My heater blows cold air

Question:

I have a 1994 Ford F-150 with a 302 V8 and 165,000 miles on the odometer. The truck runs fine except for the heater, which blows only cold air.

When the problem first surfaced I took it to a radiator shop, which applied pressure to clear a blockage. The heater worked fine for about six weeks, then the problem returned.

They then replaced the heater core, flushed and filled the coolant, and again it worked for a couple of months before slowly returning to cold air only. The thermostat has been replaced and the

engine operates within the normal limits on the gauge. The heater controls have been checked and are operating properly. Winter is coming and I’d like to have some heat. What will it take to fix this ongoing problem?

Answer:

It sounds as though there may be an ongoing source of debris that’s reaching the heater core or some part of the heating system, causing a blockage that’s preventing hot coolant from the engine from circulating through the heater core.

Two possible sources for core-clogging debris are the engine block (and/or cylinder heads) and radiator. Severe corrosion in either could put debris into the coolant that’s too small to get snagged elsewhere but too large to make it all the way through the heater core.

And, of course, once a blockage starts it normally progresses at an exponential rate because the blocked areas tend to trap more and more of the contaminants that caused the blockage to begin in the first place.

If the radiator’s innards are coming apart then the obvious solution is to replace it.

In the unlikely event that the block and/or head’s coolant passages are severely corroded there is no easy fix. You can clean much of the loose corrosion out with a very high pressure flush but unless you do something to stop or at least slow the progression of the corrosion the problem will simply return.

You could try coating as much of the coolant passages as possible with POR 15 or a similar product.

I’m not sure how much time, effort and money I’d invest in the engine if that is the source of the problem given the relatively high mileage.

Did you inspect the old heater core when it was removed?

Cutting open the old core would likely reveal what’s clogging it up. Close inspection of the old coolant after it’s flushed out can also tell you a lot about what’s going on inside the vehicle’s cooling system.