Tickover not quite right

So this is not urgent as the car still drives fine but…

With the engine running i blip the throttle, the revs rise and then fall too far almost to stall then there is a tick from something at the back of the engine bay and the revs rise bit higher than tickover then this repeats a few times until the tickover settles at the right level. This happens with the engine warm or cold. It's an 86 cabrio with DX engine and CIS injection. Now i've had a (quick!) look at the Bentleys and have a feeling there is a sensor not quite right. At first i thought i'd spotted the problem because the Aux Air Regulator was not plugged in! Unfortunately, this did not seem to help with the problem but it does make me wonder if this was accidentally unplugged or whether someone has looked at this before and disconnected this for a reason (car is new and only had it a few days so previous history unknown). Now before i get the electric meter out and start working through the manual trying to figure what's going on (this new fangled injection business is all new to me) can anyone possibly point me in the right direction or give ideas where to start looking?
Thanks
Andy

Idle that fluctuates is a couple of things
Timing, close but not quite dialed in.
Duty cycle is off.
Vacuum leaks.  CIS and Digi are abhorrent to vacuum leaks.  While you replace the vacuum hoses look at the rear of the throttle body.  There is a 7mm Brass screw between the Throttle Body and the Fire wall.
This screw has a o-ring on it that fails.  

If this screw is able to be turned your your fingers, then the o-ring is brittle and needs to be replaced.

Count the turns that it takes you to remove the screw. Write that number down. Take the screw to a good hardware store and get a replacement o-ring.  

Tip.  Wrap the Threads only the threads with PTFE tape.
(Teflon Tape) and return it to the count that you took to remove it, you will need the 7mm wrench to tighten the screw and slightly grease the o-ring.

Don't discount the injector seals as a source of the vacuum leak as well.

Try increasing the idle speed (screw at the back of the throttle body) as it sounds as if it to low and the anti stall device is kicking in and that is why the revs go up and down.
Idle speed should be about 950rpm, once the car is nice and warm increase the idle to 1100 rpm and give the throttle a couple of blips then lower the idle back to 950-1000 rpm.

As already mentioned you may find the o-ring is squashed and hard and that is why the idle screw is working it self lose.

As the car is new to you are you using Super unleaded petrol or was the timing changed for normal unleaded petrol?
I find super unleaded is best as I could not get mine to run and tick over smoothly on normal unleaded no matter how much I played with the timing.

Idle screw is here.


Old o-ring


New o-ring fitted

That's is useful info. Will check my o ring this week…..

Many thanks for the very useful pointers. Just took the tickover screw out completely and while it was not loose the O ring was very old, hard and brittle. Found close enough one in the garage and added bit of ptfe to threads then popped back in to same place. Fired up, left to warm then backed out a few flats to up tickover to about 1000. Definitely seems to have improved matters - tickover drops a little below 1000 for split second after blip and then settles but no longer get the tick from anti stall so will try this for few days and see how it goes. Will also try some different fuel at next fill up (currently just using standard). Have also ordered some O rings for the injectors so will also change these and check all the other vacuum pipes this weekend. Thanks again, Andy

When you run on super unleaded listen out for any and "pinking" under load as some one may of retarded the timing to run on normal unleaded.

This thread has helped two people now as I have just diagnosed the same problem with the idle screw o-ring!
When you say super unleaded petrol, is that something like BP Ultimate or is there a higher octane unleaded? Excuse the silly question, I've been running diesels for a long time now and my MK1 GTI is the first petrol car I've owned for years!

Standard unleaded is 95 RON. Most petrol stations will offer a super unleaded of 98 RON - by a variety of brand names.

When you change the injector o-rings, a little grease on the o-ring helps to get the injector seated correctly when re-installing.

Super unleaded is also available at some supermarkets, Sainsburys offer 97 RON super which I normally use as its on my way to work, Tesco also does a super unleaded at 99 RON.
Asda and Morrisons don't sell super unleaded.
Most other petrol stations sell some sort of super unleaded, just look for petrol with 97RON upwards.
As already mentioned normal unleaded petrol is 95RON.

If any one needs o rings

http://vwgolfmk1.org.uk/forum/index.php?page=topicview&id=freecycle-mk1-golf%2Ffree-idle-adjustment

I also found the brake servo check valve was leaking on mine (see my thread in Help!!!) and have blocked the leak which helped and I also found tonight that the vacuum hose from the vacuum advance on the distributor to the throttle body had split where it bends around on to the throttle body! Along with the o-ring which was at least as bad as the picture mk1gls posted. It was getting too late to try so I'll give it a go tomorrow!