I have an om616.912 in a w123 as well as an om617.952 i just finished transplanting into a jeep. I live in upstate ny temps right now are hovering on the 40s. When i got the car with the 616 a couple months ago i went thru all systems noticed thermostat wasnt opening based on grabbing lower hose. Pulled tstat out (80°C) tested in some hot water that i digital thermometer verified, it is working properly. As is the water pump. Heatercore lines flowing freely. System building proper pressure....this engine will run for hrs driving and never get hot enough to open the tsat. It runs at 175...yes one might think tstat is cracking open a bit and replenishing some cold coolant in its place but i assure you the radiator stays stone cold as does the lower hose. When i turn off the valves for the heater core it creeps up a bit but not much. The om617 i transplanted is mimicking the om616 cooling system to a T. Checked new tstat on 617 upon installation. I am certain there is no air in either engines system. Waterpump to head breather tube clear of obstructions/scum. Shot temp everywhere around the head on each engine, hovering around 185 at hottest spot. Thoughts? Thank you everyone as always

Both my 84 &85SDs Run at 80C which is 176F. I'd leave it if you have heat from the climate control.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

I'm with Junkman here ~ my various OM616 & 617's all run at 80 or 82* F all the time .

Only in the Desert or when climbing hills with the AC going full tilt boogie do they ever get hot .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

X3...they run on the cool side unless under load. Restricting some of the airflow through the radiator may be an option.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Are you getting enough heat when the Heater is on?

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84 300D, 82 Volvo 244Gl Diesel

Thank you everyone for chiming in. Heat works well on both vehicles...im just amazed that these engines are running at a steady 180 with what is circulating within the block for extended period of time and doing so without being replenished with coolant. I took the om616 out earlier closed off heater valve so it couldnt cirulate through the heater core and did some 55-65 mph driving that consisted of nothing but gladual to steep ascents. Pulled over 10 miles later shot temp around block hottest spot was 185°. Radiator was cold and lower hose was warm but just from heat radiating down from the tstat housing.

I think this is just the nature of diesel. I recall measuring head and downpipe temps at idle years ago on my Rabbit diesel and I had numbers like 190. Cool...literally!

Less wasted heat, more useful work.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

Another problem fixed thanks to the forum.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Thanks guys

I would'nt be concerned about these engines run at 80c. If you want it to run a little hotter you could 1.restrict air flow thru radiator with cardboard
2.Take off plastic radiator schrowd 3. Take fan blade off take your pick any one of those will increase your running temp. and 2 of them will give you more room to work on engine. I do not have my plastic shield around radiator and it increased my temp. a little over 80c.

You most definitely are getting gradual coolant from the radiator.
Of course the lower radiator hose is cool... it feeds the motor cooled water from the radiator...

Hot water leaves the engine out the top hose...

If you want to know if the thermostat is feeding, check the top hose temp...

If you want to verify this, install a flow meter in the lower hose.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Just to chime in on a solved "problem", my engine holds 190F just about all the time. Long uphill pull in 4th gear with my foot flat on the floor, yeah it'll top 200F by a little bit. Stock radiator oil cooler thermostat water pump etc.

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617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap

Not sure why OP thinks it is "running cool". If the dash gage shows 82 C always, that is perfect. These engines have a 2-stage T-stat, controlling both a radiator return tube and a bypass port. This is typical of Euro engines, so some diagnostics from U.S. engines don't apply.

Some years ago, my 1984 300D was running cool in winter, ~60 C. I tested the T-stat in hot water and seemed "about right", until I tested further w/ an IR gun and against a new T-stat. I found it opened slightly too soon, and responded a bit more sluggishly to temperature. After the new T-stat, it has run 80-84 C from winter to summer.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans