge just got an electrical jitter. Just make sure that oil level is ok. The 1980's gauges were quite jumpy and I don't exactly trust them. Oddly enough, the older ones, I feel were more accurate.

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Regards,
Aaron

Could an overfill of engine oil cause that ?

Perhaps I should post another question about the problems that having extra oil can cause in a diesel engine ...