Hi all I've read a bunch of posts about heat issues (which I am having), but not my problem. Here it is: Temp control lever on heat, blows heat for about a minute then cools down. I move the lever to cold and back to heat, and hot air starts to blow again, then gradually cools. '70 442 convert. with re-built 455 (now 469 cu. in.). Original AC car but now has no compressor. Thanks in advance!

 

Do you still have the vacuum operated hot water valve at RH back corner of the intake?

If so is its vacuum hose connected?

Is the HVAC vacuum storage ball still in place on the firewall, and if so is its vacuum hose to the intake connected?

Worst case the vacuum selector valve on the heater/AC control is leaking by or there's a hose split.

 

The heater control valve on a 1970 A-body is normally closed and opens when vacuum is applied. Either the valve is stuck closed or the vacuum controls are not working properly to open it when the control lever is in the HEAT position.

 

Yes, vacuum lines are all there and connected to intake. No splits or cracks. Vacuum source at intake splits to valve, ball, and a steel line that disappears below the firewall. I assume this goes to the transmission. Looks like the control valve is aftermarket. Here are a couple pics. .

 

Going back to Joe's comment on 1970 heater valve operation:

Other years vacuum CLOSES the valve. Very possible you have the wrong valve. Parts sources get worse about this all the time.

 

That part number is SUPPOSED to be correct for your application. An easy way to check is to disconnect and plug the vacuum line to it. If the heat works, the valve is a normally open style and would not be correct for your car. If the heat still doesn't work, use a vacuum gauge to check for manifold vacuum at the hose. You should see vacuum with the controls in the HEAT and DEFROST positions. If not, figure out why. If vacuum works properly, the valve is stuck closed and needs to be replaced.

 

I should add that I ASSUME the heater core isn't plugged. Have you disconnected the hoses and flushed the core? Note that the 5/8" hose is inlet to the core and the 3/4" hose is outlet.

 

Air bubble in the cooling system perhaps?

 

The symptoms of heat initially that turns cold quickly is typically indicative of restricted flow to the heater core. The most likely cause is a stuck heater control valve. The second most likely cause is a plugged core. Are both hoses hot when the heater is blowing cold?

 

First things first, are you POSITIVE the cooling system is full? Air pockets will cause rapid changes in heat temp.

With the engine cold, remove the radiator cap. Top off the coolant, leave the cap off overnight. If there is air in the cooling system gravity will naturally force it out

If the coolant level is down a considerable amount, start looking for a leak.

 

Thank you all for your replies. All great suggestions that I will follow up on. It will be nice to have heat since my daughter is getting married in October, and wants to drive it to the various functions. I'll let you know what I find.

 

It's the valve. Disconnected and plugged the line as Joe suggested. Heat stays on! I will order the correct one. Any recommendations for the correct part #?
Thanks Joe and others!

 

Update: disconnected vacuum line from the valve and plugged it with a golf tee.(they come in handy for caulk tubes too!) As long as I keep it plugged, the heat stays on and I'm good to go. That begs the question: if this is the wrong valve for my car, how do I make sure I buy a correct one?

 

You're kinda at the mercy of the manufacturer and whoever packages the valve to get the right part. If you have a MityVac you could take that with you to a local parts store and make sure, but that's not an option if you buy it online.

Ten years ago I'd have said find a new old stock GM part but even that's risky now because the vacuum diaphram may be dry rotted.

 

The OEM part number is 399679
I believe 35C60 and 61 are air conditioning options. C60 is manual C61 is automatic, I believe. Yours should be C60 (399679)

 

The difference in original part # valves between C60 and C61 is the one for C61 automatic temperature control (Comfortron) has a threaded fitting for the Comfortron's coolant temperature sensor.

You'll find a lot of part numbers have been consolidated and ALL have the fitting. No worries if the new valve has it. It's just parts inventory consolidation.

 

I wondered what the difference was - makes sense now. Thanks.

 

That little temperature switch keeps a Comfortron from starting until coolant temperature is 120° or so when it's in one of the AUTO positions. It could be force-started by putting it in DE-ICE, but you still wouldn't have any warm air till the engine had warmed up.

The A-body cars didn't have to deal with it since Comfortron was never offered for them. The Tempmatic (first offered in 1974 A) was a lot less complicated.

 

It’s easy enough to verify if you have the valve in hand. If you can blow through the heater hose fitting, it’s a normally open / vacuum to close valve. If you can’t blow through, it’s a normally closed / vacuum to open valve.

My experience is the threaded fitting is a different size between the two styles. That keeps things in order with the correct year factory intake manifold. Aftermarket intakes have the larger size hole so an adapter has to be used with the valve with the smaller threaded fitting, and a different year factory intake may not have the proper size hole.