Question about 1987 Nissan 300ZX

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Heat works sometimes but is only warm

There are 3 vacumn lines that control the heat. What are they connected to

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One will control the heater core valve, one will control the deflector door and one is problem vacumn in from intake manifold. If the vacumn motor that controls the valve or the door is leaking you would have this problem. Also if vehicle is low on anti-freeze you could have this problem, Hope this helps! The vacumn motors are easily checked by take a small piece of vacumn hose and intstall it on each one and applying suction by mouth, if they leak you will not be able to make them retract or they will leak down after you pull them in.

Posted on Dec 17, 2008

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Most vacumn systems that i have ever came in contact with incorporate a vacumn supply tank(nothing more then a round plastic vac. resevoir) which is used in sequence with the a/c heater controls, its purpose is to keep an even vacumn supply to all command points continously while the vehicle is being driven, without this supply one will experience the a/c coming from various different non selected dash vents upon acceleration, this is a direct relation to the tank either being defective(leaking vacumn) or its supply hose(usually vey tiny rubber/plastic tubes) being pinched or broken not allowing the vacumn to the supply tank.
listen for vacumn leaks(hissing sound), look for the obvious...broken vacumn lines or hoses, also check for a defective one way vacumn valve on some pneumatic controlled door vent systems(these valves are inserted in a vacumn line one side being the ""in" port and the other side of it being the "out" flow port,typically if these dont work the vents will also devert your a/c or heat all different ways through your dash.
to check these valves, simply remove from the vacumn hose, and apply vacumn to one side,if vacumn flows through the valve, apply the same vacumn to the opposite side of the plastic valve, if vacumn flows through on the opposite side the valve is bad, it should only allow flow from one side not the other. These valves are small and are usually white or black plastic and look like the old vacumn dash pots used on the old carbureated engines only smaller with one small hose in and one small hose out.

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You are experiencing a warm air blend door problem, possibility cased by a week vacumn diaphram.
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YES, THEIR IS A HOT WATER CONTROL VALVE, ITCAN BE OPERATED BY VACUMN, OR CABLE, VAC, TYPE WILL HAVE AVACUMN HOSE TO IT, PLS CHECK IT FOR CONDITION, CRACKS, SWELLING, IF OK, THEN WITH ENGINE RUNNING, HEATER CONTROL IN HOT POS. PULL HOSE OFF VALVE, PUT FINGER OVER END, FEEL FOR VACUME. IF THERE IS, MOST COMONLEY RESULT IS THAT VALVE NEED TO BE REPLACED.

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