Cylinder off.....
 

Done a bit of diagnosis on the S8 I picked up a couple of weeks ago. Looks like its going to take more fixing than I've got time to give it, but just before i move it on, thought I'd run past them that know more about the bonnet contents than your truely +++

1) Cylinder 6 seems to be completely ineffective. Engine performs the same with the coil pack connected or not

2) Wiring to Cylinder 6 seems rock solid, all wires trace back to where they should.

3) Cylinder misfires on both LPG and petrol - So not an injector issue

4) Swopped coil packs and plugs - no difference

5) No faults logged in ECU - Suggests not an electrical fault

6) Compression is significantly lower on cylinder 6 than the others

So, I guess I'm looking at either a sticky valve, or failed piston seal ?

One thing that really interests me, is that the plugs from cylinder 6 came out wet, and smelling strongly of petrol..... Surely, even with poor compression, the petrol, spart plug and air would still burn up the fuel, just not provide any useful power from it ?

Anyone got any troubleshooting tips that dont involve removing the cylinder head, or using an expensive camera I dont own.... If its valves, I'd be slightly tempted to fix as it could be done in situ, piston probs is a bit more major....

Same prob I have except cylinder 7
 

Well I have the exact same problem except on cylinder 7.

If you put a small amount of oil in the cylinder when it is at top dead centre and check the compression...if it goes up then its piston problem otherwise it could be valve or head gasket....

That was the advice I got...and mine turned out to be problem with piston ring...hence why I m thinking up manual conversion....

Let me know how you get on.

I was suggesting the old oil trick to Ranj. It is not conclusive but does give idea if it's more likely to be piston cylinder or head/valve problem.

How much is the compression difference?

When you say no fault code are there no codes at all not even for misfire? Does it show misfires in measuring blocks?

You do not need expensive camera/borescope to look inside cylinder you can make one out of old laptop webcam which you can pick for less the £10 from e-bay if you do not have donor.

My low cost borescope is mentioned in this thread:

http://forum.a8parts.co.uk/showthrea...ight=borescope

Ian, sorry about cylinder. Can't help much here. I think it's a "head off" job to really diagnose the thing...

D2 again. Back to basics? +++ D3 was a pawn in much larger plan? :)

Need a new compression tester to give proper readings. The one I was using read 6 bar before I even connected it (!) I'll pick one up tonight.

The best cylinder I tested read around 15 bar, the faulty one read around 10 - but I cant guarantee these numbers, only that the faulty cyclinder read relitively quite a bit less than a working one.

Nothing in vagcom at all for misfire - As I believe it is firing, but oddly there arn't errors even when I unhook coil packs. Not checked measuring blocks, but will do.

Still puzzled over the wet plugs though. If it fires, why would the petrol still be there. Is it possible to have wet plugs with low compression only - Or should I be looking even harder into the wiring ?

Is the oil test theory that the oil will help seal the piston a little, thus improving the compression if the pressure is leaking around the piston?

I'm not scared to sort out a valve problem, as that can be done with the block in situ, but if I'm removing the block, might as well put something more interesting in -but I havnt got time for that ...... :rolleyes:

Oil and water are both clean and seperated btw, so not suspecting head gasket right now.

Arrgh... This is a steep learning curve for a non-mechanic like me.. +++

Have you tried changing the plugs around to determine if it is a faulty plug?

If your mixture is too rich the spark won't ignite it

Yup... Changed plugs and coil pack. No difference.....

You should be able to connect a spare spark plug to the HT lead and make sure it's earthed to the engine and see if it gives a healthy spark with the engine running.

The 'oil test' was good on old fashioned engines with flat top pistons sitting in the vertical plane thereby ensuring the oil was evenly distributed around the rings giving a temporary seal.

On a Vee engine the oil will sit on the low side so not so useful a test.

The new way is to pressurise up the combustion chamber via the spark plug hole and listen, stethescope? for air escaping into the sump or coming out the inlet tract.

It's interesting that VCDS shows nothing ie it thinks everything is OK so my twopence worth is that a minor wiring break on the HT side is causing this.

Always look for the simple solution first and one thing at a time if you go for substitutions +++

Stew,