Loss of interior heating when moving-1968 442

[Jrbrks]

I have a 1968 442 4-speed convertible that is stock.  

When stopped, I have full interior heat, but the air goes cold when I am moving.

When I flushed the cooling system, I replaced the thermostat with no change in the loss of heat when moving.

 

What obvious thing am I missing?

 

I appreciate any help.

 

Thanks

Jim

 

 

[joe_padavano]

Is this an A/C car or a heat-only car? If this is an A/C car, have you changed anything associated with the under-hood vacuum hoses? The A/C system uses engine vacuum to operate the HVAC system, including the heater control valve on the intake manifold. On your car, the heater control valve is normally closed (thus no heat to the interior) and only opens when vacuum is applied. Engine vacuum drops as the throttle is opened, thus the lower vacuum means less force to open that valve. There is supposed to be a vacuum reservoir canister on the firewall that stores vacuum for these low vacuum engine operating conditions. If it is not connected, the system will not work properly when the throttle is opened.

 

Alternately, you could also simply be low on coolant. At higher engine speeds, there may be an air pocket forming in the heater core.

 

Do you have a temp gauge in the car? What is the engine temp reading when the heat works and doesn't work?

[Jrbrks]

Thank you, Joe.

This is a factory A/C car, and I will check the vacuum system as you have described.

 

is it also possible the heater control valve has gone bad?  

 

I had heat at acceleration, when I purchased the car about 8 years ago.  The only real change I made was to have a shop convert the A/C to R134, but  there was no other change to the factory set up, of which I am aware.

 

I live in a very hot area, so the heater is rarely used.

 

There is a temperature gauge in the cluster and it does not change from normal operating temperature, when I accelerate.  I did have the problem before and after changing the thermostat and flushing the cooling system.

 

Thank you, again.  

 

Jim

 

[joe_padavano]

42 minutes ago, Jrbrks said:

is it also possible the heater control valve has gone bad?

 

I can't think of a failure mode in the heater control valve that changes state with acceleration. Your car uses a normally closed valve that opens when vacuum is applied, so it needs to have vacuum applied when you want heat.

[Jrbrks]

I appreciate the additional insight.

 

Thank you, again.

 

Jim

[joe_padavano]

23 minutes ago, Jrbrks said:

I appreciate the additional insight.

 

Thank you, again.

 

Jim

 

Here's a thought.  Temporarily disconnect the vacuum hose going to the heater control valve and connect it to a vacuum gauge with a long enough hose that you can see the gauge while you're driving (tape it to the windshield or something). Start the car, put the control on HEAT, and drive under the conditions that cause the heat to go away.  The vacuum should be above 15 in hg at all times.  If not, that's your problem.

[Jrbrks]

Thanks Joe.

I will give that a try next weekend.

Jim