Hello

Would be grateful for any advice :

Engine management light came on recently - garage plugged in the diagnostic which revealed loss of compression in cylinder 4

Upon taking the spark plug out of no. 4, this was covered on oil. The mechanic immediately diagnosed new rocker gasket and new plugs - and as the leads were original new leads as well

However, since this was done, I now have a lumpy idle - when hot or cold - the revs just seem to drop approx 50 revs, every 5 seconds or so

Any ideas why this is so - does this sound directly related to the work done ??

Any advice much appreciated

Edited 3 March, 201016 yr by skoda675

Probably piston rings I'd say. Is the engine the AUB code? What age and mileage? Be ready for an expensive bill.

Probably piston rings I'd say. Is the engine the AUB code? What age and mileage? Be ready for an expensive bill.

But the idle was absolutely fine before the gasket and spark plug change - what makes you think piston rings ?

Engine management light came on recently - garage plugged in the diagnostic which revealed loss of compression in cylinder 4

Can the diagnostic interface really tell a mechanic about the state of compression on a particular cylinder? I have no idea but it sounds almost too helpful to be believeable!

Can the diagnostic interface really tell a mechanic about the state of compression on a particular cylinder? I have no idea but it sounds almost too helpful to be believeable!

I was there at the time and can verify this

I think the diagnostics can say "misfire on cylinder [number]" and one cause of a misfire can be loss of compression. I don't think diagnostics can directly determine whether any particular cylinder has lost compression though (I could be wrong this is just going by my limited experience). A spark plug fouled with oil would cause a misfire (as there would be no spark).

http://wiki.ross-tech.com/index.php/16685/P0301/000769

And I can't see that a rocker cover would do anything other than let oil drip down the outside of the engine?

Edited 4 March, 201016 yr by anewman

Coolant Temperature sensor - I've trawled through all the info on here about it and one of the techy guys has suggested this can cause lumpy idle. There is a guide with handy pictures on how to replace it yourself http://briskoda.net/forums/topic/61178-fabia-coolant-temp-sensor-how-to/.

I have the same problem with the other halfs 1.2htp and had a new sensor delivered from Jorily yesterday which was only about a tenner, I'll fit it and let you know if it makes a difference.

And I can't see that a rocker cover would do anything other than let oil drip down the outside of the engine?

It might flood the outside of a spark plug with oil, unless there's a drain hole in the bottom of the pockets.

I mentioned the piston rings as this is a common fault with the 1.4 16v engine and the original poster reported at least one spark club being covered in oil which may suggest oil getting past the rings. I'd have thought that the first thing to do would be to have a formal compression check done of all the the cylinders.