Blocked heater 02 Mercury Cougar

I have an 01 Mercury Cougar Sport Coupe that we bought from a friend for not too much, just to use as a daily driver. It’s got a lot of miles on it, but it still gets good mileage, which combined with the fact that we didn’t pay much for it makes it an okay car for around town.

Here’s the problem. I live in Wyoming where it’s winter nine months out of the year, and the heater core is apparently blocked. I made it all through last winter with this problem, but now it’s intolerable. The kids call the car “the Red Icicle”. My mechanic diagnosed the problem as a blocked heater core and tells me the only solution is to replace it, which costs something like $500. I can’t work on the car as everything is too jammed under the hood to get access to anything.

Do I really need to replace the heater core? The whole car isn’t worth much. Is there something I can put in the cooling system that will unblock the core reliably?

There’s nothing you can add to the coolant that will unplug the heater core. But here’s something you can try. Remove both heater hoses from the heater core. In some manner attach two stand pipes to the heater core. Take some CLR or Lime-Away and fiil the stand pipes. Let this sit in the stand pipes for a few hours to see if disolves the rust/calcium causing the restriction. Then back flush the heater core. If this doesn’t work, then the heater core will require replacement if you want heat in the vehicle.


Good advice as always tester, one other thing to check is if there is a control valve that allows coolant to flow to the heater core that is not operational.

If your heater core is blocked, you should change it. Ford heater cores tend to leak if they are stressed, so forget about flushing it, which a good shop could do. The control valve is the first thing to check.

Good advice from all the posters. The last time I had this problem it turned out to be a bad temperature control valve. Just because the cable is moving does not mean that the unit is functioning inside.

I had the same issue in a 95 Caprice, and I was able to back flush the core with a garden hose. It was easy to do, but I did have to do it at least once a year to keep it working and wait for a day the hose was not frozen.LOL

As Tester instructed, if you can isolate the heater core from the rest of the system, Filling it with Muriatic acid overnight will remove the lime/calcium deposits. Do NOT do this if the core is made from aluminum as the acid will dissolve it too! But it will not harm copper, brass, lead…The acid, dilute hydrochloric, is normally used to clean masonry…

What about something easily available like Lime-Away? Wouldn’t that have the same result? It’s supposed to be easy on most metals…?? I’m not sure what’s more expensive, but I always seem to have a bottle or two of that stuff in every bathroom, and then another in the kitchen for good measure. Wife hates mineral deposits everywhere, and since this is Texas, we’ve got them in abundance.

Lime-A-Way is simply a solution of phosphoric acid, just like Naval Jelly which is the same stuff in gel form. It is less corrosive than muriatic (hydrochloric) acid but must also not be used on aluminum. Either of these acids can get the job done.