I changed the coolant on my 85' 300D W123 and now the water pump doesn't seem to be working. It was fine before with no issues or over heating in any circumstance. The car sat for a week up on blocks after draining the old coolant out before I had a chance to get more, but nothing else that I think would effect it. The auxillary electric pump works fine, and circulates hot coolant to the heater block so there is hot air in the cabin.
How likely is it that a water pump would just stop working after sitting like that? The car has sat previously for months before I had time to make repairs, never an issue. The belt on it is not slipping and I flushed the radiator with water just befoer refilling it. Is there something I'm missing or do I have to drain it again and pull the pump, I'd rather not. I put in a new thermostat today just as good measure but no effect. Is there some other trick to try before pulling the pump? Thanks.

 

Never heard of a M-B water pump just stop working before. Mechanical device, so if the shaft on the outside of the pump is connectedthen it should be spinning the circulator behind the pump. Good news here is the pump is an easy change should you decide to investigate further. A few of the lower bolts go directly into the water jacket so make sure to drain ALL of the coolant and seal the threads on the bolts. I use a Mity-Vac pump to remove remaining coolant from the jacket before installing a newgasket and pump.

 

I thought it was very odd as well. I'm concerned about taking it apart becuase of loosing all the brand new coolant and suspect somthing else is going on but don't know what. The symptoms are cold cooland in both the upper and lower radiator hoses and a cold radiator. The engine starts to over heat if the cabin heat is not turned on to activate the auxillary water pump. Can anybody help?

 

Somebody here surely has seen this before - give it a few days on the forum. Sounds like a physical blockage somewhere, or possibly a stuck or malfunctioning bypass valve.

I would take a closer look at the coolant circuit located in and around the electric circulator pump. Drain some off if you have to. Agallon of coolant is a small price versus the cost and headache of overheating your engine.

 

Are you positive you didn't put the thermostat in backwards? I hate to suggest it, but it's easy to do.

 

Problem isn't the thermostat. I changed it after coolant would't circulate out of good measure, and put it in correctly, but thanks for the suggestion.

 

The only things that I can think of (other than the thermostat) that could cause anything remotely like this are 1) water pump failure - the impeller came loose from the shaft 2) a really damn big physical blockage 3) an air bubble, and 4) someone dumped a ton of stop-leak into the system and the whole darn radiator is plugged.

The engine block doesn't have a lot of fine passages. It's virtually impossible to clog it. The same holds true for the hoses coming off the radiator. In fact, I can't think of how the radiator could be cold since at one end it's open to the rest of the system and would get warm just by fluid exchange. I can understand one hose being cold on one side of the thermostat, but that's about it.

I think I would lean towards the big damn air bubble theory. Try loosening a top hose and see if you can't get it to flow.