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So I've had my 1998 Z28 for about four and a half years. Summer only car. I'm almost positive it's always ran at about 210 degrees at most on a highway trip. This week I noticed it's running closer to 235/240. Should I be concerned? What things do I need to look at that might need replacing? The car is at 38,000 miles so I'm assuming 99 percent of it is original.
After the fans, I'd check the shrouds, air dam, and both sides of the rad- and condenser for air blockages. Same for the air lid/air filter. Next choices would be status of cooling water pump, coolant level, thermostat stuck partially open, failed coolant temp sensor, collapsed/kink hose, air leaking into the coolant system, and contamination in the coolant. You can start the car up and idle it with the rad cap off and monitor temp with a cooking temp gauge. You should also see the point where the thermostat should open and induce flow....see 1st thread below.
You probably don't have a P/S cooler so P/S fluid sludge in the coolant shouldn't be an issue. Hot engine exhaust gases getting into the coolant? The engine being way out of tune or air/fuel way off (air leaks, failed sensors) could contribute to a temperature rise. For such a large change I would think water pump/thermostat if the fans are working normally and there are no other blockages. Confirm that the coolant temperature sensor is working correctly (infra red gun). Can untension the drive belt and see how freely the water pump turns. I assume there are no external cooling leaks noted.
Lots of old threads on this topic to search through.
Excellent advice above by Firebrian. I that would be the same process of checks that I would follow as well. Start with the basics as mentioned and then work from there.
Overall, the temp you are seeing on the gauge is not normal, especially at cruising speed, so assuming the reading is correct (good idea to verify with IR gun as mentioned above) then you definitely have a cooling system issue.
I knew the temperature gauge was right when the extra coolant tank had some boil out. Bought a 160 degree thermostat and it works fine now. Thanks for the help.
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