Ok... not heard this line of thought before regarding the oil thermostat. Wasn’t even aware there was one. So a stuck oil stat could cause over heating coolant?

Currently, the only way I can keep the van from overheating on long inclines is with the thermostat modified, either drilled or internals removed. Obviously this is just a stop-gap till I can figure out the reasoning behind it.

Hi
OM 602 and 603 are different from N/A and Turbo main différences are pistons, piston oïl jet in crankase, and air/oïl cooler, also head I think is bit different.
If it was mine I will do in order:
- check if cooling cap keep the pressure
- antifreeze 70% demineralized water 30 % antifreeze
- install air/ oïl cooler
- install 75 C water thermostat is for tropical zone.
- if not enough install a push electric fan with switch that swith on at 105 C
keep in mind this engine in hot summer run often about 100 C.
- also i read somebody change the water pump pulley with smaller diameter it is mercedes update i hope you can solve.
Alberto

I have been researching a similar problem on my OM 602.962 in sedan form. You say there is an oil cooler on the right side of the radiator. Unless the van is very different to the sedans, I am very certain that you are looking at the automatic transmission fluid cooler, not connected to the engine temp. In my studies it seems that the oil cooler on modern (post 1990ish) OM 602s is always an oil to air type, nothing to do with the coolant rad. The oil radiator seems to be usually mounted in the left front wheel well.

Since all the Mercs I have had always ran exactly on 80C I suspect the turbos, with their huge heat output, MUST have an efficient oil cooler to dissipate that heat effectively. If you did not fit an oil cooler after turboing, I would say that is yr problem.

The Utah desert temps mean that in the summer my problem has been much more severe than yours. I have lately found that the oil filter to cooler feed pipe, when the engine exceeds 100C, is literally burning hot while the return pipe is cold, no problem to hold your fingers on it for a long time. This indicates that the oil thermostat is functioning but that the flow through the cooler is probably blocked. Now trying to figure out a simple way to confirm this. Unfortunately the Utah winter is the opposite of the summer so I can't do much till Spring.

Good luck.

It a manual van so no autobox cooler.

All sorted now - did a citric flush, 85c thermostat, high pressure rad cap, new Nissans rad, new belt and pulleys, new waterpump and all running at perfect temps 👍🏻

The op did not mention the length of hills or the grade. Also did not say if the electric fan is oem or aftermarket. If it is oem then the viscous clutch should be reinstalled.

And when pulling hills you should (if you don't already) expect to gear down and let the engine rev a bit and accept that on hills you need to use your gears.

The temp rising on hills is normal as long as it stays out of the red.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.