[OFB Cafe] Engine overheating

[Rick Bowers]

Hi all,

Here is a "technical" question for the Cafe, even though it's not the 
Truck Stop Cafe. ;-)

For the past several months, more frequently now that the weather is 
hot, the engine in my Durango has been overheating.
It actually started last year, then stopped in the fall as the 
temperatures dropped. It hasn't overheated again until the weather 
recently got hot. Even when stopped at lights for extended periods.

Here are the symptoms:

- I can drive at highway speeds all day and the temperature stays normal
- If I get off the highway, or slow down due to heavy traffic, the 
temperature soars.
- If I get back above about 40 - 50mph, it cools back down.

Here is an example. I drive home from Cape Cod, about 90 miles. The 
engine is running at normal temperature.
I get off at my exit and drive about 1.5 miles on 30 mph roads with a 
few stop lights.
When I arrive home, the temperature gauge is pegged in the red, the 
radiator is bubbling, and the overflow is dumping anti-freeze.

When things cool down, I check the radiator levels. It's full.
This weekend I flushed everything as best as I could and put in new 
antifreeze. When I drained it, the initial drainage was almost black 
and as thick as oil. Ughh. But that was just in the bottom because 
shortly after it turned liquid and just brown. It did need a change.

After the flush and fill -- same problem. It still overheats at slow 
speeds/stopped.

During the flushing, I put the water hose into the return (water pump 
to radiator) hose and pushed water back through the water 
pump,  through the block and out the radiator drain.
I ran it until the water was clear. I also ran the motor to heat it 
enough to (I think) open the thermostat first.

I don't suspect the thermostat -- it runs normal at highway speeds.
I do suspect the fan clutch. During my flush/fill session, it 
appeared the fan was turning slowly, so I used a piece of 2x4 and was 
able to stop it. I could hold it still with my hands while the engine 
was idling with little resistance. The fan clutch was replaced about 
2 years ago by an unscrupulous mechanic, It may be defective. But the 
local CAP auto parts guy says "no" (even though he could have sold me 
a new one for about $125).

Any ideas?