I can't seem to post replies with quotes...

Shern, Frank and Vox_incognita, thank you for the advice on the flow restriction. I think this is definitely the case!

I went ahead and ordered a fancy-schmancy MB thermostat to replace the aftermarket one. Will throw that in as a next step.

Here are a few photos of my water pump install. This is the "old" one, which came out looking pristine. Someone else was trying to solve this problem it appears!
https://www.peachparts.com/shopforum...1-img-6335.jpg
https://www.peachparts.com/shopforum...2-img-6336.jpg

Here is the pump housing. Does this look trashed enough to reduce flow? While its a little scored it doesn't seem terribly bad.
https://www.peachparts.com/shopforum...3-img-6337.jpg

And here is my new pump install. Standard pump and gasket from Mr. Kent himself. Anything look suspect?
https://www.peachparts.com/shopforum...4-img-6338.jpg

One thing I notice is that my car doesn't have the banjo bypass tube. The plugs look factory though...maybe this is an '85 thing?

Do not understand the Youtube fan clutch test. What was that supposed to show???

You would test your viscous fan clutch with a rolled up newspaper. If it stops the fan blade, your viscous fan clutch is weak or bad.

The old water pump has the proper cast impellers; hoping that the one you installed has the same. Some new ones have impellers which are not cast - they look more like slanted metal fins and are not as durable. I do wonder however about the scoring; do not remember seeing that when I replaced my water pump about ten years ago (as it turned out the original water pump was 35 years old and was still perfect - the squealing that I thought came from a bad water pump was just a bad belt).

Interesting thread.

The pump I installed was exactly the same! I bet the P.O. and myself bought them from the same fella based on some of the part kits that came with the car. Rhymes with shmercedes shmource. If I’m understanding correctly, the advice above about the fan clutch was that when the engine is hot, the fan clutch is “engaged” and the fan should not free-wheel. So, I got the engine hot and then turned it off to see whether the fan continued freewheeling (clutch non functioning) or stopped spinning shortly after engine shutoff due to properly engaged clutch.

EDIT: Of course…if the radiator isn’t getting that hot due to low flow, then maybe the fan clutch won’t fully engage. Just a thought!

I went on a similar odyssey chasing 82C, even replacing a perfectly good water pump myself (do keep it). Short of finding a cork in the pump housing, all data points to a pretty significant restriction. Let us know when you've installed the new Thermostat.

JK:

Have a look at this thread, post #19:

https://www.peachparts.com/shopforum/diesel-discussion/416951-om617-thermostats-overheating-help.html?highlight=Bleed+passage

The cast-in bleed passage seems to be unique to the 1985 model year.

Close to 100dC while idling at operating temp is nothing to worry about. I'm assuming temp drops once you get up to highway speed?

I actually had seen that thread! When I pulled my thermostat out I checked that passage with a thin straight pick. At the right angle I could get the pick to pass thru the passage, so I'm assuming it's fine. Although, if the thermostat doesn't solve it, then I will remove the housing and try to inspect it more thoroughly.

Is that true that the bleed passage is 85 only? Very interesting. Didn't realize the '85 uniqueness was also in the cooling system.

It does not drop at highway speed! If anything it climbs a little. an hour cruise in the highway results in minimum 95C on the gauge, and more usually 105C with the AC on.

I have that bleed passage in my '83 240D. I'm going through all the same gyrations you are. Flushed like madness, latest was Thermocure for a week in there. Parts cannon loaded and shot. At least 5 different thermostats, the water pot tests, so on and so forth. I'm leaning towards the dash gauge at this point (on mine). I caught it dropping to 80°C and parking there for a few seconds once, and then it "ticked" right back up to about 95°C. Stories of bad solder joints and bad grounds on the cluster side of things. Might be worth investigating.

Done! No effect unfortunately. Here are some photos:


The unit I used...
http://www.peachparts.com/shopforum/...-img-6397.jpeg


Old thermostat removed. The inside of thermostat housing looks pretty clean. Coolant drained from the radiator clear and bright.
http://www.peachparts.com/shopforum/...-img-6398.jpeg


Kind of hard to see, but here is a picture of me fishing a straight pick thru the bleed passage in the thermostat housing to show that it is not blocked.
http://www.peachparts.com/shopforum/...-img-6400.jpeg


New unit installed. Arrow pointing upwards, spring side towards the block.
http://www.peachparts.com/shopforum/...-img-6401.jpeg
http://www.peachparts.com/shopforum/...-img-6402.jpeg


End result:
http://www.peachparts.com/shopforum/...-img-6403.jpeg


I am flummoxed. A few questions:

• I appear to have weak heat as well. With the heat on, engine temps are slightly lower. Can this be related?
• Should there be air in the radiator top hose at all at hot idle?
• Should you be able to feel water flowing thru the top hose when the engine is hot?
• Is my next step tearing apart the thermostat housing and water pump housing to find the restriction?

Any advice is welcome!

Before digging further into the engine components, give a thought to confirming that flow thru the new radiator is what it should be.

Hey there, sorry to hear you're having the same issue! But I'm also glad its not just me. :) Have you measured your engine temps with an IR thermometer? It'd be interesting to compare results. My fuel gauge is a bit jumpy (sporadically, not correlated with vehicle functions), which lead me to think it might be a bad temp gauge as well.

It might be a stack up. I took the car out today after weeks setting inside, ran the a/c, drove it briskly, temp seemed to want to park at about 86°C. At idle it would climb to just over 100°. Whatever I have done seems to have made a difference. Now to investigate the cluster and its connections. These engines (616-617) do not respond well to delayed maintenance in the coolant department. Mine was let go for a period, judging from all the rust I flushed out. Now it's running Zerex G05 50/50 water/coolant.

Howdy! Thank you all for the responses and patience! I dug in a little deeper this weekend.

Good point! I checked the dash reading with a direct contact thermocouple. With around 100*C on the dash gauge, the thermocouple read 90*C pressed against the brass base of the temperature sender. Pressed against the thermostat housing, the thermocouple read 93*C. Could point towards a small dash gauge inaccuracy! I will revisit once I solve the low coolant flow situation...

I did some exploratory surgery to find the restriction today. I removed the thermostat housing and water pump housing from the engine so that I could inspect them. No smoking guns like a rag or old gasket blocking the passages. The only possible "damage" that I found was scoring to the water pump housing conical surface. I would still be surprised that this could cause low flow, but please give me opinions! Here are some pics:


https://www.peachparts.com/shopforum...-img-6415.jpeghttps://www.peachparts.com/shopforum...-img-6417.jpeghttps://www.peachparts.com/shopforum...-img-6419.jpeg


While I had everything apart, I used my garden hose to flush out the radiator, head and block to see if there was an obvious restriction.

• I flushed water thru the radiator top to bottom and it flowed very freely. By the look of it, water was exiting at the same rate as it was entering.
• I disconnected the coolant hose that connects to the rear-right side of the head. I flushed water into there and noted that the water flowed freely down thru the head and out of the engine block where the water pump housing attaches.
• I flushed water into the engine block thru where the water pump housing attaches. The block and head filled with water and eventually water flowed freely out of the head where the thermostat housing attaches. Here's a quick video of the flow: https://youtube.com/shorts/MjN0LW6UvdM?feature=share

So, unfortunately I didn't find anything obvious.
Does this water pump housing look like it could be the culprit?