Diagnosis please
 

I warmed up my car last night before driving home because it was cold, no problems. When I got on the highway I noiced the temp. gauge falling steady to cold and the temp. dropping in my car. The vents are blowinng air but its not very hot. Once the car sits at idle I notice the temp. gauge coming back to hot. What can this be.

The rad
The heater core
The thermostat
I have no clue some help would be appreciated before it gets really cold.

Thanks.

I'd go with thermostat stuck open :nods:

could it be that simple? I hope so

having an engine too cool is usually a simple problem :D

the coolant continues to circulate through the rad and on the highway that is too much cooling for a motor.

Thank you very much for the help, I will get this done tomorrow.
I have an extra rad and heater core, so if that was the problem I was ready to install those but a thermostat is much easier to do, and would an OEM thermo be the best choice?

you didn't say...is the rad full??

Not sure because I didn't check. Explain the differences please, I will check right now.

rad is full, but resivior is empty

air lock..
no water/freeze to carry heat to from engine..

Isn't a cool running engine a good thing? People put in those race radiator caps to increasing cooling... so would a stuck thermostat be a good thing?? :) Just a silly idea...

not if its 20 below and your freezing in your car! LOL

Try thinking about this.....
Is redlining your car as soon as you turn it on a good thing?
Its cool then.... right?

If you want to test your themostat

http://home.cogeco.ca/~mpereira16/therm.jpg

:) Thanks! Like I said, it was a silly idea... :)

I run a restrictor plate instead of a thermostat in my Integra, and temperatures are steady at 60deg or so... The engine is less prone to heatsoak from my observations.

But for stock ECU's the VTEC is less likely to engage at low temperatures... If it's too cold, I believe VTEC will never engage.