Car temp going red, then cooling down while highway driving.


Asked by May 24, 2016 at 10:49 AM about the 1999 Pontiac Sunfire 2 Dr GT Coupe

Question type: Maintenance & Repair

My 2.4L DOHC Sunfire has been running hot, but once it hits the yellow or
red line, it then starts to cool down a minute later, to about mid gauge while
driving (even at 65 MPH steady for 15 minutes).

I know the fan kicks on because the first time it happened, I immediately
parked it and checked and the fan and it was running even with the car off.

I tried changing out the thermostat, but when I checked the old thermostat
in boiling water, if opened up no trouble. (new one is leaking, but I think
that's because autozone gave me the wrong gasket because the one they
gave me was labeled water outlet gasket but the clerk swore it was the
correct one.)

When I took the thermostat off, a lot of coolant dumped out, so I'm sure the
water pump is working.

Any ideas what would make it run hot then start cooling down about a
minute later?

2 Answers


Update, since it won't let me edit the question. I replaced the thermostat with the correct gasket today. Nothing leaked when car was on. I also pulled the hose of the coolant pipe under the car and fluid poured out, so that seemed ok. I then drove it to work, and it stayed at middle temp until I got almost there (about 10 minutes into the drive) and it pegged at the red line and wouldn't budge. I pulled over and parked it, shut it off. It started expelling steam from different points on the engine. Pretty sure the head gasket might be done for, since the oil light came on before I turned it off. I waited for it to cool off, then drove it to work, the oil light was on the entire way, but no white smoke came out of the exhaust. Maybe it's the water pump? I don't know.


Another update. Drove it home from work at about 50 mph all the way there. Drive was about 15 minutes. Car stayed at 100 until I neared the 13 minute mark an when I kicked on the heater. It then started slowly creeping up but never got above midway. I did smell oil burning, though. Interesting to note is that my heater never actually blew hot air. Only cool air. I think the heater core is located in line after the waterpump, since GM likes to use the ambient heat from the coolant to speed up the heating process. This further leads me to believe it's the waterpump. On a 2.4L engine, the water pump is tied in with the engine's timing system, and is located on the backside of the engine. The local shop i go to quoted 6 hours worth of labor to change. I really hope it's not the pump, but it's either that or the head gasket. Engine had zero coolant the, no steam coming from it when it heated up on the way home, so it may not be the gasket.

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