I put a new thermostat in a while back and the problem lingers. Never even goes above 50, I'm guessing that's the unmarked notch on the gauge just north of 40. Somehow I think the bypass channel in the housing works too well. As I will soon be swapping in a new expansion tank and draining coolant - I added quite a bit of distilled water on a couple of occasions - seems like a good opportunity to revisit the bypass issue. Anyone experienced this?

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1986 300SDL, 362K
1984 300D, 138K

Have you checked your temp sender? Oftentimes they can be way off.
You may want to double check your temperatures with an IR thermometer too.

Agree...check actual engine temp with an IR thermometer first. If cold remove thermostat and check opening temp in pan of water.

Good luck!!!

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

Good advice, hadn't thought of that (DOH!). It also occurred to me that I started a thread on this a while back and wouldn't hurt to review. Some good stuff here also:

http://www.peachparts.com/shopforum/diesel-discussion/403276-thermostat-housing-bypass-channel-300d.html

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1986 300SDL, 362K
1984 300D, 138K

They sell cheap thermometers (glass) for making candy and other cooking issues. I would use one of those to check the temp in the coolant recovery tank after you have driven for awhile. That will give you a more accurate reading then the IR thermometer will.
It will also tell you if your sending unit ant temp gauge are accurate.

You can also use the same glass thermometer to remove the Coolant Thermostat and test the thermostat in a pot of water (a T spoon of salt decreases the bubbling making it easier to view) as it heats up to see when it starts to open and when it opens fully.

Suspend the Thermostat above the bottom of the pot and the same with the thermometer.

It was on a gasser but I had a coolant temp sending unit get covered with white scum and not conducting the temp well.
On gassers some manual will say at x temp the them sensor will have X amount of Ohms when you check it with an ohm meter. Have not looked that up on Mercedes.

They also have more study metal meat thermometers but they cost more.

I keep the glass ones I have way back in the corner of a cabinet.

I have a glass tube type one pictured and another type of glass one and I have the digital one as pictured with the long probe which is made for sticking the probe in the meat and a magnet holds the digital read out end to the exterior of your oven.
The digital one has a more extensive temp range.

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84 300D, 82 Volvo 244Gl Diesel

Good idea, not infrequently I’ve bought a semi spendy tool and used it rarely. There’s a restaurant supply store in my normal range, I’ll check it out.

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1986 300SDL, 362K
1984 300D, 138K

Well Walmarts, K-marts and Targets have the candy thermometers and I got the digital one at Target. One of the candy thermometers I had I got at a 99 cent store.
They would likely show up in an on line search.

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84 300D, 82 Volvo 244Gl Diesel