The heater control valve on my 77 280z (non A/C type) was not working well (subject to vibration from circulating coolant at intermediate settings between hot and cold)

I got a new replacement from Motorsports. It appeared to be NOS (made in Japan anyway). After the difficult installation process done by a professional Z car garage, the new part seems to have the same problem, although less extreme.

Has anyone else had this problem?

Given my experience with two similar experiences in a row, is this a minor design flaw or am I unlucky?

Any known fixes?

Thanks!

Before you do anything else, you should verify the cable is working properly. It might be possible that it is loose on the backside of the control panel, the cable shield is clamped to the framework of the control panel. It might be loose or stripped of its covering and the whole cable assembly is moving instead of the cable connected to the heater control valve.

It should work stiffly enough that it will hold the control valve at whatever setting you choose.

I seem to recall that the 77 and higher Z's had a vacuum operated heater control unit. Check to see if this is the case and then check your vacuum hoses.

2¢

Your are correct. The factory a/c used a vacuum controlled valve but the non a/c models used the simpler cable only control.

My cable is working well, tight but moving smoothly.

Any other thoughts?

Thanks for your ideas.

Ok, the problem appears to be that the valve is "chattering" or not allowing the water to flow into the heater uniformly. Please correct me if I'm wrong.

If that's the case, then you're becoming aware of the cycling because of the warm / cool / warm not being a constant.

Since the heater gets it's water from the engine block AFTER the pump starts moving it around, a long shot would be to check your thermostat. If the thermostat is cycling open and closed because you're right at the temp that it's set for, your heater will do the same.

Kind of far fetched I'll grant you, but the valve and cable are strictly a mechanical linkage and I can't imagine the amount of vibration you'd need to have the heater control valve stutter.

2¢