overheating and no heat at idle.
 

This is the 82 300d euro. I believe no secondary water pump I believe. Manual Heat and air. No A/C .I am thinking its a worn water pump. At idle the cabin blower will go cold and the engine will heat up fast. If I leave it idle for 10 or 15 minutes it will be above 100 and climbing. I give it a rev and it drops back quickly. No issues at full power on hills or in the summer heat.

Might be low on coolant.

I agree that the water pump is suspect. The original has a cast impeller. Aftermarket pumps with a stamped impeller are notorious for not moving enough coolant.

However, I'll bet that your radiator is also not doing such a great job.

When I noticed the cold air coming from the heater during Idle, I went out and touched the front of the radiator. It was cold. Radiator is full. I hae no issues in the summer with hot weather going up hills. That leads me to think the radiator is fine. It is tempting to get a new water pump and see how that does. I am not looking for things to spend on right now, but if needed then I will.

CD:

Consider also that the pump housing, and its matching scroll surface, may have become corroded/eroded to the extent that too much clearance exists for efficient pumping at low impeller speeds. The aluminum housing will erode more readily than the cast iron impeller.

If this were my car, this is how I would diagnose:

1. With a cold engine, remove hose coming out of cyl head (by the oil filter housing) and route it into a 2 gallon bucket. Start engine and measure output/ time.

2. Step one was done at the heater inlet. Next, do it at the heater outlet. Replenish coolant, reconnect hose at filter housing and disconnect hose to monovalve and take the same measurement of output over/time. Make sure the time is the same for both tests.

Output volume should be the same for both tests. If output 2 is less than output 1, heater is clogged.

By comparing outputs to an engine with no heat issues and if the output is less than a healthy engine, then do more diagnosis. It could be worn water pump, clogged engine coolant passages, defective thermostat, clogged radiator.

This sounds more like a thermostat that isn't opening. I believe checking it, in a pot of water, before taking everything else apart would be a better approach. A lot less expensive as well.

The two highlighted statements above are in conflict.

Easy enough to test - remove the thermostat (reinstall the gasket) and go for a drive. If the overheating stops, you have a thermostat problem. The heater core won't get hot with the thermostat out of the system.

Total cost to test: $0.00

You are taking statements that are at least 6 months apart. It is easy to have a thermostat failure in that time.

Besides, rule out the easiest thing first.

You need to read the FSM. It specifically says "do not remove tstat, engine will overheat w/o it"

I'm intrigued...how is removing the T-stat any different than it sticking wide open (a very common failure on any engine design)? The point of the T-stat is to keep the block temperature up - the cooling system is oversized for the maximum load at max operating temp the designers anticipated. When the T-stat sticks open, the block runs cold due to excessive water flow for operating conditions.

A thermostat has three functions. It regulates hot water going to the radiator, so you would think that removing it will lower temps...but it also regulates flow through the bypass passage, so removing it means that at least some water circulates continuosly through the hot head, without entering the radiator. Finally, it provides a restriction which increases local pressure in the head, increasing boiling point.

Removing the thermostat can have an unexpected effect, depending on which factor dominates. Generally, Mercs wont overheat, but a GM v6 can boilover in minutes without a thermostat.

Thank you for the extremely informative explanation. Learn something new every day!

I am wondering if it was an air bubble. I did put in a new t-stat recently. It is now functioning, as of this morning, correctly. I could have just made the bubble come out somehow by driving it. I'll have to check grounds and wiring and verify it is actually hot when it says it is. The gauges have been a bit bouncy as of late. The cold radiator was on a cold day too so that may have been a factor. I will have to let it idle again for 20min and see what happens.