Heat

[rwljpl]

My 85 has a/c which works fine. The heater will blow warm air for a few minutes, then it gets cool. There is no sign of a heater core leak. What this could this be? Since I get hot air for a few minutes, I assume the blend door is working. Could I have a problem with a vacuum line? I have never seen this kind of issue before. I was thinking about replacing the heater core, but don’t want to tear into that if it’s something else. TIA for any help.

 

[Scott91370]

Does the temp gauge say the coolant is getting hot/warm? If not the thermostat is probably stuck open and you may get a little heat.

Also, feel the hoses going into/out of the heater core. Once should be really hot the other slightly cooler. Could be a blockage and once the heat in that is in the water (from residual heating) in the core is pulled out you won't get any more heat.

 

[SirRobyn0]

Check the hoses like Scott says and make sure the engine is staying warmed up, but when we see this kind of thing at the shop 99% of time it is a clogged heater core. It maybe possible to clear the cloggage without replacing the heater core, I've got a method I can describe to you if you want, but truly the correct repair is to replace the core.

 

[rwljpl]

Temp gauge indicates the thermostat is working
I will check the hoses - but it seems like if I had blockage, I’d have a leak somewhere right?

 

[Bextreme04]

Temp gauge indicates the thermostat is working
I will check the hoses - but it seems like if I had blockage, I’d have a leak somewhere right?

No, a blockage just restricts the flow through the cooler, you won't have a leak from a blockage. You can sometimes disconnect the two hoses to the heater core and blow pressurized water through the opposite direction of normal flow. If its a blockage from sediment or debris that can often blow some of it out. It is really a bandaid though. I would recommend a full coolant system flush to clear the whole system of as much debris as possible. Then replace the heater core and fill the system with fresh 50/50 coolant mixture. If you had enough junk in the system to clog your cooler, chances are you will just clog your new one also without first flushing the system.

 

[rwljpl]

Ok thanks - good info!
I’ll be working on it this weekend and see how it goes!

 

[Matt69olds]

Check the simple stuff first, is there air in the cooling system? Heat that comes and goes is a classic symptom of a air pocket

 

[rwljpl]

How do I get an air pocket out of coolant system? The temp gauge does fluctuate a good bit.

 

[SirRobyn0]

This is true

Check the simple stuff first, is there air in the cooling system? Heat that comes and goes is a classic symptom of a air pocket

This is true but unless his coolant has been low recently, or he recently had the system open for repairs there is unlikely to be air, plus in general if the radiator is full these trucks usually purge the air out of the heater core pretty good on their own. You should still check for that possibility, I just don't think it's going to be the likely culprit.

 

[75gmck25]

The amount of heat through the heater plenum is controlled by a cable, so vacuum does not affect heat. If you remove the glovebox door you can see the top of the plenum and see the cable move the door. If you remove the vertical plastic plenum piece in the center (3 screws?) you can see the core and make sure nothing is blocked.

Assuming the engine is getting hot, it sounds like a blocked heater core.

Bruce

 

[MrMarty51]

When doing PMs on trucks and had the cooling system drained. I would be real careful and blow air through the heater core, after removing the heater hoses, then use the hose and push water through, followed by more air, go to the opposite hose connector with more water then more air, repeat until the water that is blowing out comes clean, and, it would always be surprising the amount of sediment that would blow out of them cores.
Just be careful on each application of the airhose when first pushing out the water, it will build a fair amount of pressure and We surely would not want to pop a heater core.
I know this has nothing to do with Your heating problem but, it sure could not hurt to give the inside of the heater core a good flushing.