94 GMC Heater Problem ( & Winters Coming!!)

[67Blacksheep]

Gents-

I need some expertise fast! I just purchased a 94 GMC Sierra extended cab. ( After sadly having to part with my 67!! ) I LOVE this truck, but am having heater issues. It has the digital controls, and all seem to work. AC, Defrost etc. My problem is that it does not blow hot air. When I switch from cold to hot, the temperature does change, but only to a very luke warm temp. I have the proper thermostat in, the heater core is getting hot, and all appears to be normal, but it won't produce enough hot air to defrost the window, or warm up the cab. Friends with similiar trucks say that theirs will blow you out of the cab even in the coldest of weather. With the Ohio winter soon to come, I desperately need to get this resolved. Anyone have any advice on this?? Where is the valve that controls the air passage over the heater core? How does a fella even check this sort of thing? Any help at all would be greatly appreciated!!


Thanks All,
Blacksheep

[Piston]

Could have a couple of different issues.

Heater problem:
do a quick test for me... Warm the engine up to normal operating temp. Feel the heater hoses going to the heater core.
Let me know if they are:
Both hot
Inlet hot / outlet cold
both cold

From this we'll be able to tell exactly whats going on in the heater core.

Air flow:
Does the blower motor sound louder than normal?
Do you have a moldy oder in the cab on hot days?

One common factor is the Evaporator core becomes restricted with mold/debris/paper (napkins) which reduces the air coming out of the vents.

Let me know the results of the hose test, then we'll go from there.

[67Blacksheep]

Piston-

Both heater hoses are hot. In and out. The blowor motor does make a little noise, but blows a considerable volume of air. Like I said, if you feel the heater core housing with your hand it is really hot, the air just doesn't get the temperature. Thanks for the reply...I'll wait to hear back from you.

[Piston]

Ok good deal. Since both hoses are hot, we know the basic system is working! Hot coolant is flowing through the heater core and should be blowing hot air. We know this because there is HOT coolant in the return hose (no cooling from the heater core)

So, we "Should" have hot air... IF the blend door is forcing airflow through the heater core, which it is not (or atleast all of the air). First step is to find out of the heater blend door is actually moving or not. (we already know its not, but why?)

Check to see if you hear/feel the blend door motor at all. You'll be able to hear/feel the gears turning inside as you move the heater arrow from hot to cold. Put your hand directly on the motor to feel.

Here's a few diagrams to go by. I've included a R&R procedure incase you need to remove the motor and manually move the door! If the door is not moving, remove the motor and manually move the door to verify operation. If the door moves fine you could have a blend door motor failure or control head failure (VERY common).

[67Blacksheep]

Thanks again Piston. I went as far today as to remove the glove box liner and observe the little motor turning as I moved the temp range from Cold to Hot and back. It appears as though it goes from point to point. The temperature does change from cold to slightly less cold, but never to "Hot". How would the control head figure in to this??

I suppose the next step is to manually tear it down and see what's inside. It sure suprised me to see that motor turn though, I thought that the "blend door" was it for sure. Probably not though since that moves from hot to cold, right?

[Piston]

It's still a problem in the blend door though. Since there are only 3 bolts holding the motor on, I usually pull the motor and move the door manually. If air is being forced ONLY through the heater core you'll notice the heater hose return will be cooler than the inlet. Basicly there is no heat exchange going on.

There are limit switches inside the motor that turn it off when a limit is reached. If this is off for some reason, it will stop the motor before it actually closes the door, OR... if a peice of debris is in the way (Pens are usually the problem) it will lodge partially in the door's path and keep it from moving.

Manually close the door and see what you have then. Make sure when you close the door, you hear a solid thud in each direction.

[Piston]

I should have asked before but... Do you "know" for sure the engine temp is above 185 when checking all this? Does it produce "ok" heat when idling for a long period of time, but cools off when moving?

BTW: Figure 65 is the blend door. The other one is for defrost/vent.

[67Blacksheep]

I have made sure the temperature is up to snuff. I even changed the thermostat just because I have only had her a short while. I will take the motor out as you suggested. ( It is the one under the glove box liner, right? ) The truck was owned by a guy who did construction, so there could be a good chance that a pen or nail has slipped down there and is lodged. I'll try to pull it out tomorrow evening. I'll let you know what I find. Thanks again for all your help.


-Blacksheep


P.S.- You seem to have a lot of knowledge on these models. Have you tried any performance oriented tricks on one?? I had a pretty wicked 67, I just wandered if there is any quick HP gains to be had on these TBI 350s. Let me know, eh?

[67Blacksheep]

Piston-

Now I'm VERY confused. I took the motor off tonight. No obstructions in the way of the door. The motor turns point to point, and all seems normal. I dropped the bottom pan to expose the heater core and the blend door itself. Someone has been in there before, but the only "damage" is a small chunk of plastic busted out of the chanel ( not in a spot that would allow in- proper mixture. ) The heater core gets hot as fire when you run the vehicle, and once again all seems to be working correctly. Why is this thing not blowing hot air??? Am I telling you what you need to know to diagnose this???

[Piston]

Try turning the blower motor on while you have the bottom cover removed. Is there massive air flow coming out?

We know the heater core is hot so that rules out any possibility of coolant/heater core or thermostat problems.

The air isnt being diverted through the heater core (which is done by the blend door only) If you dont have good airflow through the heater core area you'll need to follow the airflow upstream and find out whats causing it not to blow through.. which in the case the only thing is the blend door lol.

Can you see the blend door? does it actually close off the passage way?

[67Blacksheep]

Yep...the blend door is completely closed. No obstructions. And...the blower motor sends a good amount of air through when I had tried it with the system open. I held the heater core up to the casing, and the air flowed through it. By this time, the coolant had cooled down a bit, so the air wasn't that hot, but it was there. I am getting nervous that I will be stuck with no defroster for the winter. Then I'll have to drive the wife's mini van....good god man you have to help me!

[Piston]

hehe dont worry... we're along way from minivan status!

Something you mentioned struck me as odd, You say that both heater hoses were hot right? Was this test done with the blower motor on high and the heater on?

Here's where im heading with this... A restricted heater core will act "OK" at lower fan speeds and hot coolant temp. Turn the fan on high and now you have luke warm air coming out. The heater core doesnt have the capacity to handle that kind of air flow if its restricted. (coolant flow)

Sorry I should have been more clear on the heater hose test... You MUST have the heater on high fan speed for this test.
I'm willing to bet you'll find that the exhaust hose is now cool or cooler than the inlet hose. What coolant the heater core is allowing through is being cooled extremely well which is why the outlet hose will be cold/cooler.