Reader Follow-Up—

May 1, 2011 | By Dave Stanton

More Advice for a Heater that Blows Cold Air

I read about Mr. Siddell’s heater problem in the December Mechanic on Duty section and it reminded me of a similar problem I once had. (While his 1994 Ford F-150 with a 302V-8 and 165,000 miles on the odometer runs fine, the truck’s heater will only produce cold air. He has used pressure to clear the system, replaced the heater core and checked the heater controls. The system produced heat for a few months and then went back to blowing cold air only.)

In 1980, I bought a 1974 Ford Country Squire wagon with a 400 engine. It had only 52,000 miles on it. The guy I bought it from said he had changed the oil regularly, but did nothing else to it. When I took off the radiator cap, there was stuff floating in the antifreeze. After buying it, I tried the heater and there was no heat.

I changed the antifreeze and flushed it out as best I could without professional equipment. I put the garden hose to the heater core inlet and outlet, flushing it back and forth and a lot of gunk came out. With new antifreeze, I had heat for one day. I pulled the hoses, used the garden hose again and I had heat for a week—sound familiar?

Adding an Antifreeze Filter

The solution I came up with was to add an in-line antifreeze filter to the heater hose just ahead of the heater core.

Now this wasn’t something I could just buy off the shelf. A friend of mine who had access to a machine shop made me a slick adapter that had metal tubes that fit in-line with the heater hose, and allowed a spin-on oil filter to be screwed onto it so the oil filter filtered the antifreeze. I had heat for two weeks, changed the filter, and then had heat for a year, changed the filter again, and after two or three years of no further problems I took the filter off, as it had finally caught all the dirt in the cooling system. Looking into the radiator, I’ve never seen antifreeze stay so clear!

If you try this, here are a few tips:

•Buy an oil filter that doesn’t have a built-in check valve meant to prevent oil backflow (drain down) when the engine is off. To determine this, look at the circle of holes in the top of the filter—if the holes are covered with rubber inside, the rubber is the check valve. Filters with a check valve will still work, but at an idle there may not be enough water pressure to open the valve, so there would be no heat at an idle.

• Insulate the filter! On really cold days at stop lights, heat was reduced by heat escaping from the surface of the filter—I wrapped some plastic foam around it, held with it duct tape and took care of that problem. Just be sure the tape and foam don’t hit the exhaust manifold as they could melt or catch fire.

• The restriction added by the filter slightly reduced heat when running at an idle, but it was not a big problem.

• Filters that have a relief valve that bypasses oil around the filter should it become plugged, should be avoided as they may also bypass dirt to the heater that the filter has taken out.

Another thought is that some 4WD GMCS-15 Jimmys and Chevy Bronco IIs from the late 1980s/early 1990s with the V6 engine had the oil filter up behind the left headlight with hoses running down to where the filter would normally be on the engine block. I had a ’91 that had this. It may be possible to cobble one of these filters (from a salvage yard) in-line with your heater core, but I’m thinking the input and output lines may not be big enough to carry enough antifreeze to keep you warm. I don’t remember how big the lines were.

—Dave Stanton