Drive heat...no drive,no heat

[radiomike45]

OK, thinking of changing the T-stat here, but before I do, see if anyone has come across this. I have a 98 Blazer. Since it has been cold in the mornings here in the Dakotas the last few weeks, I would start the engine to warm it up. However only cool air is blowing out of the heater, however, soon as I drive 100 feet the warm air starts coming out and continues to blow nice and hot until I stop , then cool air comes out again. Once I start moving, it blows warm and hot air like it is supposed to. T-stat problem..or what?? Any ideas?? thanks!

[mike2004tct]

Originally Posted by radiomike45

OK, thinking of changing the T-stat here, but before I do, see if anyone has come across this. I have a 98 Blazer. Since it has been cold in the mornings here in the Dakotas the last few weeks, I would start the engine to warm it up. However only cool air is blowing out of the heater, however, soon as I drive 100 feet the warm air starts coming out and continues to blow nice and hot until I stop , then cool air comes out again. Once I start moving, it blows warm and hot air like it is supposed to. T-stat problem..or what?? Any ideas?? thanks!

What's your temp gauge doing, does it bottom out to cold when cold air blows from the heater?. If not, you're probably low on anti-freeze, or you have air in your system.(do you hear gurgling?)
If your t-stat does go to cold, it's time to replace it.
Usually a bad t-stat works opposite of what you describe (warm/hot when not moving, cold when driving). So I'd guess low on anti-freeze or air in the system.

[Gabe25]

If your radiator and T-Stat checks out OK. You mite want to take a look at your vacuum lines going to your climate control. You could have a small leak causing the baffle to close allowing the system to switch back to vents.

[drdd]

I had a similar problem on my 97. I thought it was the belt-driven water pump dying and that it was only circulating coolant with the higher revs and not at idle.

I took the water pump off and took it apart and it looked fine. I had heard stories of the water pump blades completely corroding away.

Anyhow, what solved it was a thorough acid flush and reverse flush. Flushed the hell out it. There may have been air in there too.


oh yeah ... what's your temp gauge on the dash doing ?

Originally Posted by radiomike45

OK, thinking of changing the T-stat here, but before I do, see if anyone has come across this. I have a 98 Blazer. Since it has been cold in the mornings here in the Dakotas the last few weeks, I would start the engine to warm it up. However only cool air is blowing out of the heater, however, soon as I drive 100 feet the warm air starts coming out and continues to blow nice and hot until I stop , then cool air comes out again. Once I start moving, it blows warm and hot air like it is supposed to. T-stat problem..or what?? Any ideas?? thanks!

[dmbrisket 51]

DON'T DO AN ACID FLUSH... working at a shop, those things cause more problems then they cure by 100 fold... what you want to do (to mea sounds like maby a clogged heater core) is a selfe flush, disconnect the heater core at the firewall (from the block dont take the lines off at the wall) and put the hose to it, seal it up good, and flush it both ways untill clear water comes out, for extra goodness, pull out the block drains and flush the block out too...

[mdrush]

I had a similar problem in my '98. Only cold air until I would accelerate..Feel the heater hoses that go from the waterpump to the firewall. If they are cold then it's the waterpump as it was in my case. Of course make sure the coolant level is full.

When the waterpump is starting to fail the flow to the heatercore 'heat' is the first thing to degrade.

I replaced my waterpump ($ 68.00 for a new pump from carquest) and about 2 hours of my labor (could be less as I'm definitely not a mechanic) + coolant.

After that back to full time heat....BTW change the rad cap while your at it as the plastic Delco OEM is know the let air into the system.

The hardest part of the job was separating the cooling fan nut from the waterpump assembly.

Really a fairly simple job on the '98s.

[mdrush]

Forgot to add....

Unlike most, upon inspection of the waterpump mine did not leak through the weephole, it ran out the bottom of the housing backplate.

[BlazerLT]

If it is a 98, flush and then swap in some new coolant and a new stat to be sure.

If the coolant was never changed since 1998, it is definitely due.