Any ideas as to why the heater in my '84 4wd GL only stays hot for a minute or so after turning on the fan?? It starts hot, then peters out after a minute or so... same thing happens in B/L, Vent and "Max A/C". (Car is non-A/C equipped, but this setting blasts air nonetheless.) If I turn off the fan and then back on a few minutes later, it's warm again, but then the cycle repeats...

 

My first guess would be restricted flow through the core. It gets warm while no air is blowing through it, but not enough hot water is flowing through it to keep it hot when cold air is coming across it. Just my first impression.

I agree. If you were to feel the hoses going to the heater, you would find one much cooler than the other when it starts blowing the colder air. Classic sign of a restricted heater core.

Sometimes, you can back-flush them by swapping the hoses around, (be sure to swap them back). Sometimes, you can use some cooling system flush to clean them out.

Doesn't always work, but it's a quick try.

As far as using any system flush, I've found that NAPA's "Mac's Radiator Cleanser" works the best of any. I've left it in longer than the 20 minutes that the directions say to on a few vehicles, doesn't hurt anything doing so.

I agree. If you were to feel the hoses going to the heater, you would find one much cooler than the other when it starts blowing the colder air. Classic sign of a restricted heater core.

 

I like your idea of checking the hose temps going into the core to see if the flow is flowing. I assume if flushes don't work, a new heater core is in order? :-\

Sadly, Yes....

Run a good flush or coolant system cleaner in it first, and if that doesn't work remove both heater hoses to your heater core, go buy a garden hose repair end and install it on a short piece of heater hose. Screw your garden hose on and clamp the little hose you made onto your heater core. Turn on the water just a little, and gradually increase the flow/pressure untill you get a solid flow out of the other heater core pipe. Then put the hose on the other heater hose pipe to reverse the flow. Keep in mind that the pressure in your coolant system is usually around 13-16 psi, and your house water pressure could be 40-80 psi. SO if you just crank it up you could very well blow your heater core and fill your floorboards with water! I usually turn the water up and krimp the hose with my hand, slowly opening it up to incease flow. I have done this on my last 6 vehicle and it worked!

Thanks for the tips, guys! I'll get some flush on the way home tonight, and let you know how it turns out.

 

-S

This sounds like exactly what my hatch is doing. I've done the "water hose-reverse fulsh" and it got better but not fully. I do have hot water on both hoses, so I'm suspecting that maybe the heat control valve is shot.....

Good luck..