[larrym1961]

Lately my heater will blow hot air when it gets to operating temperatures then it will go cool for a bit then back to hot. Coolant level is to the top of radiator and there is no drop in coolant temps when heater starts to blow cool air. I don't have auto climate control just the manual temp dial. Could I be having a vacuum line issue that is causing the blend door to open and close causing the fluctuating heating temps? Any idea on what I should look for to fix this would be great.

 

[Cowboytrukr]

Some model year info would be helpful, Larry.

 

[larrym1961]

2003 GMC Sonoma ZR2 4.3L

 

[xPosTech]

I think it's more likely to have an air pocket in the cooling system. Vacuum leaks aren't usually intermittent but steady at any given RPM. When the heater blows cold are you stopped at a light or stopsign? Do you ever hear a gurgling sound from under the dash?

Ted

 

[larrym1961]

I had to replace my intake gaskets a year ago (typical GM Dex Cool failure) after filling the cooling system heat worked great the rest of last Winter. This issue has crept up since then. Could it be possible the cooling system developed an air pocket in it since? If so what would be the best way to bleed the system? I did let it run for a while with the radiator cap off a few weeks back hoping that would release an air pocket if there was one but it didn't.

I don't hear any gurgling under the dash and the air temp will change while driving as if I turned the temp control dial from hot to mid point then back to hot.

I read on a GM Truck site that with a recovery tank cooling system, trapped air in cooling system would bleed through anyway.

 

[larrym1961]

I think it's more likely to have an air pocket in the cooling system. Vacuum leaks aren't usually intermittent but steady at any given RPM. When the heater blows cold are you stopped at a light or stopsign? Do you ever hear a gurgling sound from under the dash?

Ted

I ran right side of truck up on a ramp so that side was elevated. Opened the radiator cap and stuck a funnel in the radiator opening and let it run to operating temperature. I let it run for a while then shut it off, let it cool for an hour then repeated the process. After it cooled again had to add about a cup of coolant to the radiator to top it off. Heat is working like it once was again. No cool spikes in it. So it does look like there was an air pocket trapped in the system.

 

[larrym1961]

I fixed the problem and it wasn't coolant related at all. During the Summer when using the A/C I had a problem, an opposite of this issue. A/C would go from cold to warm and then cold again after I would shut the truck off for a few minutes then start up again.

I noticed that I would only loose heat in the defrost and floor/defrost settings but not in the floor setting. So after I lost heat again I opened my hood to look underneath at possible problems I noticed the Accumulator had 1/4 inch of frost on it. So after some further investigation I found out a bad low pressure switch on the accumulator would cause this. So I replaced the switch and O ring and now heat works constantly and no more frost on accumulator. I am sure that I will find now that my A/C will work constantly this Summer too.