I hate intermittent problems... u never know if u've fixed it until it happens again

When I was driving my car earlier this summer, I noticed that sometimes the air from the A/C was blowing from the defroster rather than the dash vents. I noticed that when I 1st got in the car (when it was warm from sitting in the sun) the A/C came out of vent. As the interior cooled down from the A/C, it started coming out of the defroster. As I was trying to track down the problem, we had a couple of cool evenings and I noticed that sometimes my heat would work and sometimes it wouldn't. I unplugged the vac line from the heater control valve and plugged it w/ a bolt. The heat seemed to work fine like that, but again, it was an intermittent problem so I don't know if I actually fixed anything.

I did my research on it, checked for vac leaks, etc, and decided to replace the vac harness in the dash. So far, the air seems to be blowing out of the right places, but I don't know if it's fixed for sure since it's an intermittent problem. I took the bolt out of the vac line and reconnected to the heater control valve, but I didn't have any heat. The heater hoses were nice and toasty though. I should have pulled the vac line and tried it again, but I didn't think of it.

So what's the deal? Are these 2 problems related? I don't think it's the thermostat... the car isn't overheating and the temp gauge is right where it should be. If the heater core was plugged or the blend door was jammed, that wouldn't cause an intermittent problem. My dad thinks it's a leaking vacuum tank, but then why would removing the vac line make the problem better? And my cruise control works fine.

 

bump

 

I think you're overlooking the obvious. Replace the heater control valve.

 

But if the control valve was bad, why would the heater hoses be so hot?

If I understand this correctly, the cooling system sends hot antifreeze to the control valve. If the heater setting is selected inside the car, the valve should open and allow that hot antifreeze to go to the heater core. Is that correct? So 1 of the heater hoses should always be hot (assuming the motor is hot) and the other will only be hot when the heat has been selected inside the car?

If that's the case, then 1 of the hoses may have only been hot becuz it was laying on top of the motor. I don't know how u can check it any better though... I guess both hoses should be at about the same temp. Anyway, I'll look into it more... thanx for the idea!

 

I think that coolant is always going through the heater core and the hot/cold valve which is controlled by a cable will dirrect air through the heater core when you set it to hot. The vents are controlled by vacuum. If you loose vacuum to the controls it will aotomaticly go to defrost and wont change. If it starts comming from the defroster in all settings then I would check the black vacumm canister on the drivers side by the firewall above the brake booster to make sure it has vacuum. If that checks good then follow the small line coming out of it. It should go all the way to the passenger side and go inside the fire wall. If that line has any holes then you won't have any vacuum to power the vents and it will stay on defrost. Well thats the way it is on my 83 firebird but I think it should be the same for yours too. Hope this helps.