Question about 1985 Nissan 300ZX

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Vents blow hot air all the time and heater/ac controls do nothing

After having a competnet mechanic shop remove my original engine and install a rebuilt one, everything is good but the ac and heater no longer work. The vents allow engine (hot) air into the cabin all the time and nothing I do stops that. I want to resume normal function of heater, ac and vents. What should I do? I have very little money but I am fairly good mechanically if I have some guidance. In other words, I don't want to go and rip out my dashboard in search of the solution and have no clue where to start.

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I had this problem also, there are some vacuum hoses under the hood near the back passenger side of the the engine bay check for broken or cracked hoses.

Posted on Jun 15, 2009

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I had the exact same problem with my '85 300ZX. Turned out to be a vacuum hose under the dash that had come loose. I noticed that others were brittle and would soon probably crack, so I replaced them all. There is some good info at http://www.thumper300zx.com/z31/vacuum/vacuum.htm


Posted on Feb 01, 2009

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As per your problem mentioned. There are few solved help links to troubleshoot these problems. Go through the list and Click the link below:----

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http://whatisbyme.blogspot.in/2012/05/2000-toyota-avalon-no-heat-on-passenger.html


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http://schematicsdiagram.blogspot.in/2012/02/heater-blows-cold.html

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1 Answer

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Keep this principle in mind: Heat, flows INTO cold... When you hold an ice cube in your hand, your hand gets cold because the heat from your hand is flowing INTO the clod ice cube, NOT because the cold is flowing into your hand.

On this particular vehicle (like many modern vehicles), hot water from the engine flows through the heater core at all times, even when you have the A/C on or when you have nothing (A/C, heat or vent) on. The system uses a door that closes down over the air opening to the heater core to keep you from feeling warm/hot air when it is not desired. The reason(s) you feel that the vent air is warm even when the outside temperature is somewhat cool is/could be caused by the following:

First, although as mentioned above, the heater core is restricted from receiving air when you are not commanding heat by the control panel, the heater core ultimately ends up as warm as the engine temperature. This is generally somewhere around 200 degrees F. Because all the air coming out of your vent(s) must pass over the door that restricts air over the heater core, and because the heater core gets so hot, some of that heat is picked up by air passing over the hot door even though it is closed.

Second, the door that restricts airflow from passing over the heater core has a seal around it. After a period of time this rubber (or usually felt) seal deteriorates, allowing some air to actually pass over the heater core. Since the core is hot all the time, some of the air gets heat loaded, producing warmed air.

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Last, the vent intake opening, where the 'fresh' air comes into the car on the 'vent' setting, is located just below the windshield on the passenger side. This opening receives its air directly off of the sun-baked, engine-heated hood and because of this the air is never as cool as outside air even on a cooler night.

I hope this answers your question. Many domestic vehicle manufacturers have begun installing heater control valves on their vehicles once again as they nearly all used to have. Note that the fix for most everything I have mentioned here requires removal of the dash of the vehicle or replacement of the control head and therefore is in many cases not worth the investment if it is not an extreme problem (for example, unless the system is stuck on full heat in the middle of summer. Additionally, remember that, if you so desire, manual shutoff valves can be placed in the heater hose lines (please consult a qualified mechanic as water from a vehicle can and will scald and burn you!), allowing you to manually shut off the flow of water into the heater core, thus reducing some of the heat (Readers note that certain foreign vehicles REQUIRE water flow through the heater core at all times for engine cooling and are not candidates for this procedure!).


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