My superb 3.6 has a rough idle and a slight missfire when idling but runs fine while driving. My local dealer cant find anything that could be causing it. There are no codes coming up that wiuld suggest any failed components.  Does anyone on here know what to look for.

 

Cheers

 

Matthew

When did this start happening and anything changed/done just before it - even if it doesn't seem relevant?

 

Worth doing a compression test to check valves are sealing OK and not affected with carbon buildup (leaking past the valves does give poor tickover but better with foot down a bit and at higher revs)

 

Coil packs a consideration  although they usually show up when you try and pile the power on. When coil packs are borderline they don't always throw a code.

Edited 17 April, 20197 yr by bigjohn

Its just had a major service and they had trouble getting plugs out. Could they have damaged anything then and how would i be able to check.

 

Matthew

If it’s since the service head back to them to sort it out.

Had a Volvo 850 with full Volvo service history and I struggled to get the plugs out having to work them out/back in, absolute joke.

When  I worked on trucks and service’d the company cars we always used copper slip on the plug threads and never had any issues with plugs coming out.

Thats how the Volvo was afterwards, crack them undone and just unscrew by hand 

Edited 18 April, 20197 yr by Snapper1725

On 18/04/2019 at 07:24, 7415963 said:

Its just had a major service and they had trouble getting plugs out. Could they have damaged anything then and how would i be able to check.

 

Matthew

 

Was it mis-firing before the service?

 

Worth checking plugs have been tightened - I once left a plug loose on a Ford Zodiac V6 and it eventually blew it out, dinting some bodywork!

 

and yes they could have damaged the coil packs or damaged the plug threads

Edited 19 April, 20197 yr by bigjohn

All the above or a poorly seated plug lead/cap possibly.....

Do not assume just because  new parts ( the plugs ) are new means that they cannot be faulty 

i have just removed all the plugs and the gaps were all over the place, from 0.2mm on a bent plug to nearly 1mm.

these were fitted by a main dealer and they couldn't find the problem.

all plugs now gapped to 0.8mm and smooth running has returned.

Good to hear your sorted, bet plugs had just been taken from boxes and fitted.

some bad dealership servicing being reported lately, mine tried to get £290 from me for rear discs and pads. 

Nothing wrong with the discs, cost £30ish for gen pads 

17 hours ago, Snapper1725 said:

Good to hear your sorted, bet plugs had just been taken from boxes and fitted.

some bad dealership servicing being reported lately, mine tried to get £290 from me for rear discs and pads. 

Nothing wrong with the discs, cost £30ish for gen pads 

In my (limited) experience, even out of the box (quality) plugs are better than that, and if they are not correct they are at least consistent. That sounds like a handful of plugs which have bee  dropped/thrown on the floor!