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I have a Mk1 VW Golf/Rabbit with K-Jet mechanical injection. Occasionally when I pull up at stop lights the engine idle will rhythmically dip and climb (between 600 and 1200 rpm). This probably happens 1/6 times I stop and it doesnt seem to matter what temperature the engines at.

The car feels and drives fine otherwise.

I would think this might have something to do with a vacuum leak but if that were true I would assume the problem would be consistent and not happening randomly. That being said, I am pretty sure the brake booster is knackered so their might be some issue there.

I'm not sure about the idle control valve either as the car has no air-conditioning so nothing drawing large amounts of power at idle (is this what the IAC valve does?).

Would anyone know what could be causing this and what I should be looking for?

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I would check the following:

  1. PCV Valve Assembly
  2. Idle Air Control Valve
  3. EGR Valve

Many autoparts stores will help you swap out simple parts like these at the store itself, but obviously I would call in advance to ensure that they'd be willing to help first.

If you go through those three things, I'd recommend taking to car to a shop and having them do a smoke test on the system to see where else there may be leaks.

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    Yep you're right: Idle Air Control Valve – MeltingDog Sep 21 '14 at 23:37
  • Awesome. Glad to help out. – Sage Sep 25 '14 at 19:14
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Just for others who find this: the problem returned and this time it was the O ring on the idle adjustment screw. They can wear down and become flat creating a vacuum leak.

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