Pro2AInPA Posted May 7, 2010 Share Posted May 7, 2010 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? Link to comment Share on other sites More sharing options...
Sarge Posted May 7, 2010 Share Posted May 7, 2010 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. I know, I know. Different motor, different car, etc.. But I know how weird mazdas can be. Link to comment Share on other sites More sharing options...
Chris Keen Posted May 7, 2010 Share Posted May 7, 2010 Turn the AC off. Problem solved! 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. Link to comment Share on other sites More sharing options...
Pro2AInPA Posted May 7, 2010 Author Share Posted May 7, 2010 Never using the AC is not an option. Link to comment Share on other sites More sharing options...
Graham Smith Posted May 7, 2010 Share Posted May 7, 2010 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? Link to comment Share on other sites More sharing options...
Ronnie j Posted May 7, 2010 Share Posted May 7, 2010 try cleaning throttle plate Link to comment Share on other sites More sharing options...
Pro2AInPA Posted May 7, 2010 Author Share Posted May 7, 2010 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 . . . Link to comment Share on other sites More sharing options...
COMATZD Posted May 7, 2010 Share Posted May 7, 2010 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. Link to comment Share on other sites More sharing options...
Pro2AInPA Posted May 7, 2010 Author Share Posted May 7, 2010 Looks like there's a ton of possibilities. Guess I'll be taking it in to a shop and hoping and praying it doesn't cost much to fix. Link to comment Share on other sites More sharing options...
JoshF Posted May 7, 2010 Share Posted May 7, 2010 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. Link to comment Share on other sites More sharing options...
Pro2AInPA Posted May 7, 2010 Author Share Posted May 7, 2010 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. Link to comment Share on other sites More sharing options...
-JQ- Posted May 7, 2010 Share Posted May 7, 2010 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. Link to comment Share on other sites More sharing options...
Pro2AInPA Posted May 7, 2010 Author Share Posted May 7, 2010 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. Link to comment Share on other sites More sharing options...
shooterbenedetto Posted May 7, 2010 Share Posted May 7, 2010 I would go to a dealer and ask the parts guys... if you can, you can probably sneak up pick a technicians brain on the sidewalk where they take 15 min breaks. mostly works for me. Link to comment Share on other sites More sharing options...
BritinUSA Posted May 7, 2010 Share Posted May 7, 2010 ODB stores the codes, you don't have to have an ODB scanner connected when the problem occurs. Just check it later and it may show the root cause of the problem. Link to comment Share on other sites More sharing options...
Classic_jon Posted May 7, 2010 Share Posted May 7, 2010 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. Link to comment Share on other sites More sharing options...
Two Delta Posted May 7, 2010 Share Posted May 7, 2010 Take it to a mechanic,someone you trust. Lots of people work on cars only some people fix them! Link to comment Share on other sites More sharing options...
Ridgerunnr Posted May 7, 2010 Share Posted May 7, 2010 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. Link to comment Share on other sites More sharing options...
Two Delta Posted May 7, 2010 Share Posted May 7, 2010 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. Link to comment Share on other sites More sharing options...
Dave Gundry Posted May 8, 2010 Share Posted May 8, 2010 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. Link to comment Share on other sites More sharing options...
Bubber Posted May 8, 2010 Share Posted May 8, 2010 (edited) Never mind as I just reread your post and it only does it when AC is on. Sorry, later rdd Edited May 8, 2010 by Bubber Link to comment Share on other sites More sharing options...
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