1989 190D 2.5 - low pressure, high temp - any advice?

[tuxbeej]

Greetings, eh?

Up until mid-January, my 190D 2.5 always ran at 80C when warm and 1.8-1.9 bar idle when warm. I use Mobil 1 5W-30 and the Mercedes OEM filter for every oil change. I recently changed my oil after a 5-hour road trip for Xmas 2009 (I've done it several times before, so I know to replace the filter gaskets/o-rings and the crush washer). I was getting the same readings until now - always rock-solid at 80C, WOT or warmed-up idle.

I let my car warm up to 80C, then started driving at 50 km/h. Once I accelerated to 70, the temp started increasing past 80C but not to the tick between 80C and 120C (90C? 100C?).

I pulled over and saw my oil pressure drop to 1.5 bar.

While the engine was running, I checked all of my fluids:
- oil seemed correct (except you can't tell with oil when the engine is running). I calmly panicked and added about 100-150 mL of oil. Stupid, I know.
- I added more transmission fluid since it was a little low.
- Coolant appeared fine, but I wasn't looking closely (it was a little low, and I fixed that yesterday).

I turned off the engine, waited five minutes, then rechecked the oil. It was now coating the red Max mark, but I don't believe that's a big issue. Wish I hadn't added oil anyway...

I turned the car back on and it sat at 80C and 1.8 bar again. Perfect.

I drove it from Olds to Calgary @ 120 km/h (1 hour) and visited my friends. When I drove it back to Olds, the readings were "bad" again. I assumed it was probably a bad sensor or thermostat and I could replace it later.

I drove it from Olds to Calgary yesterday and discovered that the readings were consistently 'bad':
- temp rises to 80C on idle, but when throttle applied, it slowly climbs, but never reaches the next temp tick.
- pressure stays at 3 bar on throttle, but on idle, it will hang around 1.5 bar, and has dropped to 1.2. The car sounds just as it always has, though - no rough running or knocking.

I noticed that the expansion tank was not at the proper fill level, so I added more coolant. This did not affect temp.

For all of these settings, they 'waver' - the needle slowly bounces like it's riding a lazy wave.

Here's what I'm going to check and/or replace.
- oil pressure sender
- coolant thermostat

I have access to a recent FSM for my car.

Any other ideas? Am I picking the right things to check?

EDIT: Also forgot - prior to these problems, I added a capful (not more than 15 mL) of a diesel additive to keep it from gelling and to keep my injectors clean. I did this each time I refilled my fuel tank (so that's twice before the problems started). I don't think I'm adding any more of this stuff for the time being, though.