I replaced the thermostat in my truck last week because it was stuck, and boiling off all my collant. So it was fine for like a week. Then today it started overheating again. It overheats when I'm just babying it driving around on back roads or in town. But if I get on the highway and run it out to the top of 2nd gear at WOT, it cools down and stays cool until I get back on a slow road.

Should I just flush it? Or is it possible my new thermostat is funky too? I have heat and the coolant is cycling from the overflow jug when I shut it off.

Suggestions?
Thanks

 

Sounds like you are not moving enough air thru the radiator at low speeds. I would suspect you are having a problem with the fan clutch.

 

^^^X2 on the bad fan clutch.

 

Thanks for the reply. I'll be checking that out in the morning. On another note, World of Wheels was awesome tonight!

 

Ok. Loose when cold, and stiff when hot means it is good, right?

 

Yes. To test a thermal fan clutch.
Lift the hood.

1. Verify the front of the clutch is not dirty because the silicone fluid has started to leak out.

2. Gently push the fan blades towards and away from the radiator. Excessive play means the bearings in the clutch are shot. Some slight amount of play is acceptable.

3. Start the engine from cold. Run it at fast idle. You'll think the fan is blowing a lot of air, and making a lot of noise. As the engine becomes fully warm, there should be a SUDDEN AND DRAMATIC increase in both air movement and fan noise(about 160 degrees).

If there's no sudden and dramatic increase in fan noise and air flow when the engine is totally warmed up, the fan clutch is bad. The reference temperature is not necessarily engine temperature but temperature at the clutch.

Note: This procedure is based on normal ambient temps. The ambient temperature has a major impact on clutch operation and with very cold ambient temps the clutch may not lock at all if the engine is not under load and therefore not generating enough heat to engage the clutch.

 

Check the radiator cap. if it cant hold pressure than it will overheat, but when your are driving there is enough airflow through the radiator. i had the same problem with mine.



91 D150 5.7L hemi

 

Have you tried to burp it? You may have air bubbles in there that can only be moved through the system when under load. Very common after an overheat as boiling = bubbles...

 

If it ain't broke, don't fix it. And I guess it ain't broke any more! Maybe all of my aggressive diagnostic driving "burped" it for me.