My Truck's Gots No Heat!

2000DV4.8 · November 1, 2008

Let me start off by saying I don't know shit about anything in the HVAC system of my truck. And I mean nothing. Really. So deal with my ignorance, please?

First, and most important problem: The heat in my truck is virtually non-existent. After about 10 minutes, it will blow warm air (not hot), but only if it's above 40-45 degrees. Anything under that (40 and below), it blows kind of lukewarm air, barely warmer than the inside temperature of the truck. Well, not having heat makes me cold, and I don't like to be cold at 6:45 in the morning. I've got a half hour commute to school, and that whole time, it just plain doesn't get hot, or even warm enough to feel warm on my face. Any ideas as to what could be causing this?

Second problem: The passenger side vents down by the floor have never blown hot since I've had the truck, and my Dad always complains on ski trips. I don't mind the fact that it blows cold, but the complaining gets old.

Third: This may be normal, but it might not. Compared to every other car I've been in, the HVAC system has never really blown with a lot of 'force'. If you stick your hand in front of a vent, the difference between '2' and '5' is negligible, but the fan whirs like a MOFO. Changing the cabin filters had no change on this last year. This might just be a characteristic of the HVAC system in these trucks, but I have no idea.

My Dad's a mechanical engineer and specializes in HVAC, so he knows more about this stuff than I do, and tomorrow I'm going to pull apart the dash and see if we can find anything that's obviously broken, or not functioning properly, but if anyone has experienced any of these problems, and found a fix, let me know. (holy run-on sencence )

Thanks in advance.

97SierraGMC · November 1, 2008

i would almost say its something electrical...

maybe like the hvac controls themselves? since you not only have no hot air it maybe isnt sensing what you have want to be set at and also the fan isnt blowing right... (maybe check out the fan too)

minibike132 · November 1, 2008

Let us start with the main problem, no heat at all. Did this just start happening after the water pump? If so I'd say you got a big air pocket in there.

Bostonsfavson · November 1, 2008

Well, if you're occasionally as stupid as I am, let me ask you this question: have you checked your antifreeze lately? I had a similar issue on an old DeVille , and it turns out that the damn thing was just low on antifreeze. As soon as I filled it up, instant heat! Not my proudest moment Good luck Dan!

97SierraGMC · November 1, 2008

he said its been happening for a while even before the water pump thats why i dont think its that unless its the heatercore or whatever it actually gets the heat off of..

2000DV4.8 · November 1, 2008

Nope, this was all present before the water pump change (which had no effect on anything). I meant to put that in the original post, but I'm freaking tired.

And coolant is full.

ujtruck · November 1, 2008

You have too many mods in your truck and that's why no heat

Alright this is what I found on the net..:

Q. My heater does not get hot. The line going into the heater core is hot from the engine but cold on the return line to the radiator. Is there a switch that opens this up are could it be stopped up?

A. Chevy usually has a heater control valve mounted on the right rear corner of the intake manifold, the corner near the heater core. This turns on and off the flow to the heater core. there is a cable attached to it to help figuring out where it is. Usually this goes bad and won't open to allow hot coolant to the heater core. It's easily replaced and not expensive.

There is also the possibility that the heater core itself is clogged, either due to rust and corrosion or excessive use of stop leak.

Dan, you got a hurry up its getting cold fast.

spackelman · November 1, 2008

Have you ever changed the cabin air filters?

marko · November 1, 2008

Okay Dan, here's how it works:

Your heating system is like the heating system in most houses, a boiler heats the water and it runs over to the wall radiator, and there it passes thru a series of tubes, and air moves over the tubes, and takes the heat from the radiator and puts it in the air.

In your car's case the engine is the boiler and you have 2 radiators: one at the front of the vehicle called the radiator, the other insider the heater box called the heater core. Engine coolant circulates in the water jackets in the engine and takes some heat away from the combustion process, then it's pumped by the water pump to the radiator (and the heater core sometimes if the dial is set on warm) and the heat is taken out of the water and put into the air by the action of the fans blowing air over the radiator/heater core.

The radiator is capable of radiating more heat than the engine is capable of producing. If the coolant is cooled down too much, the engine won't operate efficiently. For this reason, there is a thermostat in the engine (at the engine end of the upper rad hose) that opens and closes. If the coolant is cold in the engine, i.e., under about 180 degrees, the thermostat stays closed and does not allow coolant to escape the engine and flow to the radiator. Once the coolant heats up over 180, the thermostat opens and lets the coolant flow into the radiator. The water pump operates constantly, as it is belt driven, but just cavitates if the thermostat is closed.

Coolant flows into the rad thru the upper hose and back to the engine thru the lower hose. Coolant flows to the heater hose thru a diversion hose drawing off the top of the engine, and back to the water pump. The heater hoses are much longer than the rad hoses, and thinner, they flow less coolant (obviously).

Temperature in the heater core is controlled by the amount of coolant which is allowed to pass thru the heater core. This is controlled by a gate valve, which is actuated either by a cable (manual temperature control) or by a solenoid (climate control). Find the gate valve and control cable on the heater hose, and actuate it by moving the temperature control and watch if the cable moves the gate valve.

Your cooling system uses coolant. The level can go down merely due to evaporation. This should be a very, very slow process but you could lose a liter or two over a number of years due to evaporation. Anything more than that and you have a leak.

The heater draws coolant from the top of the engine, to a skinny-hosed loop far from the engine, so if that circuit fills with air, it can create a "vapor lock" and coolant won't flow to it. More often than not if the coolant level in the engine is low there is enuf for the coolant to flow thru the engine, thermostat, and radiator, but there isn't enuf for it to make it all the way to the heater core.

You may have had two problems: first, the coolant level may have been low because your water pump was leaking. So, you changed the water pump and filled up the coolant. But, in a system as large and loopy as a car's cooling system, there are bound to be air pockets, and you can expect to have to let the system "burp" or "settle" for a week or so. Keep an eye on your coolant level.

If your heater core is still cold, there are four possible additional problems: first, the thermostat is stuck open, allowing coolant circulation at all times, thus never allowing the engine to warm up. You will be able to tell this from your temperature gauge, it shud be somewhere in the middle. If it's too low, change out your thermostat (and don't put it in upside down).

Second, the cable could have slipped off the gate valve or it may be seized. This is not likely but it's worth checking.

Third, there are leaves/crud on the outside of your heater core, which are keeping air from flowing over the heater core and taking heat away from it. You will have to visually inspect this and vacuum out the heater core. In addition, your truck may have a cabin air filter (search the forum for the thread) and if that air filter is blocked you won't get any air moving over the heater core either. If it's dirty/blocked, replace it.

Fourth, the inside of your heater core could be blocked with corrosion/crud, keeping coolant from circulating thru it. If the rest of your cooling system is okay this shud not be a problem, but you never know - fix this problem last if all the other ones check out.

Sounds like you'll be spending some more time in your engine compartment......

minibike132 · November 1, 2008

Wow that is quite a tutorial . I just have one modification to it and ujtruck's post. There is not a heater control valve anymore. The coolant flows through the heater core anytime the engine is running. The temperature is controlled by door position inside the air box. The temp doors change position based on the need for heat or a/c.

That said you could still have an air pocket. Do what ujtruck said and warm it up then feel the heater hoses. Remember those are the skinny ones that go into the cab of the truck. They should both be hot, not much temp difference.

If they are both hot we can move inside the truck.

VMax2007 · November 1, 2008

I'd check your canooder valve.

(sorry, like you, I don't know squat about automotive HVAC systems, but the PW in me made me post )

marko · November 1, 2008

I'd check your canooder valve.

(sorry, like you, I don't know squat about automotive HVAC systems, but the PW in me made me post )

Living in Minnesota as you do, it might be adviseable to study up. Could come in handy you know.

VMax2007 · November 1, 2008

I'd check your canooder valve.

(sorry, like you, I don't know squat about automotive HVAC systems, but the PW in me made me post )

Living in Minnesota as you do, it might be adviseable to study up. Could come in handy you know.

Nah, those days are over for me. Only reg maintenance items for me anymore. The rest gets done by the pros. I am getting lazy in my old age.

2000DV4.8 · November 1, 2008

Have you ever changed the cabin air filters?

As I mentioned in the OP, I changed them last year (I think it was last December).

You may have had two problems: first, the coolant level may have been low because your water pump was leaking. So, you changed the water pump and filled up the coolant. But, in a system as large and loopy as a car's cooling system, there are bound to be air pockets, and you can expect to have to let the system "burp" or "settle" for a week or so. Keep an eye on your coolant level.
The coolant level was BARELY low before I changed the pump, and I'm pretty sure all the air is out of the system now, as I had to add coolant after the first 3 or 4 times I drove the truck, but it has stayed at a steady level since Wednesday.

If your heater core is still cold, there are four possible additional problems: first, the thermostat is stuck open, allowing coolant circulation at all times, thus never allowing the engine to warm up. You will be able to tell this from your temperature gauge, it shud be somewhere in the middle. If it's too low, change out your thermostat (and don't put it in upside down).

The thermostat was replaced in late July, and the engine warms up as it always has (maybe even quicker), and to the temperature it always has.

Third, there are leaves/crud on the outside of your heater core, which are keeping air from flowing over the heater core and taking heat away from it. You will have to visually inspect this and vacuum out the heater core. In addition, your truck may have a cabin air filter (search the forum for the thread) and if that air filter is blocked you won't get any air moving over the heater core either. If it's dirty/blocked, replace it.

Cabin filters were replaced under a year ago, so I don't imagine they're that bad, but I'll check.

Sounds like you'll be spending some more time in your engine compartment......

What's wrong with that?

Wow that is quite a tutorial . I just have one modification to it and ujtruck's post. There is not a heater control valve anymore. The coolant flows through the heater core anytime the engine is running. The temperature is controlled by door position inside the air box. The temp doors change position based on the need for heat or a/c.

That said you could still have an air pocket. Do what ujtruck said and warm it up then feel the heater hoses. Remember those are the skinny ones that go into the cab of the truck. They should both be hot, not much temp difference.

If they are both hot we can move inside the truck.

OK I'll do that as soon as I eat breakfast.

Thanks for the help everyone.