This spring, I noticed that the car would heat up when pulling a hill. the tempature gage would go to 95 or so, and then drop quickly once the downhill portion of the drive showed up. actually that has been consistant since owning the car... as it will do the same when pulling a longer hill, (speed is higher, so it takes longer to heat up) wait, is this a case of a bad engine fan? in that it seems to heat up the most when traveling at moderate speeds. At fifty or so it will heat up but much more slowly, and cools down once the power demand is reduced. but at 35 mph it just runs kinda warm, not hot not cool but in the middle.

I Just replaced the thermostat, and radiator yesterday, and discovered that the car runs hotter than before... The radiator does get hot, so there seems to be coolant going through it, and the thermostat now appears to work, as it opens and presureizes the system at 80' So I guess the next question is how to test the fan clutch... off to do a search. But if there is some thing obvious that i am missing please jump in.
Jason

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46 WDX Power Wagon

84 300TDT daily driver

JD, We had an '84 TDT also and I replaced the fan clutch and I didn't see any difference in cooling. I NEVER heard it "cut in". I don't know if it is that type that is supposed to engage when it gets to a certain temp. Did you put in the same degree thermostat? Really anything 100 degrees C and below is fine. We get into 100's (+) in summer.
What is your antifreeze concentration? I see that you live in Vermont so you need freeze protection. I have heard that the best is 60%water/40% coolant. I know that if you over concentrate the coolant, you will lose cooling efficienty.
Make sure that the new thermostat is the same temp as the old one.

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My Truck.. 2007 DODGE, 5.9 Cummins, 6spd stick, 4X4. My car..1977 240D, OTHER WHEELS...1955 VW Oval window bug, European Delivery (Holland) with a 1700cc, 2 barrel, Porsche drum brakes. 1939 WILLYS Pick-up. 1967 Triumph 200cc Tiger Cub. 1976 Honda 550F 4cyl Motor Cycle.

The Thermostat seems to be opening at 80', and the radiator does seem to be accepting coolant... but the engine operating tempature will slowly climb to 105 or so. it will drop back a little when going down hill, but not much. I have read that theses cars are supposed to run around 80' but that is just a pipe dream for me at the moment. Of course running the heater helps but that was just a band-aid to get me home... wondering if it could be the water pump??

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46 WDX Power Wagon

84 300TDT daily driver

With a proper new thermostat and radiator, the engine should maintain a steady 90°C. whether the fans work or not. The temps in VT are not all that high.

Are you positive the thermostat is installed properly?

What brand is the thermostat and where did you purchase it?

Are you reasonably confident that there is no air in the cylinder head? Did you fill the upper radiator hose all the way till it just about overflows.......then quickly connect it to the radiator?

The thermostat came from Mercedesshop. It is a Vernet Colorstat, and I am not Positive, that it was put in correctly.. If I remember correctly I put the flat disk, engine side. How happy would I be to learn that this is the cause of the overheating!!

I did fill the cylinder head from the upper radiator hose. Used all the coolant that came out of the cooling system.. so I think that aspect is fool proof.

The cooling fan does not seem to pull much air, but then the radiator did not really get hot enough to engage the clutch, and the motor was close to 105', so Is it a coolant flow issue. (yea, I just did a complete reverse of my first post.) The radiator was warming up.... but may be there was not enough coolant comming out of the block, to allow the radiator to do it's job...? which brings me back to the thermostat....?

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46 WDX Power Wagon

84 300TDT daily driver

Yep, you installed it properly and it appears that your analysis is correct. There is insufficient coolant flowing.

Time to pull the thermostat and check it in a pan of water with a candy thermometer. It had better be fully open by the time the water gets up to about 200°F.

We've seen defective thermostats.......right out of the box......before.

Thanks Brian, I was starting to get a little stir crazy. Thinking that the water pump would need to be checked next.. Really did not want to do that, and hope not to have to. Will pull the thermostat and check. Also that may explane why the other thermostat would sometimes keep the engine at 80' and some times higher.. May be it was sticking or some thing.

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46 WDX Power Wagon

84 300TDT daily driver

Dave had one bad water pump with rusted vanes in all the history that I can recall on M-Shop. While it can't be ruled out, the factory M/B pump with the cast impeller is estraordinarily durable. My 22 year old factory pump had a perfectly good impeller. I changed it during the overhaul of the head.......just because........

I'd bet against the water pump at this point.......until more data is gathered.

BTW, changing it is not a big deal. In fact, if you get a gasket, you can check the impeller for almost no cost and about two hours work........and rule it out.......if you wish.

Did you have the thermostat that has the small hole on the outside disc part of the thermostat? I have used WALLER thermostats. I have also drilled a small hole about 1/16" in the thermostat if it didn't have one. I realize that you noticed the raise in temp AFTER you changed out the radiator and thermostat.... BUT.... Do you know for sure that the temp gauge is reading accurate?
I have tested temp gauges and senders by removing the sender, extending the sender lead and ground long enough to allow me to immerse the sender in OIL heated on a hot plate with a candy thermometer. I turn on the key and slowly heat the OIL and check the cars gauge against the candy thermometer. I use oil because I can ck the gauge and sender at temps above boiling point of water. You would be surprised that some temp gauges are not accurate.

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My Truck.. 2007 DODGE, 5.9 Cummins, 6spd stick, 4X4. My car..1977 240D, OTHER WHEELS...1955 VW Oval window bug, European Delivery (Holland) with a 1700cc, 2 barrel, Porsche drum brakes. 1939 WILLYS Pick-up. 1967 Triumph 200cc Tiger Cub. 1976 Honda 550F 4cyl Motor Cycle.

I am not betting anything on this cooling system. I flushed it last year and got huge amounts of gunk out.... flushed it again when the thermostat went in and MORE gunk came out..... So my first flushing was suspect, but may be there is a lot of rust that has been exposed from the first flushing... How it managed to cool the car Is a mystery to me, but it sorta worked for 248,000K, and the radiator was original. Bet it could be cleaned professionally, but the fins are breaking off, So I doubt it would be worth the effort.

gonna test the thermostat this afternoon.. It does have the hole in the side of it. guessing that is for bleeding air out of the cylinder head.

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46 WDX Power Wagon

84 300TDT daily driver

i had a similar problem and did the "fix" on diesel gaint's site and wow what a difference... the car now runs much cooler and the temp stays righ at 175 and not pushing 250 like it did before the fix

Thermostat checks out as "good". I guess It is time to verify if the gage/ground circut is the problem.. time to do a search, although, the gage does go down when the heater is turned on, and it will occasionally drop to 80' when going down a long hill......So I feel that may not be the cause. But it seems more likely that it is a gage accuracy issue than a water pump issue. Does that seem right to everyone? Or, am I standing on the wrong dock waiting for a boat that will never show up?
Jason

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46 WDX Power Wagon

84 300TDT daily driver

Well, I still believe that you've got a flow problem. If the thermostat checks out OK and will allow full flow at about 200°F., then the only other possibility, other than the guage, is the water pump.

Now, it's certainly possible for the gauge to be reading too high, but, I'd bet against it. See if you can find one of those infrared temperature sensors and shoot the upper radiator hose when the temp gets up to 100°C. Usually the local mechanics will have one.

If the temp. is confirmed, it's time to check the vanes on the water pump. You mentioned that you had the cooling system from hell. This might have taken it's toll on that pump.

Brian,
I think you are right. I put it back together and took it for a road test. Tempature goes to about 98' so thermostat is open, upper hose has some pressure in it. I tried squeezing the upper radiator hose shut with my fingers to feel for coolant flow. Inconclusive test, or bad water pump. Looked at the exteror of the water pump closely and it appears as if the pump has been changed. As they used a red silicone sealant...

So what am I getting at? I will be pulling the water pump tomorow. what if it appears to be ok? could there be a huge amount of sediment in the block some where? blocking the coolant flow to the impeller? Could the impeller be sliding on the pump shaft? guess I have to take it apart and find out. What is your opinion on using silicone sealant for the water pump? I usually try to avoid using it, but the I didn't plan on taking it apart, and it will be tuesday night before I could get a gasket at the earliest..? Of course, if the pump is junk then I will be sitting still until tuesday any way... May be I answered my own question.

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46 WDX Power Wagon

84 300TDT daily driver

Personally, I like to use a gasket and the gasket sealer.

But, the silicone will work fine, provided that you do not use too much. The thinnest possible layer on one surface is what you are looking for. When you tighten the pump down, you won't see but the slightest squeeze come out the mating surface.

I'm really hoping, however, that there is corrosion on the pump vanes and the flow is inadequate due to this. The only thing left is more corrosion in the block and head.........necessitating another citric acid flush.