S8 Enjin Troubles
 

So, spawning a new thread for this to get some collective brains around it.

Since getting the car back, the idle has been rough. not so rough that the rev needle jumps about all over the place, but rough enough that engaging drive is a jerky experience and its annoying me.

I'm not an engine expert by a long shot, so this is me laying down what I know in the hopes that other folk might have ideas where to look.

Info:

Car is LPG converted. No noticeable difference between fuels.
Has 8 brand new coil packs fitted
Has 8 new spark plugs fitted
Has oil / air filter change
Cleaned intake manifold
Problem is more pronounced when engine is cold
Would say it is more pronounced again when very cold outside
Compression test is good.
Timing is spot on
Heads were changed by previous owner, not sure why.
Engine is AQH - Early Facelift S8 engine 4.2 V8
Maf was changed for known working good one, no difference
Engine ECU change, no difference
Only fault code ever related to one lambda. Both lambda and cat changed for known good ones. No change
Pulls well. MPG is good. Both poor though before service new coils & plugs
Measuring blocks all seem within tolerances, but recommendation is that these are done with coolant virtually at 90 degrees, by shich point problem is not so bad.

Ideas ?

Does it have the standard camshafts fitted and if so are they worn or are the followers worn? Are the followers hydraulic or solid? Sounds like the temp thing could be something to do with it. Like the cam followers if hydraulic are not pressurising correctly until the oil is sufficiently warmed. Maybe?

Coolant Temp Sensor changed recently? (an easy job, but fiddly and easy to lose bits!)
I note you say: Remember it's a 2-part (4-wire) sensor so a good reading on the dash may be misleading.

Is the LPG system currently waiting until the engine has warmed up before cutting in or trying to operate from start? Try turning Lpg off completely until fully warmed up and then see if you get the problem using petrol only perhaps?

Torque converter dragging the engine when cold?

Sounds like a possible air leak - have you had the LPG system checked/serviced?

throttle body, change it with working one and will be ok.
If not change it will going worst.

We like this idea most at present - The car did idle notably better after the service, which I'd assumed was the sparks/coils.

It could well have been some decent new oil that was slightly thinner and helped as the old stuff was well past it.

Will run an engine flush through it me thinks asap and see what happens... +++

There is no difference between lpg or petrol, no matter what the temperature...

Yes, it starts only on petrol, but the BRC is pretty good and soon switches to gas even when the engine is still largely cold.

What do you mean by rough? Hunting up and down?

Sticky idle control valve - will be operating in a different part of its range when cold and providing a bigger proportion of the airflow.

I guess a manifold air leak might cause a ragged idle and reduce as the engine warms as thermal expansion reduces the gap, but I also think that a leak large enough to cause ragged running would probably show some codes relating to mixture control.

Hydraulic followers usually suffer from "pumping up" at higher rpms as oil pressure is higher there. My experience of Audi followers with problems is that they clatter when cold, shut up when warm and make no difference to the running.

Nick