OK - I will provide as much info as I have gathered, in hopes of getting some help....

For starters, I have read some of the posts, and done some searches, and gathered that some run HOT ... Also, I have read about the "air bubble" anomaly, and tried to "burp" the system by squeezing the top radiator hose. I don't know how effective my techniques were...

In my quest to find the root cause, I have made a couple of "temporary" modifications:

a) I have removed the thermostat
b) I have bypassed the heater control valve, and used a reinforced transparent tube as a bypass (to visually see the flow)
c) I have replaced the auxilary fan - and it works.

OK, here it goes:

1. On an average day ( not hot or cold) with the AC OFF, the car will hover around 80 to 85 degrees (remember, there is no thermostat)

2. On a HOT day, the temperature will slowly climb.. and climb.. and climb to 100 to 110. With the AC, it climbs faster - but without, it still climbs ( I don't let it go past that).

3. When the car sits idle, the temp rises a bit, but when I start to accelerate (higher revs) it drops down a few degrees. However, even at highway speeds, on a warm day, the temp will slowly climb.

4. Observation: Remember the transparent bypass tube??? Well, at idle, there is no flow through that tube (bypassing the heater control valve). The tube IS FILLED WITH COOLANT, but there is no flow at idle (so it is not an air bubble). However, as the engine revs up, there IS flow through it....

5. Another observation: I bought a non-contact Thermometer... When my car gauge was showing about 100 C, I took some readings:
a) Temp at the upper radiator hose was about 80 degrees
b) Temp at the lower radiator hose was about 77 degrees or so
c) Temp at the head (near the rear-most glow plug) was INDEED 105 C!!

So the dash gague is accurate. Could it be that my water pump is weak, and does not circulate enough through the head??? or ???

Any help is greatly appreciated.

Thanks in advance.

It is possible your water pump is at fault, but not likely.

The first thing I would do is install a OEM T stat and the way I "burp" the system is ..raise front end, disconnect the heater hose (coming from the head, drivers side) at the fire wall, wire it up, Turn the heater on full blast, disconnect the upper radiator hose at the eng and pour the coolant in the hose till it comes out the elevated heater hose, connect hoses back up. , start the engine, leave the radiator cap off, add coolant as needed, when heat starts coming out the ducts, the temp should drop back to operating range. All that shouldn't be necessary for just a T stat change, but its the procedure I use for "maxim burp", which you might want to do just to eliminate the trapped air possibility, then go from there. Also check the new T stat in a pan of water on the stove.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

Check all of your electrcal grounds (battery terminal, engine-> frame strap etc...). Forum member Palangi has a 240 Euro that he purchased which had intermittent 'overheating' issues. IIRC, fixing the bad ground fixed the overheating.

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Never a dull moment at Berry Hill Farm.

I don't think fixing a bad ground will fix an overheating engine. It might correct the intermittent or low water temp. gauge reading but not the overheating itself, and IMHO, a bad ground will show a lower temp gauge reading and not a high reading. Try to disconnect the wire on the water temp. sender, which means no power, therefore the reading will be lower or zero degrees. So, a bad ground will be a low power, or high resistance which will obviously read low.

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'85 300D Turbo - CA Version

When you get a T stat look closly for an arrow on the outer rim, as I remember, which should point streight ahead or up depending on your T stat housing. The last OEM "Whalen" (sp?, you know what I mean ) I bought did'nt have one for some reason tho.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

I, too, have a non-contact thermometer. The temperature at the outlet for the thermostat housing, just before it enters the upper hose is typically 193°F. on a hot day with the engine idling. The gauge is up near 95°C. Close enough.

However, the lower hose shows a return coolant temperature of about 125°F. So, the radiator is removing a decent amount of heat from the coolant.

Your testing shows that the radiator is removing a negligible amount of heat from the coolant. Three degrees is nothing. In fact, I question whether your testing procedure is flawed.

If the temperature of the return coolant........as measured on the aluminum tube in the thermostat housing..........is not somewhere at or below 130°F., then the radiator is the culprit. Either the internal passages are plugged or the external fins are plugged. You can clean the external fins if you remove the radiator from the vehicle and use compressed air/pressurized water/a condenser cleaner or a combination of several of these. Getting the radiator clean is a tedius and difficult task.........but your alternative is to replace the radiator.

Aren't MB thermostats bypass thermostats so removing the thermostat will cause the engine to run hotter?
So, in response to Brian's observation, the reason why the radiator is not cooling the fluid is that most of it is simply circulating thru the engine and not thru the engine and radiator.
Put the thermostat back in, make sure the bypass is sealing, and run your infrared thermometer tests again.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

We've been around the block with the thermostat issue about five times this month.

Without a thermostat, the bypass is open and a certain amount of coolant will not flow through the radiator. But, the bypass is smaller than the opening to the radiator so the loss is not anywhere near 50%.

But, removal of the thermostat seems to increase the total flow of coolant, thereby offsetting some of the loss to the bypass.

Some folks report lower temperatures under all conditions with the stat removed. Others find that the engine overheats.

I don't believe all the variables are known to provide a definitive answer.

First, thank you ALL for your replies.

Brian, I will repeat my test ( temp measurements at the input/output of the radiator).

But I have a few questions...

1) Could a weak water heater (thus low flow through the radiator) cause the temperature anomaly at the radiator ???

2) Could the "No Flow" condition at idle, that I observed with the make-shift transparent heater bypass suggest that the water pump is weak???

I'm not following the "weak water heater" description.

But, I do encourage you to repeat the tests with the infrared thermometer. However, the accuracy on the hoses is questionable. Shoot the aluminum just past the end of the hoses on both the inlet and outlet.

I will repeat the test in the next couple of days....

As a matter of fact, I will insert a thermocouple in the inlet, and one in the outlet, and give much accurate readings of the fluid temp.'

Now the other questions....

1) Is the Head temperature supposed to be THAT much higher?
2) What causes the no-flow-at-idle (but good flow with engine revving up) at the rear of the engine (where the heater control valve, which is bypassed, would be connected).

Thanks again,

Ed.

the head is where most of the heat is generated so it should be the hottest place.

i also am not following brians idea that since the bypass slot is smaller than the radiator hose it automatically flows a lot less. the thermostat closes completely so all of the flow is going through that little slot. also the slot is a direct route and the radiator is the long way around with lots and lots of resistance, by comparison.

picture a furnace with long long ductwork to push air through versus a shortcut. one way has lots of resistance the other way smaller but no resistance. air and water are both fluids but with different densities, so flow characteristics have similarities which may help understand sometimes.

like most things, i agree with the factory service manual on this one. i would not run my benz without the stat.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Hi Ed,

I am a newby, but I was having similar problems with my 617. My is turbo'd and I noticed that it would run about 115C or better on the highway. I also noticed that the turbo was making a leaking noise. So I found the cracked wastegate hose and fixed it. This reduced the problems but didnt solve the higher temp issues. I also realized that the wastegate is controled by vacuum on my 83 300D turbo. After inspecting all the rubber vacuum hoses, starting at the wastegate and working back toward the source, I found several of my short hoses were brittle and lose around thier conections. Particularly the contection to the temp switch on the water pump housing. It looks as if wastegate operation is controled by vacuum and if so, also controled by engine temp.

So my theory, the wastegate is interupted by this sensor/switch in the t stat housing while the engine is cool. With proper e temp, the vacuum switch on the t stat housing opens allowing vacuum to the wastegate to operate. If the wastegate stays closed all the time your EGT's would continue to increase because your compression from your turbo is not controled properly.

All that to say, check your vacuum controls on the wastegate to assure it's proper function at operating temp. Assuming your 240 is turbo'd.

Later and good luck!

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83' 300D Turbo - resurection in progress...

I wish it was though !

oh well too bad.

But still, check and see if some part of your vacuum system is not somehow in play here. It might be worth it.

If nothing else would flushing your radiator 2 or 3 times help?

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83' 300D Turbo - resurection in progress...