I'm possibly looking to buy a 1985 300d with 104K on it but the owner say's that it's overheating.

What things could cause these cars to overheat? He's already replaced the thermostat & it didn't cure it.

Also, would a compression check tell me if the head gasket is still in good shape?

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1980 240D (Mileage Unknown, broken odo say's 160K)
1985 300SD (213K) SOLD

That sounds like my '83 when I bought it. It was suffering from deferred maintenance- the coolant looked like pea soup. I had my doubts that the engine was any good. In the course of all the repairs I did to it, I replaced the original radiator with a 3-tube Behr unit, and flushed the cooling system multiple times. To verify the water pump is OK, disconnect the mono valve and you should have unbearable heat in the cab. That verifies the water pump is OK. The thermostat should be installed with the arrow "up". Don't rule out the fact you could also have an out of range temp. sender, I went through a couple on mine before I found a good one. I would also recommend MB coolant. Five years later, the car works fine. A true low mileage car is worth putting some $ into.

T-stat in backwards or a cooling system that needs "burping" can cause overheating. So can a bad expansion tank cap.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

If the car looks good, like it really has low miles and appears to be well taken of, no rust, offer a low price, OR, fix it for him and pay for a "running" car

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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"

Thanks for the tips.
I forgot another thing he said.

He said that the heater doesn't work very well in it (in the cab).
Could this be part of the overheating as well? (ie. a blocked coolant passage somewhere?)

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1980 240D (Mileage Unknown, broken odo say's 160K)
1985 300SD (213K) SOLD

In order to obtain a meaningful answer, you need to be more specific than "doesn't work very well."

Sorry, here's a little more info.
He said last winter it would barely produce any heat, even after driving the car for a while.
He didn't say much more than that.

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1980 240D (Mileage Unknown, broken odo say's 160K)
1985 300SD (213K) SOLD

That could be a control issue or a coolant flow issue, or a combination of the two. You could disconnect the monovalve electrical connector. That should give you more heat than you can stand. If it doesn't, the heater core is likely obstructed. Sometimes that can be easily corrected by backflushing with a garden hose.