Jumpy Idle with AC on

[janherko]

Dear fellow roccers,

is there anybody who could point me in the right direction with this issue?
Ever since I bought my Viper Green some 3 years ago, and in addition to pretty much having a good time with it (except for the episode of major engine issue forcing me to replace pistons:(...), I've been having a hard time fixing up a jumpy idle immediately after AC is engaged...rpm jump between 700-900, sometime it calms down (especially after a longer ride), but still, this hassle is really pissing me off.
All the possible causes have been checked, from the throttle body alignment to alternator pulley and AC compressor itself...to no avail. AC has been refilled, is cold enough but the damn rpm, when in neutral, simply don't want to be stabilized...right after I hit the AC button...
Any opinions?

[Roy_01]

Maybe you can raise the stationairy rpm a little with vcds to +/- 800 rpm and see if this is running more stable.

[blower]

Unfortunately there is no way to raise the idle with VCDS on this engine.

Have you cleaned the throttle body? - my idle was also a little twitchy at the lower RPM's and cleaning the throttle body with acetone helped quite a bit, of course if you do this do another throttle body alignment.

Also idle is very sensitive to the condition of the spark plugs on this engine. When were they last replaced/checked....make sure the spark plug gap is no less than 0.75 and no greater than 0.85. Contrary to some information on this forum having too small a spark plug gap can severely effect idle stability and quality - I have found my idle is noticeably smoother if I gap them to 0.8mm from the stock 0.7-0.75 that new plugs come with (the stock plugs are supposed to be 0.8, but they are always less than this when you measure them)

[janherko]

Yes, both throttle bodies properly cleaned, alignment performed...still to no avail. I'm very cautios regarding the spark plugs on this engine and the current ones are the newest spec for our engine, had them changed some 5 months/5000 km ago...though I didn't play with the gap at the time rather left it as it was when brand new...

[blower]

How many miles on this engine? - like all direct injection engines they can suffer badly from intake coking.
I cleaned mine out recently when i took the head off, it was a right mess : http://www.sciroccocentral.co.uk/forum/ ... =2&t=22332" onclick="window.open(this.href);return false;

Other than a clogged up intake, other things that could cause the idle to hunt are:

-Vacuum leak (cracked intake manifold, cracked evap/brake booster hoses, oil dip stick not fully seated)
-Faulty map sensor (it would only be the map sensor on the intake manifold, the other ones are not used during idle)
-Clogged fuel injector (poor idle)

Check fuel trims to see if there is any long term fuel trim correction - indicating vacuum leak etc.

[janherko]

Ok, will do as advised...let u know
Thanks man

Edit: 163.000 km on it

[janherko]

one more question here, can a faulty alternator cause jumpy idle on neutral when AC compressor is engaged? I have noticed that my 3-months old battery never shows voltage higher than 13.6-13.9V and though I´ve never recorded any fault code related to alternator itself, my mechanic is somewhat skeptical with these outputs and says the whole idle hesitation could be due to faulty alternator...most mysterious thing is that once I disengage AC, the idle on neutral has always been super smooth.

Also, when scrolling up windows, at the very last motion as it closes up, I can clearly notice a half-second flash of footwell lights...is this normal?