My used 2005 MC drives fine - no temperature problems UNTIL I've been driving it over 60 miles at one time. Then the needle spikes and the "HOT TEMP" light turns on. I pull over w/safety concerns and check the engine - no leaks, no excessive heat and no noises - it appears to be fine! So, my mechanic replaced the thermostat; same problem! I drove it 90 miles, needle spiked, "HOT TEMP" light came on, again. Engine appeared FINE and everything under the hood was a NORMAL temperature to the touch. Mechanic replaced the water pump (because he couldn't figure what else it could be...) and that was a week ago. I don't think that fixed the problem because when I'm stopped at a traffic light, I see the heat sensor needle starting to creep up just a bit. I do NOT want to be stranded far from home when this thing happens on a long drive again.

What's WRONG with it??? No one knows what to even check for! The engine's NOT hot, there's no percolating noises coming from the radiator, no steam, no leaks, no oil seeping from the head gasket, all fluids are clean and at their normal levels - NOTHING's WRONG when i pop the hood to check it out. But it DOES "overheat" (i.e. "HOT TEMP" and spiked needle) once I've been driving it over an hour, non-stop and won't start back up until it's cooled down for a LONG time. If I never drove it on long trips (over 60 miles at once); I'd never have this problem since I drove it for 3 months prior to a lengthy trip and it was fine. Any suggestions on what my two confused mechanics should be looking for? Thanks

 

Without being around the vehicle, the first thought I have, next time the temp spikes, pull over and open the bleed valve for the cooling system.

If you have a lot of air coming out, that will explain a couple of things. If it's a one time deal, then you just had an air pocket in the system, but if it's more then a time or two that you repeat this process and get air, there may be larger issues.

Best idea I have at this hour.

 

Well that's no good to be overheating. Can you see if there is any debris building up in the radiator and a/c condenser fins? They can get clogged over the years and cause big time overheating problems. If you live in an area with tons of flying insects they tend to block air flow by embedding themselves into your cars cooling fins. Watch a You Tube video on this problem and how to clean the areas of debris. I just backwash, spray the area from behind the radiator in the engine compartment, let it soak for a few then just be prepared to get wet and really spray the heck out of the fins trying to dislodge any leaves, bugs etc out the front side. They have a fin cleaner to, but I think it's just a glorified baking soda concoction.

You did not mention anything about your fans, is it safe to assume that they are coming on or you would have mentioned that to us?

Is the air dam under the front of your car still in place? It's a long black plastic inverted spoiler looking object running the width of the engine bay just under your radiator area. Sometimes people pull up to far to the cement parking stops, sidewalk curbs and such and can break off their air dams, that too can cause your car to overheat by not catching and diverting cooling air up to the radiator face.

You need a new mechanic, he has numerous ways to test if your water pump had failed, he just cost you a chunk of money by his lack of ingenuity and skillzzzz.