Idle surging

[Mustang Sue]

So my issue has been a little bit of an ongoing one for a while and so far I've replaced the below list of parts due to various codes and misfiring issues etc when I first got the car along with general maintenance. Its all stock so nothing questionable done to it as far as I know!

Its a 2007 V6 4.0 5 speed manual and while driving my idle speed will sit at around 750 rpm for most of the time when I stop, but randomly it'll rev up to 2000rpm + when I'm at a stop sign or lights or when I push the clutch in before it settles back down again to a lower number. Sometimes if I've stopped somewhere and turned the car off then started it up again for the rest of that trip it behaves normally, so it does seem to be pretty intermittent!

When replacing the throttle body I ran through the idle relearn procedure from the manual as usual, and for that trip - a near 40 mile drive with speeds from 20mph - 70mph it ran perfectly so I was hoping it had settled down but two days later on a drive it started randomly revving again!

Any ideas on what I can try to solve this problem or has anyone noticed it on their 4.0?

Parts replaced:

Throttle Body
Battery
Spark Plugs
O2 Sensors
Leads
Coil Pack
TPS
EGR Valve
Fuel Filter
Oil & Filter
Air Filter

Hopefully there's someone out there who has had similar issues to this and can shed some light!

Thanks

Darren

Did you get your altenator tested?

 

[DarrenN]

Did you get your altenator tested?

Hi there,

I checked out the alternator last weekend, it did seem to be out putting the correct voltage, but that was when it was just sitting and idling. I've not been able to test it when any surging has been happening though. So tomorrow I'll test it again on a drive and see if the live OBD II scanner picks up any weirdness between the voltage and any RPM surges if it does it.

I'm currently thinking (whether correctly or not!) that if the car has been sitting for a few days now, then the battery will have drained a little to have a lower voltage and the issue may return then if it is a low voltage thing. As Juice mentioned- "The battery volts low is interesting, and may point to the alternator. Pcm maybe raising idle to compensate for low system voltage. A possibility, monitor voltage see if it is low when idle rises."

If after the car has been sitting for a few days and the PCM is raising the idle to compensate for the lower voltage until it can come back up that may point to the alternator - it does seem to generally happen when the car hasn't been driven for a few days and when it's a cold start. When it's warmed up it generally behaves normally... I'll see what happens tomorrow and hopefully have an idea of where to go with things next