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Discussion Starter #1
Hello All!
So after some top-down driving in temperatures around the freezing point, my wife convinced me that perhaps I should look into fixing the heater :)

So here is the story. It is a 74 Spica equipped Spider.
The car runs well, and engine temperature at operating remains steady at 180F.
Heater fan runs, but just blows cold air.
In an attempt to debug what is going on, I decided to flush the heater box to assess the situation.
Undoing the hose from the manifold and water pump yielded a first clue....the hoses seemed dry.....(so this may have been the original cause of the lack of heat, as I have realized (through researching this forum) that I did not properly bleed my system upon coolant replacement earlier this year, so upon reassembly, I will bleed properly (let's park that for a moment)
Back to flushing...........
Flushing the heater box with water didn't yield the nasty rust color water I was expecting. So that was encouraging.
When respecting the actual "direction of coolant flow", I was able to use a garden hose to flush the heater box, and the water flow coming out equaled the water flow in. No perceived restrictions there, and aside from some white calcium deposits, nothing nasty coming out of there. Playing with the heat lever inside the car also seemed to reduce or increase the flow accordingly. So that was encouraging.

Next, I attempted to reverse the flow of water. In other words, I attached the garden hose to the return line, which is normally connected to the water pump. Notice the deliberate use of the word "attempt". This resulted in a very wet tshirt, and a teenager laughing at his dad. I could not get any water to flow in this direction, which I qualify as the opposite flow from normal system operation.
So this long diatribe brings me to my questions :

1) Is this behavior normal? ie unidirectional flow? If so, then great. That means the heater box is ok, and I can concentrate elsewhere.
2) If this is not normal, I assume that the culprit is probably the 47 year old heater valve. Is that correct?
3) If I should suspect the heater valve, then can someone please help me with pictures and procedure to removing/checking/fixing said heater valve in my car?

Thanks in advance everyone!
74 Spider

Premium Member
213 Posts
Usually the valve either leaks or won't turn.
Im not sure if the water is a one way street or not. I believe that there could be a one way valve in the heater control valve. I would inspect to see if the heater valve is installed backwards. If it is backwards and there is a one way valve, it could be that someone did that on purpose because there was a leak somewhere and they did not want to fix it.
The heater system on our spiders is fairly simple, theres the water going in,heater core, valve, and out. Plus the fan. If the valve works, the pipes arent blocked, and the core isn't leaking or falling apart-you should be good to go.

If you do some searching you'll find a thread with pictures with the heater valve, I know i have seen it here somewhere.

Premium Member
2,638 Posts
Since you didn't comment on wet carpet, the core may not be at fault. Temperature is controlled by the amount of water entering the heater core. Less water, lower temperature, more water, higher temperature. There is a rod on the valve that controls the opening, remotely connected to the lever on the console. If this is out of adjustment it can effect the heat. To adjust, loosen the lockbolt securing the rod travel, fully open the valve by hand, move the lever to the corresponding location, and tighten the lockbolt. This is the full open position.
A bad diaphragm in the valve or a aged hose that is collapsed inside also can effect the temperature.
If the valve works correctly and the water flow is normal, the heater core may be plugged. This seems unlikely but if all else fails that is another option.
Cheers, Jon
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