Weird idling

[Mrpopper239]

I own a 2016 mustang gt. And I noticed my car when driving and at a stop the car just drops idle like crazy, sometimes to a point where the car sounds like it’s going shit off. Took it to a mechanic and he said the alternator is bad. Is this possible for a unstable idle?

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[FruityJudy]

You should be able to see your alternator output in your gauges screen

 

[I Bleed Ford Blue]

Try a can of sensor safe carb cleaner and give the throttle body and IAC valve a good cleaning. Try that first and see if your problem goes away, if not then look at the battery before you change out the alternator.

 

[Echo4papa]

Test your battery. Same thing happened to my 2015 before the battery died.

 

[Mrpopper239]

Try a can of sensor safe carb cleaner and give the throttle body and IAC valve a good cleaning. Try that first and see if your problem goes away, if not then look at the battery before you change out the alternator.

2016 mustangs don’t have IAC valves, pretty sure idle is controlled through the ECU thank you though

 

[Mrpopper239]

Test your battery. Same thing happened to my 2015 before the battery died.

I did and battery is fine it’s new, and it’s not failing I had it tested.

 

[ugstang17]

2016 mustangs don’t have IAC valves, pretty sure idle is controlled through the ECU thank you though

Correct. IAC's have not been used on Mustang GT'2 since 2004. Idle is controlled as you mention through throttle torque management control via ECU.

Alternator output will effect the system. If AC output is too high (should be minimal as in less than .250vac) there is a problem. When DC voltage goes too low the ECU cannot then correct for the change in pulsewidth on the fuel injector. FI's have a table(s) the tune that correct for voltage fluctuations which effect the amount of fuel going into the cylinder at given RPM. IIRC 13.8vdc is the target. Above that (then system is charging) the ECU will shorten pulse width to ensure proper fueling and below that the pulse will lengthen slightly for the same purpose. If you are seeing high levels of AC on the charging circuit your alternator. If the battery is bad and causing the alternator to pull down (likely the car would not start if this be the case or struggle to start) replace the battery.

As mentioned already - Check the more common things as well as mentioned such as cleaning the MAF with MAF cleaner and the TB with TB cleaner. Do not use TB cleaner on a MAF.

 

[Dfeeds]

I'd spray around the hoses to see if there's a small air leak or check for a sticking pcv valve.

 

[Diocletian]

Check for vacuum leaks. I chased this exact gremlin for MONTHS, replaced battery, MAF, thought it was an issue with my tune, with gas quality, intake filter. Turn out I had a vacuum leak at the airbox, fixed it and haven't had a single issue since.

 

[Mrpopper239]

Check for vacuum leaks. I chased this exact gremlin for MONTHS, replaced battery, MAF, thought it was an issue with my tune, with gas quality, intake filter. Turn out I had a vacuum leak at the airbox, fixed it and haven't had a single issue since.

Yeah just finished changing the alternator, mechanic says battery is fine...and still the problem persists. The voltage also isn’t changing to where it should be which is 13-14 while the vehicle is operational. Still stays under 13 volts...any suggestions? Thank you

 

[ugstang17]

Since you never mentioned it everyone is assuming the car is completely stock....correct? If not what are the mods?

If not you need to paint the whole picture. Crystal Balls only work in the movies and allowing people to assume the car is stock is only wasting peoples time when the problem ends up being related to a modification that was done.......happens more than not.

 

[Mrpopper239]

Since you never mentioned it everyone is assuming the car is completely stock....correct? If not what are the mods?

If not you need to paint the whole picture. Crystal Balls only work in the movies and allowing people to assume the car is stock is only wasting peoples time when the problem ends up being related to a modification that was done.......happens more than not.

Has a cold air intake and Roush exhaust that’s it.

 

[steveo1960]

Has a cold air intake and Roush exhaust that’s it.

Not Mustang specific, but carbon build up on EGR valve is a common cause of poor idle, especially when coming to a stop. Often no code gets set. Just a suggestion.

 

[Mrpopper239]

I'd spray around the hoses to see if there's a small air leak or check for a sticking pcv valve.

Nothing like that...also cleaned the MAF sensor and throttle body...still the car can’t hold an idle..it keeps dipping so bad...I have done almost everything to diagnose it..last thing to do is maybe put a brand new battery?

 

[Dfeeds]

Nothing like that...also cleaned the MAF sensor and throttle body...still the car can’t hold an idle..it keeps dipping so bad...I have done almost everything to diagnose it..last thing to do is maybe put a brand new battery?

Eh, not sure a battery would cause a bad idle, but I also wouldn't have suggested the alternator either. A bad ground could definitely cause some anomalies, but anything along those lines would have other electrical issues or lights dimming at idle. I'd check the spark plugs and coils before the former. The TPS (throttle position sensor) is a possibility. I'd also put the stock intake back on and see what happens, if you're out of ideas.

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