I have a 2001 Civic 4dr EX that has 212,000 miles on it. On my last trip across Iowa, the analog temperature gauge held steady right in the middle. After 300 miles @ ~72mph and 3400rpm, I clutched, coasted 1/2 mile, and came to a stop. When I started again, the temperature gauge pegged out, and my cabin heater went freezing cold. I figured that it was my thermostat, and since it was time to change the water pump and timing belt again, I did all three. I still have the same problem. Now the engine temperature gauge comes up to normal temperature pretty fast, but the cabin takes a good 10 minutes/miles to get warm.
I fIgure that I have blockage in the radiator, but I don't understand why the cabin temperature gets freezing cold after coasting, and then starting up again, and why it takes so long to warm up.
If anyone has had this issue before and could save me the learning curve, that would be great, however, any help on this one would be greatly appreciated.
THX.

 

Has the coolant system been bled? Air in the system can cause the heater to not function properly.

 

It would also cause the engine to overheat, even at idle.

I would guess is has to do with the heater core, but I'm at a loss at the moment. If I think of something, I'll chime in later.

 

After looking over the service manual yesterday and thinking it over, the cold rush of air after clutching/idling for a few minutes, followed be revving up again, might be due to flow of coolant through the "water bypass hose" rather than the radiator at low rpm's(path of least resistance). Now I don't pretend to be a gear-head, and I certainly don't understand all the different paths and directions that the coolant is flowing through the engine block; but let me ask a fundamental question, and I think I might know the answer:
When the thermostat is closed (cold engine) the water pump is obviously still running, so doesn't the coolant bypass the radiator, flowing throught the "water bypass hose", and right back into the engine block, until the engine warms up and the thermostat opens?
Now if this is a true statement, then when the flow through the radiator is restricted, then at high rpm's, there may be just enough flow to cool the engine, but not at low rpm's. I guess I'll start with the radiator to see if I'm getting the proper flow. If you know the answer to my question, please let me know, and/or let me know if I should do anything differently.
THX.

 

I am guessing that if there is that much restriction in the radiator, you would have overheating condition at higher RPM since the engine is creating more heat.

If coolant is used at proper mixture ratio since new, I doubt if there is that much blockage inside the radiator. If for some reason, "stop leak" had been used in the past, it could be different story.

There is a valve that controls flow to the heater core, I believe. I would make sure that is functioning properly.

 

I checked the valve on the heater block and it is functioning (min to max).
It's going to be a busy week but I hope to check more...I'll start by seeing if there is any trapped air. Then I'll start checking flows.
Give me a week and I'll get back with my findings.
THX.

 

Bleed the cooling system. A stuck closed thermostat would prevent coolant flow through the radiator, not the heater core.

 

Yes, the heater core should always flow unless there is a bubble of air in the engine. In that case the heater may only flow when revved up and the coolant starts sloshing around. If you keep getting air in despite repeated bleeding, there may be a small leak in the head gasket.

 

I don't have the equipment to check for a blown head gasket so it is scheduled to go in Thursday morning.
Thanks for all the help.

 

Thanks for the help!
You nailed it.
I had a small leak in the head gasket in three of the four clylinders.
It was leaking from two of the coolant ports into the combustion chamber on each cylinder. Only a few thousands on gasket thickness missing, but enough to cause the air-lock at low rpm's (causing loss of flow through the heater core and overheating on the engine, enough to warp the head) and still go unoticed when checking coolant level in the radiator.
Thanks again!