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This is actually a 2004 Acura EL, but same as a 1.7 Si
Around 156000 km or almost 100k miles

Car does not blow hot air until it is driven, once I come to a red light for extended period of time with fan at full speed, air temperature will start to decrease until it becomes cold and this repeats.

I know most people would say this is low coolant problem.
I checked the overfill tank. It's beyond the full mark and up to the top of the filler neck. ie. I can see the coolant when taking the cover off the filler.

After warming up the car and having the fan blow at max. The fan blower in front of the engine do not turn on. I felt the coolant return line that goes from the top of engine back to the engine(I think that's the flow) did not get warm at all.

I have a few questions before I take it upon myself to fix it or take it to my mechanic.

1. Is the fact that the coolant is up to the filler neck an indication of a clog?
2. Can bad thermostat cause this?
3. The coolant has not been replaced before and from the manual, it says doesn't need replacement till 192000km.
4. What colour is the coolant supposed to be? I know toyota's is red, but not too sure about honda.
5. Should there be a burnt smell in the coolant? If I have a headgasket problem, shouldn't there be no coolant in the overfill tank?

Any other suggestions of what to check would be great. Thanks all.

 
Rep Power: 811

old honda coolant is green, new is blue. headgasket can cause coolant to overflow out of the reserve tank sometimes. other times it burns coolant in the engine. start with easy fixes first like thermostat and new radiator cap. its possible you also have a clogged radiator or heater core.

 
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ok, so the coolant colour is right.
What method is there to unclog radiator or heater core?

 
Rep Power: 811

well you can try a flush using one of those kits, but usually its hard to get all the deposits out if theres rust and other buildup. worth a try tho.

 
Rep Power: 0

If your car has never had the cooling system opened, there is no reason for it to be gunked up. I can guarantee it's your head-gasket as I've had this exact same problem. I would drain the coolant from the reservoir so it was back to the correct level, then bleed the system. A few weeks later, the problem was back. If I had the car run til the radiator kicked on, you could see the coolant flowing into the radiator (cap off), there was tiny, I mean tiny little bubbles coming up. This is because the head gasket was leaking and the compression from the engine was forcing air past the gasket and into the cooling system. After you get so much air into the system, the coolant has to go somewhere, right into the resi. Problem is gone after replacing the head-gasket.

You can try those "smaller" things, but it's a waste of time. I replaced the cap and thermostat as well, to no avail. Head-gasket FTW.