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Overheating Paseo

I have a '92 Paseo that keeps overheating when I drive it at all intensively for over thirty minutes (highway driving at 70 etc). I’ve had the coolant system flushed fully twice, and every mechanic I go to says that the fans are working fine, the radiator doesn’t have any problems and that the thermostat is working properly. What could cause the overheating? Especially since it’s so infrequent?

Maybe leaky head gasket.

Define “overheating.” Does the coolant boil over? How high does the temperature gauge get? What does the temperature gauge indicate normally?

Has anyone tested for a leaking head gasket?

Likely possibilities:
Radiator coated inside core; this can substantially reduce the ability of the radiator to dissipate heat.

Thermostat not fully opening. Have you tried changing this $15 item?

Water pump impellars eroded. Flow can be tested.

Unlikely cause:
bad headgasket. That comes with other symptoms not in your post.

Radiator hose with collapsed lining. Uncommon, but it does happen. As with the T-stat, these are cheap to replace.

Summary: the radiator is 17 years young. I’d be surprised if it was not problematic inside. However, a T-stat is $15. You could not be faulted for changing this part first and seeing what happens.

The heat gage (which normally stays right below half) starts rising dramatically so that over a period of five to ten minutes it will hit red line or sometimes above. (I try to pull over as soon as it starts getting above 3/4) The car generally smells hot when I pull over and check it, occasionally has a slight sound of steam with no visible precipitation.

I don’t think anyone has tested for a leaky head gasket that i know of. would that mean the head gasket would need to be replaced?

well i’m hoping it’s the thermostat, i already bought one and will try and replace it. Thanks everyone for your help.