This is a question for auto's and manuals even though my car is an auto. When I am in stop and go traffic, which is a lot on the way to and from work, I notice that the engine normally idles around 700. Every few seconds, I can feel a little vibration as the rpm's drop a little below 700, say 650 or so. It seems to do it more with the AC on, but I think this is because the AC tens to pull the engine down just a little more than without. Now, don't get me wrong, I love this car, and wouldn't trade it for any other car, but it concerns me a little when I feel this. I read a post before where someone described what I thought was a similar problem, I think it was described as a little miss in the engine. The explanation if I remember from Ford was the high compression causing a little rough place in the idle. Now, I am not going to say wheter or not I have a tune, but I will say the car runs really hard and runs really smoothly in every rpm other than really low rpm's. My question to everyone is, have any of you noticed anything similar, and do you think what I am describing is the same as the high compression excuse thrown out by Ford?

 

Hard to say for sure what's causing that. Could be as simple as your choice in gasoline.

 

It's the fan turning on and off. It pulls high amps on startup and that makes the alternator pull and this is what causes the idle irregularity.

Don't sweat it.

 

I wondered about this too. Cause when I am driving and my a/c is on, if I come to a stop at a light my RPMs drop to like 500 then surge back up to the normal range. It's only when I come to a complete stop and the a/c is on.

 

2004 and earlier mustangs id say Idle Air Control needs cleaning...but yours is throttle by wire...so unless there's a issue there,id say it is made to act this way...

 

I live in Fl. with the a/c on all the time. Just had a tune installed. I never notice ANY change when my compressor cycles (before or after the tune )at idle (I have an auto). I can't hear it ,or feel it. Another person on here just had his compressor seize. Not sayin yours is going, but your under warranty, and I don't see how they could attribute it to a tune (if you have one). My idle has been silky smooth since day one (4500 mi. now). I wouldn't accept anyones BS about rough idles, although some may not be as smooth as others.

 

What kind of tune do you have GJM20115.0? Was it a SCT tuner with one of the tunes preloaded from Bama, Brenspeed etc..? I am not saying my car has a tune, but if it did, I would just reload the stock and see if this problem stays or goes away. The car seems to have developed this over the last few weeks, so I am thinking something may be going wrong. I will probably just drive it for now and see if this gets any worse. Can anyone on here from the North Georgia area recommend a dealership in case I need to take it in. I bought the car from Paul in Rochester, so I don't have the option of carrying back unless I want to drive 900 miles......not.

 

I don't think the tune police are on the TMS lol, but yes, I have a SCT with a Steeda tune. I think if it is your tune (if you have one) , the problem would have happened(right after the tune), and not developed slowly.I am by no means an expert on this-just MO.

I went to my local dealer, ( I bought my car in Pa.) told the service manager I was getting a tune, and he said no problem. The service depts. could care less where you bought the car. If it's a warrenty issue-not related to an after market-no problamo. I think the burden of proof falls on them,but I think most want to help you-not deny you.

 

If you think it is tune realted then just load the stock tune back on it and see what it does.

 

I think that's it.

Yesterday I was backing down my driveway. I had the AC on but also the window down a bit. My driveway has enough downhill slope that I coast down it - so my foot wasn't on the gas pedal. I noticed the engine stumbled a bit and then revved on its own. And I heard the fan coming back on at this same moment.

Mine is stock - no tune.
If I remember right, most tunes have the fan coming on more often - so the alternator being tasked could occur more frequently.