Not sure how else to describe the problem. I start the truck...drive around a while with the heat or a/c (both have the problem) blowing out the dash vents.

A while later...no air flow at all even though the blower is still happily blowing.

A little later...here comes the air again.

It's as if the vents stop blowing for a while then open back up again.

Thoughts?

Will

 

The ducts are controlled by vacuum. There is a vacuum reservoir under the hood. Check the vacuum lines to it and other vacuum lines also. Do a search you will find your answer.

 

Yeah I read a bunch of threads but didn't see any where the problem would come and go.

I'll dig around a bit today if it warms up above 20 degrees.

 

I have somewhat of the same problem. I'll be driving around with the heat on set to the lower vent setting and then for no reason the dash vents will start blowing heat and no heat from the lower vents. After a min. or so it will go back to the lower vents. When this has happened I've tried changing the controls and nothing. I was thinking it must have something to with the vacuum?!?

 

Strange thing if it is a vac leak is my cruise works all the time. I would think that would cut in and out as the vents change position too.

Assuming it is a vac leak. Why do the vents start working again after a while? I think my truck is possessed

Everything works fine when the truck starts...just goes bad after 15 minutes or so.

It is almost 25 degrees. I'm going to run the engine for a while to warm things up and go snooping.

 

Get yourself a vacuum gauge and 'T' into the main vacuum line to the HVAC control module under the dash. Monitor the vacuum while you drive.

Do you have the manual or automatic HVAC system?

 

automatic system

Where is this vac line located?

 

It is under the passenger side dash. Pull the lower hush panel and look for some multicolored plastic vacuum lines.

 

Your Cruise Controll is Electronic so, it will not affect that. Therefore You shouldn't allow that to mess with your thought/trouble shoot process.

 

I have the same problem with a 1999 Blazer 4.3L. Everything works fine with heat or AC then it's like a damper shuts after 15 min. The blower is still operating but no air delivery. Why would the vacuum system start to malfunction after a period of time rather from the get go?

Found a thread with some schematics...

https://blazerforum.com/forum/showth...hvac+schematic