erratic idle when in gear (MT)

hey i just purchased a 2002 P5, car is in excellent shape, (first P5) and i noticed then with the car in neutral, clutch in or out, the idle is fine, but when i put the car into gear, the idle jumps between 1000 and 1500, with the clutch pressed in, car drives fine and has no running issues, as soon as the trans is taken out of gear the idle returns to normal, searched for an hour and came up with nothing, any help would be appreciated, thanks!

for first gear and reverse?

Is the idle high while driving i.e. you let off the gas and the RPMs never go below 1500-ish?

[shrug]

hey sorry for the late reply, no its when the car is at idle, not in motion, if the car is put into any gear (M/T) the idle will come up about 400 rpm. bought the car used, and found out the guy had actually taken the bulb out of the CEl... turns pout there was an angine code in there. P0507 Idle air control system higher rpm than expected. any help?

someone has messed with the adjustment screw on the throttle body. That screw needs to be adjusted again to bring the idle back down to normal. Someone has probably did that as a band aid to another problem...probably covering up a vac leak or something

the car will still idle alright once warmed up, but the idle will still increase when placed in gear, could this be a combination of a too high idle screw and vaccuum leak? or do you think there is actually an IAC problem?

play with the adjustment screw to see if that lowers it any. Normally a vac leak or malfunctioning/dirty IAC will cause a crappy and low idle

There are two throttle body screws to adjust idle...one is the physical stop bolt for the throttle plate...hopefully the previous owner didn't mess with this...adjusting it will adjust the throttle position sensors calibration for 'no throttle' and you can get things quickly out of wack with that...and its very hard to put back to normal without some electrical equipment...

but there is also a AAS screw, which is just a little phillips screw on the firewall side of the IAC, on top of the throttle body...that screw adjusts the air by pass passage for the IAC...when adjusted with a jumper wire in the engine bay diagnostic port, you can set your idle speed at something outside of default...if you adjust this screw without jumping the panel; it simply changes the IAC's behavior to maintain idle...but idle speed will remain the same...

IAC 'too high' probably means the PO messed with something he shouldn't have...but you can first try closing that AAS screw a little at a time and see if it helps...closing it reduces air by pass from the IAC...making the IAC open a little more to maintain the same idle speed...it could be that the AAS is out too far, and the TB plate is open too much at zero throttle input...that even with the IAC completely closed its letting in too much air...closing off the AAS my help this, but its a little unlikely...

Whats probably happening with putting it in gear...is that the PO messed with the throttle plate...so the computer never sees 0% on the throttle position sensor...with a clutch, it doesn't really disengage as much as you might think...at least immediately...so when you put it in gear the spnning clutch disc still 'sticks' a little to the flywheel...this will load up the engine briefly...couple that with the engine computer thinking you are lightly pressing the throttle pedal...its adjusting ignition timing thinking you're about to start moving...and that will slightly race the engine...

sorry i don't have a direct fix...its probably going to come down to you playing with the TB plate (closing it off VERY slowly) until you can get that into workable parameters...and this is also verifying as best as you can that there are no vacuum leaks...however, a leak will often throw a too lean code, as well as 'minor or major' vacuum leak codes...if you're only getting an IAC code...its most likely because of what i mentioned...

Hey guy, again sorry for such a late Rey, put the idle issues on hold while I have been tracking down 2 water leaks, I will look up some specs on that AAS this weekend an let you know, thanks alot!