Any good mechanic on here, I have question about my car that has to do with the A/C and a possible vacuum leak?


When i put the a/c on my car dies down in rpm. One issue i think is the A/C Compressor is going bad. I also think i have a vacuum leak in the same area were the compressor is because there is a hising sound. Sometimes my rpm is goes above 1000 and sometimes below at idle. I also noticed that the shifter is more harder to shift. I have a 2004 svt cobra and it is modified and makes 640 rwhp so I don't know if any of that has to do with it. The car never overheats and oil temp is a little above normal but the gauages and reading seems to be correct.

Is what i wrote seem accurate or could it be something more?


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  • When you turn on your ac this engages the ac compressor clutch which then puts more drag on the pulley and accessory belt driven by the engine. Which explains why at idle the rpms will dip a little bit and then recover as the engine tries to find idle again with the additional power demand from the AC.

    If you had a Vacuum leak, you'd be getting a bouncy idle or at the very least a "system too lean" or misfiring depending on where the leak is coming in from.

    I don't know what mods you have or how much more HP in terms of % than stock but a heavily modified car will never idle or run as smooth at the lower end of the powerband, simply the factory tune / equipment was not engineered for the added power.

    • stock 04 cobras are 390 HP. Mine makes 750 HP with the water-methane kit, but i have disconnected it for now because it looks like it somehow malfunctioned and some water got into the engine (but i did an engine flush and oil change). The car never really idles smoothly, it jump around from 900-1000 rpm (Without AC).

      PRIOR to this i had a MAF issues and i have replaced it but i was an idiot and forgot to reset the system (so just disconnect the battery). But it seems good. I do still however hear a hissing sound from where the ac compressor is located. So all in all i would have to replace the whole compressor correct?

      Also i was driving yesterday with 2nd gear in the parking lot, the car was jumpy when accelerate.

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    • Remember, for every $1,000 of mods you invest in the car, you decrease the value by -$500.

      Best way to try to recoup money is the just part out all the mods and return the car to stock.

    • people have been telling me that. The supercharger is not factory, but it has bigger injectors, fuel system, fuel pump, bigger everything basically.

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  • You have to find the leak then repair vacuum and charge, check the compressor clutch and nice cool air out of the vents.