Done a search but not come up with anything on here.  Think I have narrowed down where the problem is coming from by chance due to the hot weather.  Switched the AC on late last week and the idle smoothed out. Its not bad but frustrating when you know how smooth the engine can be when its not happening.

 

From what I can see online the options seem to be drive belt tensioner or more likely the alternator clutch pulley starting to play up.  It does sometimes make a strange noise on startup like a dry rubbing type of low pitched screech but only momentarily for maybe half a second which makes me think it might be the alternator pulley.

 

Anyone had expereince of either starting to fail?

Are you certain that your Vrs has the one way clutch pulley fitted to your car, as standard fitment is usually a solid pulley? - this does not mean that you have not got one, as somebody may have fitted a higher output unit to the car at an earlier time.

 

The AC has a switch in the system to lift the idle speed to allow for the load of the compressor running, so that may just be the fact that the higher idle speed is correcting your issue and hiding the cause. Have you checked all your vacuum and breather pipes are in good condition and securely fitted?. How does the car run once you are rolling and driving?.

This is the problem when trying to Google a problem.  Found lots of sites talking about clutch pulleys on 1.8t engines causing the belt tensioner to bounce

https://www.google.co.uk/search?q=alternator+clutch+pulley+1.8t&ie=utf-8&oe=utf-8&client=firefox-b&gfe_rd=cr&ei=pudHWcekNLTHXsOSr9AE

Anyway, have checked the vac lines and from what I can see none have perished or are leaking but not had any of them stripped off the car.  When the AC is engaged the revs don’t increase although you can feel the engine compensate for the extra load.  Sometimes there is a wobble on the engine when idling other times it is silky smooth but a raise of 100rpm or further from idle and the issue is gone.

I was unsure if any other issue like a faulty PCV valve could be the cause.   Car pulls and drives well.  But am considering putting it into JBS to have a diagnostic run on the dyno and see what they think. 

When was the throttle body last cleaned?. Is the car mapped or standard?. Also, what sort of mileage is on the car?.

Edited 19 June, 20178 yr by kentphil1

18 hours ago, kentphil1 said:

When was the throttle body last cleaned?. Is the car mapped or standard?. Also, what sort of mileage is on the car?.

 

100k, not mapped and about 4 weeks ago.  TB was fairly clean when I looked in so gave it a wipe in situ and idle issue was there before I gave the TB a wipe.  Serviced in the last 4 weeks including new plugs, air filter, MAF cleaned.

In view of the low cost of a pcv valve down by the filter housing, it may be worth a go, however I would be certain that all your breather hoses are sound under the inlet manifold and check that they have not rotted and split.

 

I would double check all your coils are sound and that the wiring to the coils has not gone brittle and degraded the insulation causing shorts.

 

If you can post a piccie of your alternator pulley we should be able to tell almost immediately if it has the one way pulley or not for you.

Thanks KentPhil. Will get a pic when u have a spare minute which might be this weekend. Not sure how I can test the coils but will have a good look at the wiring and go over the breathers again.

You can't really check the coils for operation without diagnostic software like VCDS or similar, but what you can do is closely examine them to make sure that none of the shielding tube that covers the plug is loose or breaking up, also check that the main circuit board is intact and not showing any signs of the sealant on top of the coil starting to degrade and let in moisture.

 

There is a resistance figure that the coil should give when tested with a multimeter listed in the skoda works manual somewhere, I'll try and find it and post it later if I am successful.

I have a multi meter at home so could have a look at the resistance if I know the values.

 

Would basic VCDS lite be enough to check things like the coils.  I have been toying with getting a cheap cable and a copy

Just read the manual, and it only gives a resistance figure for coils with an output stage like the older packs that used to use HT leads, which our Vrs type coils have in built so are unable to be accurately checked.

 

If I remember correctly you can only do this with either a full version VCDS or a registered version of VCDS Lite, but to be sure I would ask one of the guys in the VCDS section on here, as I have the full version, so have never dabbled with VCDS Lite. You can read codes, but I don't think you can interrogate systems in depth with the basic VCDS Lite, in the same way that you can in the full version. Pop up a question to be sure though.

 

 

Did you ever get to the bottom of this problem? My Octavia 1 1.8T has just started to exhibit the same symptoms - definitely not the hot weather! I just took it to a service centre and the VAG box didn't indicate any fault codes, so I'm hoping you may be able to shed some light...

Cheers - John.