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RPM'S Running High

My car has 181,000 miles on it and has never had a major problem. However, the last couple weeks I have noticed that my RPM's run at about 3500 when cruising down the interstate about 70-75 mph. That is much higher than usual. I have also noticed that the air conditioner is not blowing air very well and it wont change from feet to defrost or any other setting for that matter. I can deal with the air problem but the high running RPM's I cannot. Could these two problems be related? If so what can be causing it?


  • edited September 2010
    They are probably not related. Sounds like it isn't shifting into 4th gear. Is your check engine light on? When you are driving at this speed, try shifting from "D" to "3" and see if anything changes. If not you'll have to visit a transmission shop to have it diagnosed.
  • edited September 2010
    The direction of the vent air flow is controlled by a little door in the ductwork. In some cars this door has an electric motor, and in other cars it is operated by engine vacuum. The blend door, which controls the mix of hot air from the heater core, and cold air from the vent or A/C, is also controlled by vacuum. In some cars, the transmission senses engine vacuum, and you have insufficient vacuum, the transmission will think you are under a heavy load and may keep you in a lower gear. Also, if the car thinks you are under heavy load, it may shut off the A/C compressor.

    So my 2? diagosis is, you have a vacuum leak.

    What year is your Grand Prix and is your Check Engine Light on?
  • edited September 2010
    It's a 1999 and yes the check engine light is on.
  • edited September 2010
    Thank you for the help. I will try that and then write back.
  • edited September 2010
    For starters, many auto parts stores have a code reader that they will plug into your car for free and read off the codes. If you post the exact codes here, we may be able to help more. (They will be in the format P0001, P0101, etc)
  • edited September 2010
    The codes that came up were P0507 and P0171
  • edited September 2010
    Well, given those codes I'd say StrongDreams... was onto something.

    P0171 is about running too lean (too much air in the mix, though it could also be too little fuel)

    P0507 is just about the idle being higher than expected - a very common cause? Vacuum leak. Is it idling too high?

    You still haven't given the year so no one could say for sure but the vacuum problem messing with the transmission is a possibility.

    Put that with inoperable HVAC controls and I'd say that there is probably a vacuum line going from the engine through the firewall to the HVAC that is either disconnected or broken. You can just look all along the firewall very carefully to see if you notice anything amiss. But if you're not used to looking at all of that stuff under there you might need an experienced eye.
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