why am I over heating??
 

car ran good all winter.

three weeks ago started to run hot, 115 C, but never hit 120.

took it to an indy, who pressure checked the system, and installed new theromast, and new radiator cap.

car ran ok for a while, and now is back to over heating.

today, after the radiator cooled down, checked the level and it is right at the top.

water pump is not leaking

no air in the cooling system

this is a used car and I have no maintance records.

coolent is not full of gunk, oil, etc

oil is black, and no water in it

I can only think of two possible things:
-- radiator is clogged and will need new one
-- something in the engine is blocking the flow of the coolent, but what?

suggestions?

Bob
1981 240d

To see if it is the radiator, wait until it is quite warm, shut it off, and then feel different areas of the radiator. There shouldn't be any significantly cool spots. It may get a little cooler towards the bottom, but not much.*

Does it have a new water pump? I've heard of some aftermarkets not being close enough in the tolerances, and this causing poor circulation. MB's remans. are good, so you don't need new necessarily.

Once it is hot...the radiator hoses aren't plump(under pressure), are they?

Next time you're at the DIY carwash, grab that want and spray the radiator off (from the inside towards the outside). Probably won't cure anything, but it can't hurt.

* If it is not too badly "clogged", you may be able to get by with a citric acid flush.

Good luck with your hunt!

time to try a citric acid flush to clean out the system

Your gauge might be the culprit too.

good point, how would test?

Somewhere, on this site, is a post/thread that tells you what the resistance at the sensor is, at certain temps. In other words, when cold it should read xxxohms, and when warm xxx ohms, and when hot xxx ohms.

Trouble is, I have no idea where this thread is buried - some good search skills would come in handy. (sorry)

Maybe someone reading this remembers the thread, or some detail of it.

water pump impeller could be freewheeling on shaft too.

also could have air in system.

radiator could have plugged fins. washing from back to front with a good blast of water can do a good job of clearing it out.

tom w

Not the water pump.

Unlikely that air would cause such a rise in temp unless it's a massive amount.

Agreed on the radiator. They get hopelessly clogged with debris over the years causing a massive reduction in airflow.

Washing from back to front in the vehicle is useless. The radiator must be removed from the vehicle and a powerful condenser cleaner must be used on the fins together with compressed air. It's a lengthy procedure because the spaces between these fins are very narrow.

Hold the radiator up to the sun and peer through the fins.........you'll see the problem immediately.

I've been having troubles with my cooling system-- acting just like yours.
I quickly found that the trouble was the gauge and that the cooling system was working perfectly, but the gauge was defective.

I used a laser operated thermometer on the engine and found that water entered the radiator at 80c and left it at 60dc. The guys in the truck garage pointed out to me that if I really was running at 120c there would be no question about it if you lifted the hood.

So for the moment I have a mechanical temp gauge installed.

Check the cheap things first!