Weird idle when engine's warm. What could it be?

[general_piffle]

Hey, just as the title says. From a cold start my car (16v GTI) starts and runs no problems at all. But when the engine is warm and I start her up sometimes (not always) the revs rise and fall at idle. Like a rhythmic rise and fall as if I'm gently revving the engine then taking my foot off and repeat.

If I dab the accelerator idle settles back down to normal. Has been doing this for about 2 years and no idea what's causing it?! Any thoughts? Thanks

[Simeon]

Presumably you've already cleaned out the Idle Stabilisation Valve (ISV)? These become gunked up over time and cause a fluctuating idle.

[general_piffle]

Presumably you've already cleaned out the Idle Stabilisation Valve (ISV)? These become gunked up over time and cause a fluctuating idle.

No I haven't Simeon, where is it located? Thanks.

[itavaltalainen]

It's the cigar on the top of engine.

[general_piffle]

Thanks!

[Simeon]

Yeah, it's under the plastic HT lead holder. Whip out the three allen head bolts holding that down and you'll see the ISV. Pop off elec plug and pipe going to airbox then take out the allen head bolt holding the bracket onto the inlet manifold and the ISV will come out with a decent pull.

Refitting I find it easier to put the rubber bung in the manifold then apply a little wd40 and push the ISV back in, making sure the bung stays seated properly.

Google cleaning the ISV, petrol or white spirits or similar will do the trick

[gasketface]

Also check for vacuum leaks,especially the brake servo vacuum pipe at the servo end as the plastic goes hard over time and cracks although the soft rubber pipe at the manifold end can also perish and leak so check both ends.

Whilst you are there you may as well check the whole pipe lol