2001 Honda Civic EX with 180K. started overheating so replaced thermostat, both hoses leading to the housing, new radiator and coolant, flushed, back flushed. now it will get about 80% of the way up the temp gauge while at idle with AC on. will reduce to normal temp after excellerating through 20 mph...if you turn heat on (no a/c) it will blow ambient temp out of vents and not effect temp gauge ... also coolant fills resevoir to brim... just removed thermostat and boiled and it did open up and then close once i puled it out of the water.

original head gasket, water pump, timing belt, and heater coil.

any suggestions

 

Lack of heat from the heater.
Getting hot unless in motion.
Reservoir (this is the key) filling to the brim. Is it overflowing, or just filling?


Have you replaced the radiator cap?
Does the radiator fan work?
Did you thoroughly bleed the cooling system for trapped air?
Tested for exhaust gases in the coolant? (combustion leak test)


Sounds like a blown head gasket, though. (worst case scenario)



Your timing belt should have been replaced a long time ago.
The water pump should have been inspected or replaced when the timing belt was replaced.
You should probably replace these when you replace the head gasket.

 

ok resevoir did overflow prior to bleeding the system the other day. now it just fills to the brim. got a brand new radiator cap when i purchased new radiator last week.

radiator fan works. it will kick on and actually bring the temp temporarily down to about the mid point on the gauge.

i bled the system the other day but for about 5-10 minutes. should i try for longer?

i have not tested for exhaust gases...any idea where to get accomplished and for approx how much?

 

Based on what you're saying, I'm going with "bad head gasket" as the culprit.

When you bled the system, how did you go about doing it? It's recommended to start when the engine is cold, and to park on a slight incline. (or jack the front of the car up) You should also have the HVAC temperature setting on full-hot.

You can buy a do-it-yourself combustion leak test kit. The kits range from $45 - $75.

How much it would cost to have someone (such as a shop) do it for you, I have no idea, I've never had to do it. It may be less expensive to have a shop do it, so I'd suggest making a few calls and getting some price quotes.

 

x2.

+ warped head is likely... as this is the issue in almost all of these cases.