Cooling problem confusing me (heater core) | Ford Explorer Forums - Serious Explorations

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Cooling problem confusing me (heater core)


Well-Known Member
September 11, 2007
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I can’t figure this one out. BEFORE I did any work on the cooling system, the heater was blowing warm air, but not as hot as it should. Lukewarm air is more like but, but it would heat the interior sufficiently given time. Now it doesn’t AFTER rehabbing the cooling system? How come?

The before: Rusty brown coolant, engine not getting hot all over, heater working, temp gauge inoperable.

What I did:

Completely drain the old coolant, used a Prestone flush kit (hooked it up to the inlet hose going into the heater core) then filled with water, let it drain. Whew, that was a lot of crud! Then poured a bottle of Zerex coolant cleaner, ran it for 10 minutes.That was when I found the thermostat was no good – I suspect it was stuck closed as the radiator hoses were cool; the upper hose never get pressurized. So I have been running the radiator cleaner partially through the system. Great. What a waste.Then I drained the water with the cleaner and replaced the thermostat. Flushed it again to get all of the old coolant and the cleaner out.


Filled the radiator with the new coolant, then started ‘er up. The engine was getting hot everywhere, so I thought the new thermostat must be doing its job already (I verified the new thermostat in a pot of boiling water beforehand, so I know it works) and proceed to feel the hoses. They all were equally hot. When I went in the cab, I saw the temp gauge was functional! It was sitting dead center, where previously, it barely registered anything. I at that time thought everything was well. Turned on the heater, and…nothing. Cold air. I checked the heater hoses, and they were good.

How did this happen?

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Could have a clogged / bad heater core. They are amazingly easy to replace. Remove the glove box, the 4 screws. Someone at Ford was thinking on this design.

how long have you ran it so far? you could also still have air in the system

I agree with the clogged heater core idea above. When you flushed all that junk out of your system, some of it probably plugged your heater core. Just change it -- it's a 30 minute job and the heater core should run about $20.

Some air, yes...wouldn't air in the system cause overheating?

I attached the tee to the inlet going to the heater core...I probably plugged it up on the second job. The first flush, it was still fine. Go figure. Off to replace it, hope it'll be above freezing this weekend...

I had the same problem, went crazy for a while, got full heat back after changing heater core. Even though heater core was hot & flowing freely it seemed to have an even coat of corrosion which prevented heat from transferring to passenger compartment. I purchased mine from Check out my post for more info:

Hope it helps save you some time & money....

Perhaps your new thermostat is stuck open with the crud from the old coolant? I havd the same symptoms on my Intrepid and the thermostat would get stuck open so I got the thermostat replaced and it fixed the problem.