1968 442 - issues with the heating system

[Jrbrks]

I have had this issue previously, and have not yet been able to solve the problem.

 

I have a 1968 442 convertible with a 4 speed and factory A/C.

 

The 442 is stock.

 

At idle, I have excellent heat.  As I accelerate to 2500 RPM or higher, the heat goes completely cold.  This loss of heat can occur while driving or sitting still and revving the engine to a fixed RPM reading.

 

I have replaced the vacuum cannister/reservoir under the hood, and it did not have any impact on the problem.  I re-installed the original vacuum cannister that is the correct unit.

 

The vacuum lines under the hood all appear good.

 

I have vacuum gauge tee'd after the vacuum cannister on the small diameter vacuum line that goes though the firewall to obtain vacuum readings, while the engine is running at different RPMs.

 

At idle, the vacuum reading is approximately 18.5 psi (700-800 RPM).

 

At 1,500-2,000 RPM, the vacuum increases to approximately 21 psi, but the heat/temperature remains constant.  I am not losing any heat at this point.  I am using a thermostat placed in the vent, with the heat directed through the vents, and the fan on its highest setting.

 

When I accelerate to 2500 RPM or higher, the vacuum reading remains the same (approximately 21 psi), and the heat begins to drop from approximately 175F to only 80F or less. 

 

Reducing the RPMs to less than 2,000, and the temperature increases, while the vacuum reading remains at approximately 21 psi.  Dropping from 1,500 RPM to idle (700-800 RPM), results in the vacuum dropping to approximately 18.5 psi with no change in the temperature or heat being produced.

 

Again, I am measuring vacuum after the vacuum cannister and before the small diameter vacuum line enters the interior through the firewall.  It does not appear the vacuum cannister/resevoir has a leak.

 

If this information helps, when turning the heat to "Off" on the dash, I can hear a noticeable release of vacuum near the switch on the dash.

 

I would appreciate any thoughts or ideas to solve this problem.  When I first purchased the car, approximately nine years ago, the heat worked fine.  The only repairs to this area were a replacement of the blower motor a year after I purchased the car.  I recall the heat was still working fine.  I can not associate the loss of heat that I am experiencing with any particular incident.

 

Thanks,

JIm

 

[joe_padavano]

Temporarily remove the heater control valve from the back of the intake and replace it with a nipple. If the head still goes away at higher RPMs, the problem is low coolant level or blockage somewhere. If the heat works at all times with the nipple, the problem is the heater control valve or the A/C control head or a vacuum problem.

[Jrbrks]

Thank you Joe

Adding the nipple as a replacement for the heat control valve is meant to permit bypassing the valve ?

Jim

[joe_padavano]

9 hours ago, Jrbrks said:

Thank you Joe

Adding the nipple as a replacement for the heat control valve is meant to permit bypassing the valve ?

Jim

 

Yes. It bypasses the valve and the vacuum system that operates it. This is a way to isolate the problem to one system or the other.

Edited March 22, 2020 by joe_padavano (see edit history)

[Jrbrks]

Thank you, Joe

I appreciate the help and advice

Jim

 

[Jrbrks]

I removed the vacuum line for the heater control valve.  I blocked both the vacuum line to the heater control valve and the opening for the vacuum line to the heater control valve.

 

After doing this, I could get almost no heat through the vents to the car.  It probably required more than thirty minutes to have the air temperature at the vents reach 110 F.  Revving the engine to 2500+ RPM  (car was not moving) did not result in a drop in temperature below 110 F.  Additionally, both the inlet and outlet lines near the heater core were warm but not hot and not nearly as hot as the top radiator hose.

The engine operating temperature was normal.

 

When I reconnected the vacuum line to the heater control valve, the air temperature at the vents immediately spiked to over 160F.  Both inlet and outlet hoses near the heater core became very hot.  And, revving the engine to 2500+ RPM or driving the car resulted in a significant loss of heat as measured through the air vents.

 

Does this seem like a heater control valve issue?  If it were blockage in the heater core, it seems I would have the same issues with low temperature at the heater core outlet hose, regardless of whether the heater control valve was attached to the vacuum source?

 

Thank you

Jim

[joe_padavano]

3 hours ago, Jrbrks said:

I removed the vacuum line for the heater control valve.  I blocked both the vacuum line to the heater control valve and the opening for the vacuum line to the heater control valve.

 

After doing this, I could get almost no heat through the vents to the car.  It probably required more than thirty minutes to have the air temperature at the vents reach 110 F.  Revving the engine to 2500+ RPM  (car was not moving) did not result in a drop in temperature below 110 F.  Additionally, both the inlet and outlet lines near the heater core were warm but not hot and not nearly as hot as the top radiator hose.

The engine operating temperature was normal.

 

When I reconnected the vacuum line to the heater control valve, the air temperature at the vents immediately spiked to over 160F.  Both inlet and outlet hoses near the heater core became very hot.  And, revving the engine to 2500+ RPM or driving the car resulted in a significant loss of heat as measured through the air vents.

 

Does this seem like a heater control valve issue?  If it were blockage in the heater core, it seems I would have the same issues with low temperature at the heater core outlet hose, regardless of whether the heater control valve was attached to the vacuum source?

 

Thank you

Jim

 

The valve on your car is normally closed and opens when vacuum is applied. Sounds like that part is working. Here's another easy test. Remove the vacuum line to the heater control valve an plug the hose. Run a temporary hose from the heater control valve directly to a manifold vacuum port. This will apply vacuum at all times and should hold the valve open. Of course, at wide open throttle, manifold vacuum goes to zero and the valve will close. Ideally you should tee a vacuum gauge into this temporary hose and watch what happens when the heat stops.