Did you ever get to the root of your overheating problems, sounds similar to mine, swapped an om617 turbo into an '83 toyota pickup. Soo far have a new oem headgasket, rebuilt head that was tested for cracks, i made it 1200 miles on the interstate, before problems. Radiator is from summit, but was recomended from doomsday diesel who fabbed the engine mounting plate and has used them on many builds. It seems to intermittantly overheat, usually going up hill, before the engine is up to operating temp. It is a bit smoky and i think timing is retarded, waiting to test and see how much. Since then i have tested and replaced thermostat, cleaned bypass tube, new water pump(the old ones impeller was sliding forward on shaft). And, bled the **** out of the cooling system a few times w the nose in the air. I have also flushed system and heater core with water. went a month or so n thought that fixed it, but then Last time it overheated i had just started hauling a load of compost and i parked it. Once it sits overnight and cools i can start and drive it w no overheat, problem is very intermittent. I know thread has been unactive for awhile, but any input would be much appreciated! Its been two years trying to figure this out, argh

Almost forgot, pump is stock and alda is hooked up and unmolested(by me anyways). And, i have an egt gauge and boost gauge to plumb in after i do 2mm method for chain stretch and maybe an offset woodruf key.

I always drill two tiny holes in Tstat for air to escape.

I also built a pop tester and rebuilt my injectors when i did head, and for back story i drove from califirnia to kansas on the swap, it burned an exhaust valve so thats why i sourced a head to get rebuilt, they found hairline cracks in the original. But, it never had overheating issues

I think so far folks have been trying to come up with one solution to two problems....I am feeling there may be two solutions needed, one for the fueling issue, the other for the overheating.

Although there are many people that work on these cars/engines, and know how to put coolant in them, some don't. The proper way is to fill the expansion tank with your favorite mix of coolant/distilled water, and then fill the block with the same mixture through the TOP RADIATOR HOSE. Just filling the expansion tank will not work! Drilling the thermostat, and other methods of getting the air out are not necessary in my opinion. Just do it right the first time the easy way, and it will work perfectly....Rich

I wish these fellows well on making this work. Fantasizing about various engine transplants is something to which I devote probably too much time. But it is attrative, and it's the whole spirit of innovation that drove the advancement of the auto and other tech to begin with.

OTOH it is seemingly fraught with peril. Hard to match the engineering and resources of a major manufacturer for ironing out the details. But ironing is what we do here I guess.

Never had an issue with my swap, and I'm using the factory 300SD radiator. I have a filler neck plumbed into the top radiator hose, which is the highest point on the system.

Overheating going up hills, and when you have a heavy load in the back, which would tilt the back of the truck down, speaks of an air pocket to me. Most thermostats have a tiny notch in the sealing surface to bleed air, always make sure to install them with the notch pointed upwards. If it doesn't have a notch, I'll drill a single small (1/16") hole at the top to provide this function.

Use the better thermostat, the one that fails in the open position. Using Hank's Discount Thermostat to save three bucks is a bad deal compared to your motor.

Thanx for the reply. I believe it is a mercedes thermostat and i tested it prior to use. Ill have to check on orientation. Would it be possible to to make it 1200 miles with an air pocket before it became a problem? It seems telling of something, but im not sure what, that it has only overheated if i hit a hill on a cold engine, almost like something is keeping the good thermostat from opening, but i dunno. I guess its possible that i somehow cracked the new head, or blew the new gasket slightly, but compression is good and i didnt detect any combustion gas in the coolant with the color change fluid. Still stumped but all the input helps

Just to clarify, the first 1200 miles was before the new head and gasket, and the second 1200 miles was after the rebuild and then the problems started. In between it was always a very steady temp.

Thermostatic fan may be not up to scratch. Should have a shroud as well on the radiator. Those themo coupling fan units can get weak with time. There are ways to test them in the archives.

There is a casting in the housing for literally exactly that. Mutilation of the t-stat serves no function.

I think I need to look at boneyards for that exact housing. My 300D still runs cold. That housing on mine could be too porous. I really should get on top of it. New stat had no effect.

Totally agree!

Just how cold are we talking here? What coolant did you use?

I had a buddy with a Ford van that wasn't running enough antifreeze when it started getting cold. The radiator would freeze and block up, while the engine still had liquid coolant in it. Get driving and it'd overheat as the iced radiator passages didn't allow any cooling. Shut it off, let it sit a bit, everything melted and it cooled fine. We added more antifreeze and all was well.