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HPsauce

Occasional erratic throttle response, ongoing mystery ideas?

While this is a problem on a D2 S8 so many of the relevant systems are common that I hope other model owners can come up with some ideas.
(Lurkers in the A8parts forum will have seen this all before)

I've had this S8 about 2 years and soon after I got it I noticed the odd "kangaroo" moment, especially when cold.
My previous S8 (PF, cable throttle) never had this, but this is fly-by-wire which I think may be relevant in some way.

Typically (but not often), soon after driving away after a cold start and more likely (but not always) after negotiating a T-junction the car moves off sluggishly. Releasing and pressing the throttle resumes normal behaviour - always and immediately.
If I just keep my foot in the same position it will speed up slowly and change up gears, but at higher than normal revs.

I have observed this a few times when the car is fully warmed up, but that's pretty rare.

It's been going on for nearly 2 years now and a lot of work has been done on the car in that time including gearbox fluid/filter change, full cambelt job (water pump, aux belt etc.) and normal servicing. The two garages (a local Audi specialist and a larger well-known Audi non-franchised specialist) have found nothing wrong.

No warning lights and nothing ever shows on a VCDS scan.

I'm looking for ideas here as I'm stumped and so are the people on the A8parts forum.
Searider

Have you tried disconnecting the throttle body plug and spraying the contacts with wd40?

I had some erratic throttle moments on light throttle and this sorted it.

Had been advised that an ageing MAF or diverter valves could have caused the symptoms.  Changed those anyway although no difference.

Make sure you've done all the simple stuff first!
HPsauce

Searider wrote:
Have you tried disconnecting the throttle body plug and spraying the contacts with wd40?

No, I'll give that a go.

MAF and PCV valve have both been changed. Throttle position sensors checked several times with VCDS and all seem fine.
Quote:
Make sure you've done all the simple stuff first!
Any suggestions as to what that is - I'm listening.  
C J

Try giving the throttle body and disc a good clean with some carb cleaner and a small paint brush.
Nice simple cheap thing that usually helps.
HPsauce

OK, so here's a really INTERESTING bit of diagnostic data.......

The nearside catalyst was replaced yesterday and not long after I "thought" I detected a hint of the problem, but in a few short journeys since it's been OK.
Then an hour or so ago I had the opportunity to take a non-optimum route somewhere but one that often creates the symptoms. And it did! So, not the catalyst as a cause.

Now, it always behaves fine on cruise control, so I had wondered - what happens if I switch cruise on while it's having a "go-slow".

The symptoms appeared as I turned slowly into a main road with a 30mph limit, so I let the car accelerate gently (throttle pedal pressed some way down to do that) then hit the "set" button at 30mph without moving my right foot. I don't recall exactly but it was probably near 3000rpm when I did that.
Wallop!  It instantly set off like the proverbial scalded act!
Took my foot off and it eventually slowed and settled back down to 30mph on cruise.

So, what does that tell us?
I don't think it took the time to do anything in the gearbox, the response was literally instant, going from gentle to heavy throttle, as if it had reread or reinterpreted the pedal position.

Any and all thoughts welcome......

I'll try the same test again whenever it happens and it's safe to do so. Anything worth noting at the time or changing?
27litres

I'm a little bit late to the party as I don't frequent this forum often...
Seems to be am issue of communication between the accelerator and throttlebody.
Short of being an ECU issue (which is possible due to its intermittent nature), I would be looking at the potentiometer on the accelerator pedal.
It'll be up high on the pedal box.
Try doing the usual unplug, spray contact cleaner, replug procedure and see if that helps.
Intermittent problems are often caused by bad contacts.
If that doesn't work, see how much A8Parts charge for a second hand potentiometer and try that out.

The problem with modern diagnostics is that there's a lot of items not monitored by the ECU or other modules, so you still need the mark 1 diagnostic tool (your head)!

Good luck
HPsauce

27litres wrote:
Try doing the usual unplug, spray contact cleaner, replug procedure and see if that helps.
Intermittent problems are often caused by bad contacts.
Thanks Marty,
When I went to look it's a bit of a swine to get at. Can't even get at the plugs/wiring!
The knee panel is easy enough - I've had that off many times for various reasons - but there is ducting in the way that I just can't figure out how to remove. I'm waiting for better weather to be honest.  

I've already acquired a replacement potentiometer from A8parts at a good enough price, as at the price it's a good diagnostics tool - if I can ever fit it!    But I suspect the problem is elsewhere due to the odd way it often relates to sharp turnings out of side roads.

Not much has happened lately, it's been a bit variable for a short way after starting from cold, but otherwise OK. No repeats recently of the "really sluggish" mode.
Also, I'm currently running on BP 95   - I was desperate last time I filled up and haven't been far enough since to empty the tank.

As you say, I'll continue to use my head, but its getting very confused on this one.
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