PhaTTy Posted February 18, 2014 Share Posted February 18, 2014 Alright.. I have a fun one for you. My friend has a 76 280z L28. He started experiencing an odd issue today. When he presses in the clutch or leaves the car in neutral, the car free revs up to about 3,000 rpm by itself. He says if you put your foot under the accelerator pedal and pull up on it, it stops. He then gave me an interesting tidbit of information. He had the fan on. He said if he turned the fan off, it goes back to normal. I asked him what the top lever on the A/C control is set to and he had it set to "Vent". I told him to switch it from "Vent" to "Fresh" and it also stopped. My question is.. is the A/C engaged when "Vent" is selected and not engaged when "Fresh" is selected? I can't think of anything else that would make this stop by just switching the fan off or switching from vent to fresh. Let me know what you can come up with for ideas on what could be causing this. Quote Link to comment Share on other sites More sharing options...
ttodhunter Posted February 18, 2014 Share Posted February 18, 2014 The HVAC system is controlled by engine vacuum. As you move the lever, you should hear a momentary hiss as the vacuum pulls certain doors closed. If you move it back to "vent" and the hiss continues, there may be a broken vac line in the system that is causing a vacuum leak. There is also a vacuum line that goes through the firewall next to the battery; you could try clamping it and see if the problem persists. Quote Link to comment Share on other sites More sharing options...
PhaTTy Posted February 18, 2014 Author Share Posted February 18, 2014 Don't hear a hiss that continues. As I said, once you move the vent / (a/c) / bi-level lever from the left side ( A/C , vent ) to the right side ( fresh, defrost, etc ) , it fixes the problem. Moving the bottom temperature control from cold to hot does not affect it. Turning the fan off also fixes the problem.. This makes me think it is something electrical and not vacuum as turning the fan off wouldn't stop a flapper/door vacuum leak, would it? Would a faulty air regulator do this ( may also be known as Auxiliary Air Valve? ) cause the idle spike? I am referring to this: I can't think of what else on the engine would cause it to free rev when the A/C might be engaged. Does this seem logical, or could it be something else? Quote Link to comment Share on other sites More sharing options...
NewZed Posted February 18, 2014 Share Posted February 18, 2014 There's a mechanical hook that pulls the throttle mechanism open to idle up when the AC compressor is activated. It's on top of the intake manifold and obvious. It needs adjustment. Probably described in the AC chapter. Quote Link to comment Share on other sites More sharing options...
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