Doug, I assume when you did the test of the temp sender in a cup of boiling water the engine was not running. If so, the readings you have may be different (lower) than if the engine was running. Afaik the temp gauge runs at battery voltage and is not regulated. The voltage when the alternator is charging is typically 13.9 to 14.5 v. , with engine off battery voltage is around 12.2 to 12.7 v. The difference can affect the gauge reading.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Your assumption is correct that the car was off when the test was performed.

But, the dash gauge does not read any differently when the engine is running normally vs when it is stopped (obviously with the ignition keyed "on").

It did occur to me that when I did the test the engine was cold, and that if the wiring from the sender to the gauge were faulty (internally corroded) it could generate excessive resistance as the engine heats up, giving false high readings. So, I bypassed that wire with some 10 ga wire from the sender to pin #2 on the instrument cluster. The readings were identical.

A replacement radiator (Nissens) is on the way along with the waterpump housing gaskets. I'll probably pull the waterpump this weekend for inspection.

I pulled the water pump this morning, its fine. Also the inside of the waterpump housing is squeaky clean.

FWIW, the entire removal process was very, very easy. No rounded off/broken bolts. No problems at all.

I still think its inadequate coolant flow, due to restricted passages (t-stat, radiator or engine (god I hope not).

So, I cleaned up the water pump and reinstalled it (had to wait for the water pump to housing gasket), degreased the fan clutch (it was dirty), installed a new Nissens radiator, and refilled it with recovered (filtered) coolant.

It still ran at just under 100 C.

I finally broke down and swapped the thermostat out from my 85 300TD (which has always run a rock steady 87-90 C).

Sure enough, now the 84 300TD runs at 87-90 C and the 85 300TD runs at just under 100 C!

Both thermostats are Behr, stamped 80 C, made in Germany. They look identical but the one from the 85 300TD has a very small MB star stamped on the "foot" plate. It was in the car when I bought it 5 years ago.

What I would really like to do is test them side by side by immersing them in hot water and watching 1) at what temps do they begin to open, 2) at what temps are they fully open, and 3) if they open by the same amount.

If I do get around to collecting this data, I will certainly share it.



Thanks to everyone for all the suggestions.

Thermostats are funny devices. Testing in a pot of hot water does not always tell how ot will perform when installed in an engine. I had a 192F thermostat (hotter than stock) for another car that opened at the correct temp when in a pot of hot water, yet it would run cool installed in the engine (cooler than a stock 178 F thermostat) Replacing it with another 192 F thermostat solved the problem.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I thought my other personality wrote this thread when I saw it. I had the same events happen to my '84 300D and I finally solved my cooling issues based on Doug's story.

My car would run hot and creep up to 100C during hill ascents on the highway, but would always eventually settle down to 85C when I was off the throttle. I immediately degreased/acid flushed my system and replaced the thermostat two years ago when I bought the car, but had no improvement. This last summer, I tried degreasing/acid flushing again and also tested the t-stat on the stovetop with textbook results (84-90C / closed-opened). The next part replaced was the radiator with the idea it was still so fouled up that it impaired flow...no improvement. I removed the water pump to find a healthy looking impeller with no deposits. My '83 300D runs perfect, so I swapped the temperature sensor...no change.

I finally swapped thermostats between the two cars, like Doug, and now my car runs right where I want it. Both t-stats were stamped "80C", but they had minor physical differences (the good one looks like the Behr part on Fastlane). I didn't go through the trouble of installing the bad one on the other car, but I have one on order through the M-B dealership for $27 in hopes I can restore the '83 to its previous cooling glory without re-swapping

Cheers,

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Dave

2002 Audi S6 Avant
1983 300D
1984 300D (sold)
1990 560SEL (sold)

Well, it looks like the genuine MB part from the dealer also has a different operating characteristic. The car that was previously known to work correctly gave her thermostat for my other car and now works great. I put in the MB thermostat (even had a star on it) and now this car is operating a bit over 90. I got it up to 100 on the highway the other day so now I'm back to where I was originally. I'll replace the temp sender units soon on both cars to see whether they are really just skewed off and maybe the "good" thermostat is really opening too early. Till then, I've had enough BS with these cooling systems!

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Dave

2002 Audi S6 Avant
1983 300D
1984 300D (sold)
1990 560SEL (sold)

Call me crazy....go ahead I know you want to..but I am not sure why all replies concider this to be an issue. My 83 300SD has ran between 90 and 100 as long as I have owned her. It also had a new Tstat, rad, WP and hoses when purchased 7 years ago. Seems to me there has been a lot of time spent solving a non issue problem. Don't get me wrong I am fully aware of the potential wear and tear saved by running an engine a bit cooler but on these diesels my experience is that it runs better hotter or at the very least does not make much of a difference.
I say just drive and enjoy it.

let see you flushed the system installed new thermostat,how old is the radiator is it original no repairs ever done twenty something years old,and from my own experience I would say you have a stoped up radiator find you a new one,get one brand new for a 150.00 , as far as the water pump either its working or its not never heard of impellar on water pump wearing down if it,s loose or leaking you replace it. I have seen many a radiator over looked when it was the problem, if you presure test the system and you have no leaks system is not leaking down it is not the engine. now for the fan clutch, when engine is cold it is loose when it is hot it is working and forces more air and is hard to turn by hand. Jerry

something wierd is happening with some of these new thermostats. i have the exact same issue, except i narrowed it down to the thermostat a little quicker.

"stant thermostats" or "temperature regulation 101"

I know Stant are no good. And apparently the ones from the dealer may not be either:

anyone know of a reliable brand to use which will keep the temp near 80?

I'm wondering if the "Pull-n-Save" brand thermostats might work better (Just check the reservoir coolant color before you buy)

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Dave

2002 Audi S6 Avant
1983 300D
1984 300D (sold)
1990 560SEL (sold)