jell36 Posted September 16, 2012 Share Posted September 16, 2012 Hi guys, my car has a rough idle when the aircon is turned off. I know everyone says it is nothing to worry about but I find it annoying and it is getting more rough as time goes on. It is as if the engine kicks every now and again. So today I thought I would unplug the MAF to see if the engine runs smooth without it and that will then help me to diagnose the rough idle as a bad maf. So I unplugged it and fired up the engine, as expected the CEL light came on but I wasn't expecting the revs to jump around so much, I took a video of the rev needle so you can all see it below, you will see that every now and again the revs jump and drop, is this supposed to happen when you unplug the MAF? P.S. In the video I don't touch the accelerator pedal at all. It normally idles at 650rpm with the MAF plugged in. Quote Link to comment Share on other sites More sharing options...
docwra Posted September 17, 2012 Share Posted September 17, 2012 Cant hear it but I guess the revs are rising and falling with the needle? I shouldnt worry too much, particularly if its from cold, it will be using a base map rather than actually calculating fuelling and its possible that it could cause that. Ill try with my one a bit later if you like and see what happens If you have idle probs related to the aircon my first stop would be the idle control valve - older Nissans used to have an auxiliary idle control for power hungry applications that sometimes failed giving an erratic idle, sounds like you have the reverse problem. The ole pedal dance or getting it plugged into a diagnostic machine might show if theres any issues there, its a decent start anyway Quote Link to comment Share on other sites More sharing options...
Husky Posted September 17, 2012 Share Posted September 17, 2012 Unplugging the MAF for a cold start will give a poor idle, no surprise there. Have you checked for vacuum leaks? Have you fitted a plenum spacer recently? Quote Link to comment Share on other sites More sharing options...
jell36 Posted September 17, 2012 Author Share Posted September 17, 2012 many thanks for your reply, the video was with the engine having been warmed up and yes the revs match what the needle is doing. If you could check what it is like on your car that would be greatly appreciated (it will light the CEL though so you will need to be able to clear fault codes afterwards). Do you know if there is anyway to check the idle control valve or where it is located? Quote Link to comment Share on other sites More sharing options...
CrustyCrab Posted September 17, 2012 Share Posted September 17, 2012 I forgot to plug my MAF back in then I changed my air box. it really screwed the idle up. I guess its because without the sensor it doesn't know what’s going in so it can’t compensate. either way I turned it off straight away and plugged it in. it really sounded like it was doing some damage Quote Link to comment Share on other sites More sharing options...
docwra Posted September 17, 2012 Share Posted September 17, 2012 Its a safety feature, if the car cant meter its air then it goes into "limp home" mode, basically running rich and not letting you exceed 4K rpm, Ive invoked it deliberately when the AFM was playing up a while ago. It wont damage the car, just makes it drive worse than a 1.0 Metro Ill go have a check in a bit Jell, its nothing to worry about even if its hunting when warm though, it means the management is doing what it should do Quote Link to comment Share on other sites More sharing options...
jell36 Posted September 17, 2012 Author Share Posted September 17, 2012 Unplugging the MAF for a cold start will give a poor idle, no surprise there. Have you checked for vacuum leaks? Have you fitted a plenum spacer recently? The engine had been warmed up for 30 mins before turning it off and unplugging the MAF, I have tried looking for vacuum leaks but can't find any and no plenum spacer installed. Quote Link to comment Share on other sites More sharing options...
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