Heater Problems

[76/85 Truck]

This winter has been a cold one in Montana... and all winter I've dealt with a wimpy heater. At the end of fall, my blower motor went out, so I replaced that, but now it seems like I have no heat. When I put the switch on "low" it gets fairly hot by the floor, but when it is on high it basically blows cold air.

My first thought was that the blower motor is blowing the air past the heater core too fast, and as a result the air doesn't have time to heat up. What should I do? Should I replace the heater core with a new one and see if that will put more heat through?

Thanks for reading! Another thought was the core could be plugged or resisting flow somehow. Thanks in advance!

[tucsonjwt]

I like the simple approach. Get the engine good an hot, then hold the heater hoses - both to and from the heater. Are the hoses warm to hot? If yes, then the thermostat is opening and allowing heat to go to the heater. Next, pull back the carpet under the passenger side floor - is it wet and/or showing signs of previous leaking? If yes, then your heater core is leaking, or (possibly) you have leaking clamps/hoses on the other side of the firewall - so check for leaks around the heater hoses. If you need a new heater core get a copper one, not an aluminum one.
If the heater core is OK, then you may have a blend door that is not functioning
properly. This will likely require removal of the glove box and some ductwork - a bit of a hassle if you have A/C. You should be able to see the cable operate the blend door when you move the knob on the dash. No movement indicates disconnected/broken lever or cable. You can report back when you get to this point because others have more info on these repairs and where to get parts if you need them. Good Luck.

[sandorjoben]

The new blower motor may move more CFMs of air than the one that died. If the aforementioned hose test passes(it'll tell you if the core is plugged) and the flap that directs airflow between "HEAT" and "VENT" positions still functions properly(AND seals properly), it may very well be that the new motor is moving more air and some of it is just passing around the the core.

If you think this may be the case then you will need to make sure that any space immediately surrounding the heater core is properly sealed so that the air MUST pass through the fins of the core. Adhesive backed foam weatherstripping will do the job of filling any gaps.

If the heater core is broken, then I agree, find a copper replacement if you can over an aluminum one, the aluminum ones are touchy to install because they're so soft.

[76/85 Truck]

The flaps are functioning good and are sealed good.... It must be air passing around the core instad of through it like you said. I'll take a look at it tomorrow and see if I can seal it. The hoses do get warm... So it must be working right. Like I said, on low it blows very warm air. if it would blow that warm of air, but on the speed of "high" I would be extremely satisfied. I love my truck and would love to be warm instead of froze. Montana winters are harsh!! (Gotta love it here though!).

Thanks for all your help!!! Do you possibly have any pictures of where to seal it? Thanks!!!!!



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[tucsonjwt]

No sign of leaks under the carpet or around the hoses? Coolant level is OK?

[76/85 Truck]

Coolant level stays the same, under the carpet looks good, no leaky hoses, and I don't smell anything like antifreeze... Today I'm gunna warm it up and put the heater on high and hold the return hose that goes back to the intake manifold and make double sure it is also warm... Thanks for your help! Keep the suggestions coming!
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[tucsonjwt]

Good idea. The hose to the heater should be about the same temperature as the engine coolant - like 160-180 degrees - almost too hot to hold on to. The hose from the heater back to the engine should be a little cooler when the heater is on. You really only have 3 things to go wrong:
1) Heater core leaking or clogged up
2) Thermostat not opening all the way
3) Door/cable in duct work not opening all the way
You could have some odd things wrong, like duct work not connected properly, allowing air to not come out of the dash openings. Do you have A/C? If so, then you have some other gizmos like vacuum switches which could cause a problem. I think even no A/C trucks might have a vacuum switch somewhere.
On A/C trucks, the ducts under the dash just barely fit together (made too short, IMHO). I had ducts not fitting together properly and all of the heat/cooling went down to the floor. I got the ducts back together and put HVAC sticky silver aluminum tape on all the joints to keep it together.

[76/85 Truck]

My truck is a non-AC truck... The lever that moves the cable to adjust between defrost and floor works great and is nice and tight... If the thermostat wasn't functioning right, wouldn't my engine temperature be different? My truck has been running at 205 degrees since I got it. So I think the thermostat is okay.


Thanks for all the suggestions. Ill start my truck and see if the return hose is a little cooler... How would I check to see if around the core is sealed?
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[tucsonjwt]

The heater core comes out from inside the truck. The heater core is held inside the duct work with metal straps. The whole heater core/duct assembly gets bolted to the firewall - meets up with the blower motor assembly on the other side of the firewall. As I recall, there is foam weatherstripping around the heater core duct opening where it meets the firewall. I don't think you can see this weatherstripping very well unless you take the duct and heater core out - it gets compressed very flat when installed. You might be able to stick your hand up around the heater core duct work edges to see if air is blowing out around the edges. If your heater core leaks, then this weatherstripping is usually ruined, at least on the bottom. So, this weatherstripping is usually replaced (just get some from the hardware store). I don't think bad weatherstripping is enough to allow a lot of air sneak out of your ducts, assuming that the duct is still bolted tight to the firewall.
I replaced heater cores on all 3 of my squares because they leaked, not because they were clogged up. If your core is functioning properly, then when you put your hand close to the floor duct when the engine is hot you will almost burn your hand - think 160 degree + air. Air at the defrost ducts and upper dash ducts will be cooler because it travels farther (but will still be quite warm.)
A new heater core will cost about $30 - make sure it is copper if you buy one.

[sandorjoben]

Use this hastily mocked up pic as your reference for sealing the area around the core.

The black lines represent the black plastic heater core housing with the two flaps inside(one switches between Hot/Cold Vent mode and the other Heat/Defrost) from the firewall side view with the box removed. The Flap that selects between Hot/Cold Vent is directly behind the core as pictured, the bypass(cold vent passage) would be to the left of the core image and is cut off of the pic.

The Grey vertical MS Paint lines at either end of the core represent the screwed in core hold-downs.

The Red painted lines surrounding the core represent the area immediately surrounding it that need to be properly sealed, if it isn't the air will simply blow around these edges, then behind and out the heat passage. I ended up with an aftermarket aluminum core when mine failed and the fins were packed so tightly in the design that the air just went around the core when the heater was selected. So I used household adhesive backed grey foam weather stripping that was roughly 1/2" thick by 1" wide and it works just fine now. I've also attached an image of what I mean by household weatherstripping. Hope that clarifies what I meant somewhat.

Dave

[tucsonjwt]

So, when you select heat and put the fan on high, do you get a lot of force to the air that comes out of the dash, or is it just a weak flow of air?

[76/85 Truck]

The air coming from defrost is wayyy hotter than that coming from the floor. What should I make of that? The floor air is pretty much cold. The defrost is hot. But on low, the floor heat is also hot... Hmmm
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[tucsonjwt]

Disregard temperature for the moment. How much force do you have to the air? Is it blasting out like a hurricane when on high, regardless of temperature, or is the force low, like a gentle summer breeze? Sounds like the blower motor may not be getting the signal to run at high speed from the fan switch.

[76/85 Truck]

The air pressure is great. On high it blows super fast and hard
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[tucsonjwt]

Do you have bi level setting on the heater control, or can you put the lever halfway between heat and defrost? Try that.