Erratic throttle response when cold
 

I'm still trying to get a good feel for what goes on and when, but this is what I've noticed so far - more than once in each case:

1. Driving away from my house when starting the car from cold usually involves reversing out then driving off uphill, a definite hill but not especially steep.
I've noticed the car surging then slowing then surging forward again without me changing the position of my right foot. "Kangaroo petrol" :tuttut:
(I may have experienced this elsewhere but not 100% sure as I'm usually driving away gently on level ground)

2. Pulling out of a side road and accelerating the car only speeds up "gently" not "urgently" as I expect. No ABS/ESP warnings or symptoms. Only seen within the first minute or so of driving the car and not always by any means.

I haven't yet tried starting it on the drive and gently pressing the accelerator pedal to see if revs vary.

Problem #1 is a bit weird but #2 is potentially quite dangerous.

No codes showing on VCDS. No "slipping" or increased revs, just varying power so I suspect engine-related not gearbox.

It's an FL S8 which I understand is "fly-by-wire" though what the relevant components are and where I don't know.

Suggestions please on possible causes and how to diagnose.

have you checked that your manifold flaps and actuators aren't sticking and are operating properly? just a thought. . .

Pretty sure, I checked the other day (as noted somewhere here when I first got similar symptoms) and the links are definitely connected properly and the vacuum units operate as they should.

What can you check easily other than position when off and when started - they certainly move instantly.

A little more input on this problem.
It's been suggested that it might be a temperature sensor somewhere playing up and causing under-fuelling, so to try just leaving the car to idle and warm up somewhat before driving off. Then when it's cold again plug in VCDS and look for suspicious readings.

Did the idle warm-up this morning not touching the pedal, letting the needle move to about 60C before driving off; it behaved perfectly.

Though I have VCDS I wouldn't know where to start looking for sensors that can be checked, or which ones are likely culprits? Help! :o

Check engine temperature when it's cold if it shows much more them ambient the clearly it is wrong.

Well yes, but apart from the gauge on the dash (which seems to operate normally and has 50C at the bottom) where would I go (in VCDS?) for a more accurate figure? ;)

Measuring blocks and cycle through the channels to see different readings

OK, I'm with you, didn't realise it would explain what the measurements were. Haven't explored VCDS much so I'm a novice.:o

Did that and found nothing that looked odd, temp 15C about 3 hours after last use in 3C ambient which seems very plausible.

Started the engine and watched various measurements as it warmed up; again nothing looked odd to my inexperienced eyes. Though it did feel a bit "lumpy" when I revved it up to get it warm.

I think I might need to set it up to graph the throttle position among other things and go for a drive (with an assistant).

Any further thoughts anyone on a systematic way to investigate this and nail it down? It's getting annoying. :tuttut:

I went for a short drive yesterday, starting off by "pootling" down the road (250yds downhill) and turning left into the main road at the bottom.
Luckily it's only a 30mph limit as at the first attempt it just dawdled, so I didn't pull out. :angry2:
Second go a few seconds later was fine and I then turned on cruise control for most of my journey which is in a 50mph limit.
After a 2-mile drive it was parked for maybe 30-40 minutes then started up and driven home - no dramas at all, behaved normally.

Checked codes today and there's only the intermittent 01119 Gear Recognition Signal problem in ABS (03). Could that be in any way related? :Confused:

are all the breather pipes in good order and not leaking anywhere? just a thought as it sometimes causes lumpy running etc. . .