Helping out a buddy. He has a sb w/stock shroud and radiator but an electric fan. Temp has been okay since rebuild about 10K miles ago. Now, when he is idling, the temp comes back down to normal, but when driving the temp gets into the red zone. He changed the thermostat, flushed the system, and put new coolant/water mixture in. It's slightly better, but still overheats when driving. This is a weird problem. Whenever I have had overheating problems, it usually gets worse as I slow down and idle is the worst. His problem is showing opposite symtoms.

Any suggestions or comments appreciated.

Mike

What year car? Is it a Mechanical fan, or electrical, Does he have the correct shroud installed?

Some of the third and fourth Gen Camaros are very sensitive to changes in the lower spoiler and air dams at the bottom of the radiator. If they are missing the car will almost always overheat at speed on the highway, as it lets air bypass the radiator and pressurize the engine compartment, which lowers the amount of air that will actually pass through the radiator.

Sounds like the car is running lean at highway speeds.

68 with electric fan and shroud kit. The shroud was made for the fan - I think it's a derale, but the shroud looks like and aftermarket from someone like vintage air. Anyway, the fan and shroud fit tight and perfect.

Like I said, things worked great until a few months ago and now it's just acting weird. I was at his house yesterday for a bbq and we took a spin around the neighborhood - sure enough, idle shows reasonable temp - but when we started cruising it got hot and when we hit a 50mph road for a few minutes it started climbing and kept climbing. I was thinking pressure leak - like a bad hose or something, but??? After the cruise, we brought it back to the house, at idle it started to cool down and I could not smell any antifreeze - so no leak there that I can find.

Thanks.

I'll agree with Mark on the too lean at cruise and add that an over heating transmission can pass heat to the engine as well. Also check water pump, it could be moving water at idle but cavitating at above idle rpm and not moving fluid through the system.

Dennis and Mark have you covered here....most common cause of overheating at cruising speeds is too lean of condition (carb jets and/or intake leak), bottom hose collapsing at speed, or poor coolant conditions which it looks like you have ruled out! Sometimes partially stuck open thermostats will cause this as well.

Keep us posted.