Start car cold - 3 to 5 minutes it warms up and temp gauge looks great (in the middle). Idle OR drive for 25-30 minutes and all of a sudden the gauge spikes to hot in an instant and stays there until car turned off. I changed thermostat. I've checked fluids. Any ideas? Thanks

 

Is the radiator staying full to the top? Take the cap off when it is cold. Don't just look in the plastic bottle, you need to check the radiator itself.

 

Yes - Rad is full. Changed sensor in thermostat housing (fan switch) and the two sensors under the dist cap (temp switch and temp sensor). No luck. Let idle for 20 min's and fine - took for a ride and 11 min's into ride it pegged at hot in an instant. Any more ideas would be great. Thanks.

 

If it only overheats in traffic, fan may not be working. Unplug the switch on the thermostat housing and jump the pins in the plug. The fan should now start immediately when you turn the key on. If it doesn't, check fuse, motor, and relay. Test drive with the fan running all the time and see if there is overheating.

Your new thermostat could be defective or improperly installed. Does the heater always work?

 

Heater works great. Fan works.

 

Another question. Does my 2000 civic have a coolant bleeder screw? If so, where?

 

No, they didn't have bleed screws after 1996 I think. To get air out, park car level or slightly uphill and run the engine with the radiator cap off. Heater temperature dial should be set to full hot to open the valve to the core.

If the heater always works, that says the engine is staying full of coolant and it is flowing around the engine. Overheating under those circumstances suggests the thermostat doesn't open or the flow through the radiator is blocked.

 

OK - Thanks - I've replaced thermostat. Was fairly easy, I think. Can't see how I could have messed up installation. I'm an admitted novice w/ cars. Can you think of how I could have messed up thermostat installation? Radiator was replaced 7 months ago.

 

The vent pin in the thermostat flange needs to be at the top. The copper center core needs to be on the side facing the engine. I don't think it's possible to do that wrong though. The thermostats with rubber on the back end piece that covers the bypass hose port can get stuck. Most of the new ones have a metal disc there.

 

Confident I'm OK w/ thermostat. Running car now for 45 min's w/ radiator cap off and fluid filled to top. Temp is good. I see a small amount of air bubbles (small in size air bubbles too) coming to radiator fill sporadically. With cap on maybe bubbles can't escape and that is issue? I think I've seen you guys say head gasket? Other ideas. Much appreciated.