So, we've had our first real white winter in Arkansas and my heater's not working in my 88 Dak. Joy. I've backflushed it with a water hose a few times, drained and refilled the system, and replaced hoses. Both hoses are hot, and both are have good pressure. The fans work on all settings and the blend door seems to be fine; I've searched the forums and can't find any info on what it may be, the heater is fine on low, or even off the heat sort of just creeps in, but if you turn the heater up the heat just slowly fades out and then the heater has to be be flushed and then it works for about a day and slowly fades out again. Any thoughts? Maybe an air leak somewhere? A pinhole in the core itself?

 

If you had a hole in the heater core you would more than likely have coolant/water on your floor. Maybe you have air in the system from flushing it.

 

I considered that, and have bled the system and burped the hoses.

 

try isolating the heater core from rest of the cooling system and flush just the heater core both ways. i had a very similar problem two years ago and it was a build up of chunky stuff that looked like rust scale that was blocking the heater core tubes. just flushing my cooling system didnt get it out

 

That's what I've been doing, disconnecting the heater hoses and attaching a water hose to the heater outlet and inlet and flushing one way and then flushing the other, lots of orange gunk the first few times and now it's generally just water and some coolant bubbles, I've been tempted to try filling the heater core with a mild acid or cleaner like CLR and then blowing it out with compressed air and listening to whistling. I figure it could be a very small leak on the core that opens when the temperature changes and closes when it goes back again.

 

i dont think it could be a leak of any kind because the cooling system is under pressure, if there was a leak you wouldnt get air in, you would get coolant out. it sounds like a problem with coolant flow into the heater core. im wondering if maybe there is a problem with the the heater control valve, maybe a loss of vacuum or the valve itsnt opening all the way. if you try the air compressor route make sure to set your pressure regulator to about ten psi and fill the core with water and give the air few short shots both ways through the heater core. any more pressure and you risk blowing out the heater core.

 

You can try maybe CLR or paint thinner. Just make sure you wash all of the chemicals out.
The next option is to try a self car wash style pressurized hose. It is possible for it to be to strong and blow out the heater core. I've done it with no problems by the way.

 

Hmm, well, I can say I'm stumped. I was under the impression there wasn't a heater control valve on my Dak and that the coolant constantly circulates. It's not in the Haynes manual, or anywhere that I've traced the lines to, where would it be then? I hadn't intended to use more than five or ten PSI, but I hadn't thought of the self carwash idea. I'll see about using some chemicals today when it warms up and try the compressor to see if it helps.

 

On my stepson's 1988, the heater control valve is in the heater inlet hose. It's the vacuum operated valve close to the brake booster.

Your problem may be a plugged valve (easy enough to fix - use a 5/8" brass garden hose repair kit and place it where the valve is now for now) or it may be bad vacuum (again, easy to fix - see above). In any case, the one on his truck is bypassed for the winter ...

We have another problem also. Our thermostat seems to be hung open - it barely gets above the "C" tick mark on the gauge. We'll be doing the thermostat Saturday, along with a back flush of the heater core.

RwP

 

Just an update. I moved and didn't really have much time to do any work on my Dak', but now that I have time to work on things it's warmed up again, lol. While I was replacing my inner tie-rods I thought about tracing all the lines and still couldn't find the heater control valve. There's no valves or anything near my brake booster, just the hoses and the heater inlet and outlet. If it's in the cab that would make more sense but I'd like some confirmation before I tear into the dash to try and find a control valve, especially when I don't necessarily need the heater anymore and have other things to work on.