Overheats Quickly At Idle
 

This is on the 79 300TD.

Lately, if I sit still or even drive slowly (30mph and under), my temp gauge rises towards the top after a minute or so. If I get back to highway speeds it will drop back to normal within 10 seconds or so. Since it has been getting chilly here, I have used the heat and the heat (or lack of) corresponds as well meaning at idle it starts blowing cold while the temperature rises.

I have the servo unit and it works properly (seemingly). Of course I will check the Aux water pump but that should not cause over heating, especially in 40F weather, correct? So perhaps something is blocking/restricting the flow? What else could cause this? Air? I have no overflow tank so I'm not entirely sure how full the radiator should be. What do you think about the main water pump as a cause?

I checked the fuses and there were no obvious ones out. I guess I'm not sure which ones would be directly related to this issue anyways.

I am hoping to park the 79 for the winter soon. I'm waiting on the tranny for my 94 6.5 so I can get the put in and drive that for the winter. Once I park it I plan on doing the maintenance. I will flush the coolant, check the Aux pump function and will check out the main water pump.

Thoughts or anything else I should check?

Have you check for proper coolant level? A seriously low coolant level can produce funny symptoms.

If coolant level was low, I would suspect the cooling would not improve with more rpm and airflow. Have you checked the fan clutch?

I have not checked the fan clutch. Thanks for the advice. Anything in particular I should be doing while testing its functionality?

Loss of cool
 

Cooling problems due to a stuck thermostat or low coolant should not improve with engine/road speed. I suspect the water pump. If the pump is badly worn (either not moving coolant or allowing it to backflow within the pump) it will pump badly but might improve as engine speed rises.

At the temperatures and engine speeds you are seeing, the clutch should really not be a factor. Once the car has run for a couple of minutes, the fan should be rather loose on the clutch -- a rolled-up newspaper should serve to stop it. Also, if you press the STOP lever on the engine, it will stop immediately while the fan should continue to spin for a few seconds. At or above 100C the clutch should stiffen and the fan will then run at engine speed up to about 4000 RPM when the clutch should then disengage.

The servo could cause lack of cabin heat but it should not cause engine overheating. The aux coolant pump could be a contributor, especially if it is clogged with something. A clog or partial clog could also cause your overheating problem.

Jeremy

Fan is spinning at idle. Seems like coolant is not flowing through radiator. Could a bad thermostat cause this? Also what level should coolant be at? I have no expansion tank so I'm not sure.

Bad thermostat could cause overheating; would the problem get better at higher engine/vehicle speeds -- I don't know. Radiator should be full enough that the coolant level is just below the neck of the place the radiator cap screws on. The inside plates/fins should be covered.

That is what I'm suspecting right now as well (worn water pump) but I was hoping it was something really simple.

So far, it seems I've ruled out a fan clutch (as it was spinning full speed at idle with the temp rising starting at operating temp) and coolant level as I could see coolant at the top of the radiator (unless I'm checking the level incorrectly).

Next I'll look at the aux pump. I really don't want to dig into the main water pump until I park it as my 6.5 is inop right now but we'll see.

Maybe a tad low but doesn't seem to be the main cause. The radiator (only checked the top) felt cold while the temp was rising. So it seems the flow is the issue.

The only function of the auxiliary water pump is to improve heater performance at low engine speeds. It isn't large enough to cool the engine...Robert

Yes you are correct it is not the main cause of the overheating (unless it is blocking flow somehow which is unlikely), but if it is not working I still need to swap it out with a spare on the shelf. :)

If the auxiliary pump is blocking flow, it won't matter. The monovalve is blocking the same flow when it's energized. I would suspect a low coolant level where the pump does not have the head pressure at idle to circulate coolant to the radiator, it's simply circulating through the bypass circuit. A slipping or inoperative WP will cause the engine to heat under load due to lack of coolant circulation. A partially stuck thermostat will do the same. The only other thing that will cause heating at idle speed is poor air flow, and it sounds like that scenario has already been eliminated.

I'm pretty certain I don't have a monovalve. I have a servo. I'm still learning all the differences on this car as I'm used to the SDs.

I didn't realize water pumps were so cheap for these cars. I'll order up a new one and a new thermostat at the same time.

before you pull parts out, park the car with the radiator as high as possible, a steep driveway would be perfect. then open the radaitor, and look in there... if everything is not fully covered, and coolant up to the top of the radiator, you are low on coolant. fill it in this position.
report back.

The servo blocks coolant flow too, that's how it controls heater output.

-j