Another issue. When I'm driving at least 50-ish, it pumps out plenty of warm air, when I slow to less than 40 the heat cools off and can just blow cold, same if I go somewhere and park with the motor on, it blows cold air. What could the problem be?

 

Put a new thermostat in and you should be good. Sounds like yours has failed, thus allowing full flow through it.

 

Hey I was just about to post a new topic about my 2004 Blazer but then i saw this thread and its very close to what problems im having.

My blazer was doing the exact same thing, it was pumping out less and less heat when i was parked but it would pump lots of heat when i was driving. After about a week of that i noticed the heat would be less and less even when i was driving. Now it doesnt matter if im parked or driving or what speed im driving, whether im on the highway or in the city I have just barely the slightest warm air blowing.

I talked to a friend of mine and he said my heater core might be plugged. Im not sure what it would be plugged with or how it gets plugged or if thats even the problem. I was thinking it could be the thermostat but i dont want to make any repairs unless im sure of the problem as im between jobs right now and im short on cash.

If it helps anyone to diagnose the problem I know that my engine temp has not changed since the problem started, and I had my rad fluid checked and its good till -45 celcius, and the blazer still runs fine and there are not any warning lights on.

If anyone has any ideas, im welcome to listen. Thanks guys

 

* and girls too.

 

This is definitely a plugged or partially plugged heater core. It is a common problem on these trucks and luckily is easy to fix.

There can be several causes as to why it happens. Do a look through for information on the dexcool coolant that is used in these trucks for one of the causes.

You are going to need to flush out the heater core somehow to get your heat working again.
The best way to do it is to pull off both the inlet and outlet lines going to the heater core. One attaches to the water pump and is the return line. The other attaches to the block on the passenger side of the engine. Both run to the core through the firewall on the passenger side of the truck.

Normally you would flush the core out with a garden hose through first the return line then through the incoming line.

However being winter time and freezing outside (it is here anyways) that may not be a feasible option for you. There are cold weather alternatives though if you want to do it yourself and not take it in to get done.

The best it's below freezing outside alternative is the wet vac method. To do this you fill a bucket with warm water and dunk the return/intake line into it. Then hook up a shop vac to the other hose and suck the water through. Then reverse the hoses and repeat and reinstall the lines. (mixing some cooling system cleaner in with the water is also a good idea)

 

Thankyou for the reply Awe341, you gave me some hope. Iv been looking in the forum here on replacing the heater core and it looks like something im not sure i want to do myself. Yes its cold here too so i will try to suck warm water through with a shop vac, i think my dad has one. I will try that in the next couple days and ill get back to here and let you know how it went.

 

Before you do anything make sure the heat problem is not due to low coolant. A low coolant level can cause low or no heat.
With the engine cold take the radiator cap off and be sure the radiator is full.
If the radiator cap is bad (or if there is a leak between the radiator cap & reservoir) the reservoir can be full but the radiator/engine can be low on coolant.

 

Thanks Terry for the info, i had no idea that the rad level could affect the heat. I just got back from a couple hours of driving so i will wait a few hours and then go out and check the level. Thanks again

 

I experienced this same problem with this same cause. At idle, no heat whatsoever. Above 2k RPM, it would be fine. I was a quart or two low. This was solved at my next flush and fill (being sure to burp the air out of the system).

 

I just checked my rad fluid level and its right topped up and the overflow is at the proper level too so i think im going to try flushing out the heater core tommorow, its supposed to be plus 4 celcius here tommorow.