Here's what's going on:

  1. The temperature remains constant on the highway.
  2. The temperature remains constant for short trips.
  3. The temperature generally remains constant when the car is not moving. (It sometimes creeps up at long lights.)
  4. For longer trips, the temperature increases when I accelerate from a stop.
  5. Eventually the needle will get to about 75% - 80% of maximum. Then the temperature will drop back to normal. Then the process will repeat.

So far I've replaced:

  • Engine: Head gasket was venting into the coolant. (The shop did a "sniff" test). I got a re-manufactured engine put in.
  • Water pump: Replaced it when the first radiator went to heaven.
  • Radiator
  • Thermostat
  • Various hoses as they fail.

Any thoughts about what could be going on?

  • 1
    What parts have you replaced?
    – jzd
    Jul 2, 2011 at 13:52
  • I updated the post to list the components I have had replaced.
    – braveterry
    Jul 2, 2011 at 14:02

3 Answers 3


It looks like the problem was caused by a blockage in the hose connecting the radiator to the reservoir. Coolant was not always flowing back into the radiator as the coolant cooled. The repair shop cleared the blockage, and the temperature remains constant now.


I had somewhat similar symptoms with a Truck that had a replaced Radiator, Water Pump, Hoses, and Thermostat.

I believe the problem was too much air being trapped in the system as the thermostat was not allowing the air to bleed through. (I drilled a tiny hole in the thermostat and it appeared to fix the issue).

However, a few weeks later it started to overheat and I had significant amount of coolant start leaking out of the intake gasket. Fix that and haven't had a problem in 20,000+ miles. That might have been the root issue all along.

Edit based on updated question:

  • Check that the cooling fan is coming on at the right time.
  • Also make sure that the temperature sensor is working correctly.
  • Does the temperature sensor affect the operation of the cooling system, or is it just used by the temperature gauge on the dashboard?
    – braveterry
    Jul 5, 2011 at 15:24
  • Just used in the gauge on the dash. So your engine temperature could be fine, but your gauge could be reporting otherwise.
    – jzd
    Jul 5, 2011 at 16:12

I replaced a bad cooling fan relay that would come on and off at leisure. The system had a lot of gas in the cooling system, I bled the system while adding coolant. Runs like a charm now...

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