Only blows hot when moving?

[cochino12]

So I finally got my truck to blow air through the defrost and not just the floor vent but now it will only blow hot air when the truck is moving. At a long stop light the air goes from hot to cold and then will warm up again when I start moving again. Any ideas of what is wrong? Thanks

[beaterC10]

I would check the coolant flow. Could be low coolant, tired water pump, or faulty thermostat. Might see how it behaves when the truck is stopped and you accelerate the engine. Sounds like you aren't moving enough coolant through the heater core at idle.

[ocbaud]

mine is the EXACT opposite. if i were to warm my truck up for 10 minutes in the morning, the heater would be nice and warm and perfect. as soon as i start driving, and even worse, get on the highway, the heater starts blowing out a lot colder.

i've got a 180 tstat in there.

[Devil'S Ride]

Hey, I understand your problem as well. I fire the truck up let it get to operating temperature, and on cold or cooler days, I have NO heat at idle thru about 2600 rpm. If I am on the free way, turnin about 3000, then I have warm air. not hot, just warm. if it is warm or hot outside, I have really good heat. I flushed the heater core both reverse and forwards, replaced heater lines, an coolant. still no improvement.

I think mebbe it is because at free way speeds I am forcing more air thru the system than at idle? I am unsure....any help or ideas someone has would be appreciated I know that.....it is starting to snow here an havin no heat sucks.....

[loweredd]

cochino12, I had the same problem on my 79 Impala, only with the air conditioning. It was a broken vacuum line at the reservoir ball. I would check all the vacuum lines in the system.

[1976K5CHALET]

The problem is a really easy fix...i had the same probs in my 79 years ago when i lived in phx...if your heater line is hooked up to the waterpump...you need to move that fitting to the intake manifold...and then the problem will be solved...

Any plans for Blazer Bash or EJS in 2005?

DW

[cochino12]

Originally Posted by 1976K5CHALET

The problem is a really easy fix...i had the same probs in my 79 years ago when i lived in phx...if your heater line is hooked up to the waterpump...you need to move that fitting to the intake manifold...and then the problem will be solved...

Any plans for Blazer Bash or EJS in 2005?

DW

I'll have to check that, thanks

loweredd: All the lines are new, thanks for the help

[Devil'S Ride]

hmmn, guess I'll give that a shot.......wonder why I never noticed that before....must be from when I swapped in the big block....thanks for the ideas....

[bigd65]

I think I would try the VACUUM BALL that stores the VACUUM for the vents. I would probably bet that it has a leak or the IN-LINE CHECK valve for the VACUUM BALL is eather missing or defective.

[Devil'S Ride]

well I know my vents move, I can get transfer of air, and I know the temp vent moves because of the difference in air pressure (more pressure when on cold than on hot)

I also looked and the 5/8 inch heater hose goes to the water pump, the 3/4" goes to the radiator. So if I move the 5/8 to the intake will that solve my problems? an will it cause any difference in engine coolant temp. I just dont want to switch the lines, and then have somethin go south on me.

TIA