Heater blowing cold air.

[MCCNAT1959]

Howdy, New member. I have a 78 280z without factory air and heater will not get hot. I have replaced the thermostat and checked all operating controls and vent flaps. They all seem to be working. Any suggestions?

[Zed Head]

Check the vacuum lines to the white bottle under the hood. The valves for the heater flaps are vacuum operated. Pretty common for old tiny vacuum hoses to crack and break off. The vacuum is controlled by solenoids (Nissan calls them magnet valves). It's all described in the Air Conditioning chapter (Nissan calls heating AC also).

[MCCNAT1959]

Thanks, but no white bottle. I have read the flaps are vacuum operated but cant find any vacuum hoses going through the firewall to the interior. Also I have checked all vacuum hoses arount the thermostat and intake and replaced one but still no heat.

[Zed Head]

No white bottle, no vacuum, no heater operation. It's just the way it is.

[MCCNAT1959]

Thanks sooooo much for attaching diagram. Will check at first light and try to locate vacuum tank. Is that the white bottle referred to?

[Zed Head]

Yes, it is. It's pretty big too, over on the passenger side, hard to miss.

The diagram is from the AC chapter - Index of /FSM

[jmortensen]

On the 240s the heater control valve, listed as water **** in the manual there fails pretty commonly. I've replaced a couple of them. Sometimes you can pull it off and clean it and fix it that way.

[Zed Head]

jmort brings up another good possibility. On my 76 I also found that the cable that actuates the heater control valve (the one that determines how much heat, cold to hot), was bent and not actually doing anything when I moved the lever, just moving the bend in the cable. I had to take the control panel off and dink around with it, and even after fixing it the valve is more off/on than gradual.

[MCCNAT1959]

O.K. Here's the rundown. My Z has manual (no vacuum) controls on the heater control valve. The valve was faulty when I checked it and water would only run through one way. I bypassed the valve with a connector and the heater core got hot and the heat started working. However, the water will still only run one way through the heater core. Is there such a thing as a directional heater core? The heat is working fine, the engine temp is working great, water (antifreeze) is circulating through the radiator and the block, so what is the purpose of the heater control valve? And before I get chastised for rigging it, this is just a temporary solution to get heat before it gets cold, if it ever will, here in TIDE country.