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My car is revving itself when idling with the AC on


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This just started yesterday. The car is question is a 2004 Mazda 6. When I have the AC on and am idling (foot not on the gas at all) the engine is revving itself up and down.

This is really bad when stopping because the car wants to accelerate itself when I'm trying to stop.

When I turn the car off and start it back up and don't turn the AC on at all, everything is 100% normal.

What is going on?

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WOW! This is so cool for a totally different reason. I had an RX-7 that used to do this if I held the gas at 1000 RPM. It would rev to 2000 on its own. It would pulsate up and down all day long if I let it. Mazda never did figure it out. The guy I sold it to thought it was cool. :wacko:

I know, I know. Different motor, different car, etc.. But I know how weird mazdas can be.

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Turn the AC off. Problem solved! :rolleyes:

Seriously, a friend of mine firmly believes that you should always turn the AC off when you start the engine & when you stop it. Less wear and tear on something or other. I wasn't really listening.

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This may not be it but... When the AC compressor kicks in, there is increased drag on the engine (which turns the compressor), this can drop the RPMs and it may be that the car is reacting to that drop.

Clearly something changed. Perhaps there is a problem with the compressor that is causing even more drag and threatening to stall the engine. Perhaps the car is over reacting.

Have you taken it to a mechanic?

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This may not be it but... When the AC compressor kicks in, there is increased drag on the engine (which turns the compressor), this can drop the RPMs and it may be that the car is reacting to that drop.

Clearly something changed. Perhaps there is a problem with the compressor that is causing even more drag and threatening to stall the engine. Perhaps the car is over reacting.

Have you taken it to a mechanic?

Not yet. Just started yesterday and I work. The engine certainly isn't threatening to stall. It sounds/feels more like it's trying to take off down the road on its own . . . :wacko:

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This just started yesterday. The car is question is a 2004 Mazda 6. When I have the AC on and am idling (foot not on the gas at all) the engine is revving itself up and down.

This is really bad when stopping because the car wants to accelerate itself when I'm trying to stop.

When I turn the car off and start it back up and don't turn the AC on at all, everything is 100% normal.

What is going on?

Normal operation of modern vehicles have the idle increase when the AC pump kicks on. My truck feels like it gets rear-ended at a stop when the idle kicks up.

Now the reason why your's is suddenly fluctuating is different. Is the pump cycling on and off the same as the idle jump and drop? If so you could be low on Freon - Pump kicks on (idle up), line pressure doesn't rise enough so the safety switch kicks in and shuts the pump off (idle down), then it tries again.

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Typically it's from an Idle Air Control (IAC) Solenoid trying to compensate for the load and the air passage it regulates air through is filled with gunk. Cleaning the throttle plates / Throttle body will generally resolve this. This is of course assuming no engine lights came on.

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Typically it's from an Idle Air Control (IAC) Solenoid trying to compensate for the load and the air passage it regulates air through is filled with gunk. Cleaning the throttle plates / Throttle body will generally resolve this. This is of course assuming no engine lights came on.

Nope, no warning/indicator lights came on.

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I think most of the stores like autozone will let you use their ODB tester for free. Then write down the codes and look them up.

The GF's Honda does a similar type thing when in park. It is a sensor that is ~$300 but doesn't really do much...just don't sit in park for a long time - until you decide to fix it. Hopefully yours is something fairly benign too.

I'm not a mechanic - nor do I play one on tv.

Good luck.

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I think most of the stores like autozone will let you use their ODB tester for free. Then write down the codes and look them up.

The GF's Honda does a similar type thing when in park. It is a sensor that is ~$300 but doesn't really do much...just don't sit in park for a long time - until you decide to fix it. Hopefully yours is something fairly benign too.

I'm not a mechanic - nor do I play one on tv.

Good luck.

Problem is it's not just when I'm in park. It's when I'm in motion and idling.

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I had a similar thing happen on my old Mazda 626 when one of the "valves" that meters the exhaust gases that are fed back into the manifold from the Catalytic converter went bad. That also caused a solenoid to go bad that maintained the idle speed when stationary.

I had to pay a shop to track down what all was bad but then ordered the parts and did it myself after they showed me and told me what was wrong. Using them to diagnose is not a bad idea but if it is something simple, see if you can do it yourself and save some $$. A lot of that decision though depends on how mechanically inclined you are and if you are able or willing to take the time to do it yourself.

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It may or may not be normal. Almost every veh will see a difference in idle speed when the a/c cluth engages and disengages. This can actually happen even when running the defrost as the a/c clutch does cycle on some vehicles in that mode. Typically smaller engined vehicles see the largest difference in idle speed etc. IF its happening very frequently or enough that the car wants to move while foot on brake the a/c kicks on you have a problem. I have even seen too much freon in the system cause the a/c to cycle like crazy. First call a dealership and see if there any TSB's on it(technical service bulletins)..thats notice dealerships get of listed on going problems with certain models. If none still take it in if it appears to be something that just doesnt feel normal.

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It may or may not be normal. Almost every veh will see a difference in idle speed when the a/c cluth engages and disengages. This can actually happen even when running the defrost as the a/c clutch does cycle on some vehicles in that mode. Typically smaller engined vehicles see the largest difference in idle speed etc. IF its happening very frequently or enough that the car wants to move while foot on brake the a/c kicks on you have a problem. I have even seen too much freon in the system cause the a/c to cycle like crazy. First call a dealership and see if there any TSB's on it(technical service bulletins)..thats notice dealerships get of listed on going problems with certain models. If none still take it in if it appears to be something that just doesnt feel normal.

TSB's are public knowledge and can be found searching the internet.

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You will have to take it in and have some proper troubleshooting done, but a few ideas that come to mind:

When the a/c compressor kicks on, the engine will bog down a bit. The vehicle compensates for this by increasing idle speed, either by letting air bypass the throttle body via a solenoid and vacuum lines, or by opening the throttle plate a tiny bit more on drive by wire cars (where the throttle body is your idle air control system). Either the throttle body is dirty (needs cleaning), confused about it's role in life (needs replacement), or if it's using an older style bypass solenoid system, there is most likely a vacuum leak - which should result in a CEL and bank 1 lean codes. If the engine speed is rapidly fluctuating and the frequency matches when the a/c compressor kicks on and off, you most likely have a leak in the a/c system and the pressure level is just tickling the low pressure cut off. If the a/c compressor is staying engaged, I am more inclined to believe there is a vacuum leak or confused throttle body.

Hope that helps, let us know what they say.

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Never mind as I just reread your post and it only does it when AC is on. Sorry, later rdd

Edited by Bubber
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