Cupra revving in cycles from idle

Odd problem just appeared about a week back on our Cupra mk2, hoping nothing critical!

Instead of idling smoothly at just under 1k rpm (I think it was about there anyway, maybe more like 750-800?) it's started revving up to 1.5k over about a second, then holding there for two seconds, then dropping back (over a second) to about 800 for two seconds, and repeating. This is in neutral or in gear with the clutch in (I've not tested whether it will accelerate the car on its own if the clutch is out but the accelerator not pressed).

Found a couple threads, one referring to DSG gearboxes in the mk3 so assuming that's not related, (although sounded a similar symptom) and one referring to a missing oil filler cap being the problem - so I'm going to double check that when the OH brings the car back - but any other ideas in case that's not it?

Thanks in advance!

(referred to threads: mk3 DSG - http://www.seatcupra.net/forums/showthread.php?t=423517
and oil filler cap - http://www.seatcupra.net/forums/showthread.php?t=426319 )

Thanks, that's great, I'll check that out! Think Dave@Sere is going to have an enquiry in his inbox in a mo...

(Interesting idea on the PCV delete kit, thanks @iDan - does it have any benefits or drawbacks besides not having to worry about the PCV going again? Seems like there must be drawbacks, otherwise why have the PCV in the first place?)

Keithio said:

I had the exact same problem and it turned out to be the oil filler cap extension. These are just glued to the engine in order to bring the filler cap level with the engine cover and the glue had come away causing a vacuum leak. It was easy fixed I just did away with the extension and screwed the oil cap straight on to the engine. Might be worth checking yours.

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Particularly handy little tip this one, I'll give the extension as well as the oil filler cap a good inspection!

Hmm. It doesn't seem to me that Forge's blurb really explains the potential downsides - after all, presumably there IS a reason to have the valve there in the first place. Think I'll probably have it replaced stock, but I'll be interested to see what your results are like in 6 months!

So the oil filler cap was on just fine. With the engine running, I pulled out the dipstick and massive suction noise and the engine starts running rough. Which is classic faulty PCV right? Anyway, I've ordered a replacement from Dave (as well as the charcoal filter which also came up as a fault) so should be right as rain soon enough...

So thanks everyone for the tips and advice, all very much appreciated!

vfr400racer said:

If engine runs rough with dipstick out then pcv ok aslong as engine returns to normal when dipstick replaced. May still not be at its best though

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No it hunts when you put the dipstick back in. But it runs really rough if you take it out. You don't think that's the PCV then?

Blimey, I bet! I've ordered a new PCV so we'll see if it fixes it, otherwise a hose inspection is next I guess!