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I've got a leak somewhere in the cooling system on my '84 300D. Because I just discovered this yesterday evening, I haven't figured out the source.

Short version: I think it's the water pump. The W123 service manual CD makes it look like servicing/replacing the pump is pretty involved. Can't I just get a pump, take off the old one, put on the new one?

Long version: It started when I noticed my temp guage was fluctuating quickly and wildly. Instead of the even rise to the basically horizontal position I expect it to come to. Upon looking under the hood, I eventually discovered that my A/C belt had self destructed and wrapped itself around the fan spindle (what are the odds of that happening?) making it difficult for the fan to operate. I removed the remains of the belt and noticed the coolant was low, so I topped that up. I drive it several days with no problems. Then yesterday, commuting to work and home, the temp guage is going nuts again. When I get home I notice the ground beneath where I had parked the night before is wet. And no coolant is visible in the reservoir. So I crank her up, top up the reservoir (it takes over a gallon), and go for a short drive. Everything appears to be operating correctly again. I go out to look at the car this morning and nary a drop has leaked. I can't figure out what would cause an intermittant leak. Where do I need to look first? Did the A/C belt cause the problem, or was that a symptom of another problem?

Thanks in advance for any suggestions or comments.
 

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Just a guess, I am not a mechanic, but, I would look at the thermostat as the culprit. I think it might be sticking. If it is sticking you may be getting hot and the water is boiling out of the overflow but not at other times. Hose leaks are a very possible source, as well. With the engine hot I would inspect all the hoses. There is also a possibility of the waterpump leaking from the drain hole at the bottom of the pump. Hard to see, but stuffing a paper towel under the pump and driving it around then checking it would tell you a lot. I am sure there are many here that could be more helpful and I look forward to hearing from them. More to come. -ED
 

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I've got a leak somewhere in the cooling system on my '84 300D. Because I just discovered this yesterday evening, I haven't figured out the source.

Short version: I think it's the water pump. The W123 service manual CD makes it look like servicing/replacing the pump is pretty involved. Can't I just get a pump, take off the old one, put on the new one?

Long version: It started when I noticed my temp guage was fluctuating quickly and wildly. Instead of the even rise to the basically horizontal position I expect it to come to. Upon looking under the hood, I eventually discovered that my A/C belt had self destructed and wrapped itself around the fan spindle (what are the odds of that happening?) making it difficult for the fan to operate. I removed the remains of the belt and noticed the coolant was low, so I topped that up. I drive it several days with no problems. Then yesterday, commuting to work and home, the temp guage is going nuts again. When I get home I notice the ground beneath where I had parked the night before is wet. And no coolant is visible in the reservoir. So I crank her up, top up the reservoir (it takes over a gallon), and go for a short drive. Everything appears to be operating correctly again. I go out to look at the car this morning and nary a drop has leaked. I can't figure out what would cause an intermittant leak. Where do I need to look first? Did the A/C belt cause the problem, or was that a symptom of another problem?

Thanks in advance for any suggestions or comments.
"Wrap-around-the-fan" you mention would affect cooling. So, that is contributory to over-heating and fluid loss. Diesels of your vintage were equipped w/two water pumps. While primary pump cools the engine, the second pump resides behind the battery tray; provides cabin heat even when engine is at low RPM, like at a red light. It is very small and has a few, tiny attached hoses. Well-known for leaks and failure. Worst case scenario is a blown head gasket indicated by air bubbles in overflow tank and/or milky white engine oil (at oil change) due to water mix. MB diesels have always been strong in this area.

Intermittency is caused by constant, not so obvious pressure change in cooling circuit due to ambient temp, etc; very natural.

Hope for the best.......best bet is to 1. again, visually check and 2. perform a radiator pressure check to identify and isolate problem. Here, too, care is advised as this pressurizing, by itself, can rupture a weak hose.
 
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