The problem may be a stuck valve which switches between A/C, heater, and defrost vents. My car is stuck in defrost, and is suffering a vacuum leak that is also impacting the idle speed. I was looking for the vacuum diagrams as I found this thread. Good Luck.
I had the hot air problem and recently replaced all the smallest vacuum lines (about 1/8") under the hood. There is one vacuum line from the vacuum reservoir that goes through the firewall. I removed the glove box and while lying on my back, head forward, could reach the "t" connection. I removed that hose end and used a machine screw with the head cut off to attach new hose. I attached the other end of a sufficient length of new 1/8" hose to the "t" and had a helper pull the vacuum line into the engine compartment while I fed the remainder. Re-connected to the vacuum reservoir and all works well.
Dude, this is a VERY common problem in all Z's. The easiest fix is to disco/bypass the the heater core (in the summer) and hook it back during the winter months. Good luck.
People are not being very helpful here. There are two major causes to this specific problem.
1) There are three vacuum lines between the intake manifold and the climate control system: First, a supply line that runs from a vacuum outlet at the right rear of the intake manifold to a vacuum canister just behind the right headlight. Second, a switched vacuum line from the canister to the hot water valve on the firewall. And, Third, a vacuum supply line from the canister to the climate control system behind the dash. All three of these lines get brittle and crack/break from age, causing leaks. Since the hot water valve requires vacuum to turn OFF the hot water, any loss of vacuum leaves the hot water ON all the time. If you haven't done it, its always a good idea to replace these three vacuum lines with new hose. For the line to the interior, just cut it off with 6-12in of slack and connect a new line to it with a plastic vacuum coupler. This will fix the hot air problem, even though there may be other climate control problems.
2) The Z31 Climate Control system uses three vacuum servos to control air doors [which controls air flows in different modes]. These vacuum servos use large rubber diaphrams that also go bad over time and fail, leaving the servo inoperative. One of these servos is behind the glove box and controls intake air and can be replaced easily. But the other two servos are behind the climate control air box, up against the inside of the firewall. Replacing them requires a complete tear-out of the seats, steering wheel, console, and dashboard. This is not a job for the faint at heart. I've done it now on two '86 300ZXs and had perfect climate control operation ever since.
It could also be a faulty heater core, check to see if you are leaking any from the passenger side, it could also be the vacums, i sugest you don't do it yourself as in fix it inless you deffintly know what your doing, the instructions people give you arnt always the best to go by, i have replaced my heater core in my 85 but it turned out to be alot more then what the instructions originaly said, so don't do it inless your sure you can, because there is alot of eletrical and parts under there.
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