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Bill in Jupiter

Temperature gauge reading high intermittently at different speeds. Toyota Corolla 2005

  1. There were no issues with temperature readings, but after a check engine light came on, our local mechanic replaced one ignition coil.
  2. A few days later, the car was driven long distance for 2 hours at highway speeds. At a toll plaza where the car slowed, about 1 mile past it, the temp gauge started rising from mid to near peak.
  3. Car was towed 20 miles to service station. Tow truck driver or service station added about 4 cups of coolant.
  4. Next morning, the service station diagnosed the need for a new thermostat. After replacing it and a coolant flush, the car passed a test drive,
  5. The car was driven about 20 minutes at highway speeds and the same temp readings occurred. The car was returned to the same service station, but no problem was found even after a more thorough test drive.
  6. Soon after the car was driven at highway speeds for about 2 hours. Upon exiting the highway and returning to lower speeds around 50 – 60 mph, the same issue occurred about 10 minutes from home. All this time prior, my son was driving. I drove my other car to meet him and we switched. I drove his car slowly at first and after getting around 50, it started heating and I prepared to pull over, but instead, I just slowed to 20 mph and then the temp went down to normal relatively fast. I raised the speed again to 55 and there was no problem. Drove for about 7 minutes at 55 mph, no problem. Noticed a thumping or vibration at around 55 mph. Increased speed to 64, the sound went away and no heating. Reduced speed to 55 mph and the noise returned and so did the heating. Slowed down, returned home.
  7. Several hours later, while in park, let the car rev to 3000 rpm. It raised to normal temp and stayed that way. Noticed clicking sound in engine started up after about 3 minutes. Still no overheating. Also noticed the idle is fine in park, but in reverse with brakes on, the idle is rough and even worse when in drive with brakes on. Radiator fan cycles on and off normally with AC.
  8. Planning another test drive.

I sounds that no pressure test was performed on the cooling system to try to determine the cause. A pressure and leak down test would spot a head gasket breech or a crack in the head.

Unless this is done you will get into these problems again; you did not get a permanent fix.

This planned test drive should be to a really close shop to have proper diagnostics done before more damage to engine is done.

Does the fan ever run when the engine is hot and the AC is off? The fan has an electronic sensor in the radiator that turns it on and off depending on the temperature of the coolant. This is not the heat gauge (warning light) sensor.

Also, if the sensor for the gauge is faulty, or the wire from it to the gauge has an intermittent short, you will see this spiking.

While I agree with the advice you got from @Docnick and @“VOLVO V70” I also want to suggest possible cheap and fairly easy ideas, too.

I’d invest in a new radiator cap. They are relatively cheap and are often the source of overheating at highway speeds.