Erratic throttle response when cold

[HPsauce]

Well, I've done quite a few short journeys today and all was going well, even at places that "often" show symptoms. The car was really quite "sprightly".

Then I did the short journey that is almost guaranteed to give problems and.....

Originally Posted by steamship

It was almost as if you were driving with the handbrake on.

To explain, I leave my house (which is on a corner of a side road so there's a bit of reversing and wiggling initially) and drive "down" my road to its end, some 350 yards downhill in a 30mph area steep enough to require minimal throttle or braking until stopping at the end where it joins a main road.
I then stop and turn right (mostly) onto a single-carriageway "A" road (still 30mph) and very often get these symptoms. It's slightly uphill and a fairly busy road so you need "reasonable" throttle, though this evening the road was empty so I didn't rush.
I drove about 200 yards with the engine revving around 3000rpm at under 30mph (and at least one gear change) before I got bored and "blipped" (released and repressed) the throttle at which point it reverted to sensible behaviour, pulling well around 1500/2000rpm.

Subsequent journeys this evening have been through very similar junctions (it's a hilly area) without problems.

[steamship]

Bummer! I was hoping you'd finally sorted this, since I experienced something similar recently. I haven't had the 'scalded cat' effect yet, but then I still have issues with SAI and fuel mixtures running on LPG.

Your description of your drive to the main road was very interesting as it sounds similar to mine, the only difference being mine is about 3/10 mile long and involves avoiding potholes in the lane.

[Delboy]

I can't help but think these issues are gearbox related , have you tried leaving the house in tip mode and tried different gears like first all the way to the junction 2nd all the way etc etc

[HPsauce]

Delboy, I've had similar thoughts and tried Tip mode, though not consistently (I keep forgetting ) so it's not proved anything yet.

None of this is reproducible, it's intermittent - despite what I said about the "almost guaranteed" route which is one (due to traffic conditions) that I would only do at most once a week. Normally I go a different (only 50 yards longer) way to avoid a right turn onto a busy A road, whichever car I'm in.
I'm a creature of habit on which route I choose so don't think about it.

I can go that way later this morning, so I'll try and do it "Tip only", assuming I remember.

[HPsauce]

OK, so here's some relatively good diagnostic info. Today I have made 4 short journeys, all about a mile long and all in Tip mode.

1. About 9am with a cold engine, including my "dodgy right turn", seemed fine.
2. About 11:30am, with some residual heat from 9am, seemed OK.
3. Midday, engine still warm, thought I felt a couple of "twitches", decided to go out again 5 minutes later.
4. Just now (12:10) with a warm engine I had definite problems as described below.

Headed downhill from my house, tipped up to 3rd then 4th gear and stopped at main road very briefly, it dropped to 1.
Saw a good gap in the traffic and headed off, turning right it was definitely showing 1. VERY sluggish (for an S8)!
Changed up to 2nd, still sluggish but eventually got to approaching 30mph and around 3000 revs (does that compute?).
Blipped the throttle (release, press) and it instantly felt responsive, revs unchanged but started to increase revs/speed with normal power.
Changed up to 3rd and 4th all normal, then slowed for a roundabout which it pottered round in 3rd; cruised around the block normally at 30mph in 3rd/4th and went home to write this.

Thoughts welcome.

PS. I guess the thread title may be a bit off the mark as well.

[Nuisanceneighbour]

Just out of interest, what happens if you continue to press the accelerator during the sluggish periods? Contrary to what I posted the other day I am still experiencing symptoms of a very similar nature to what you describe but on the 32v engine.

On occasions of sluggishness I have depressed the accelerator in frustration which results in the CAT warning light illuminating and a huge loss of power twinned with a jarring thud to the transmission. Less than a second later the warning light has disappeared and power delivery is restored albeit in the same sluggish form as moments earlier.

I have noticed at times of poor performance the car seems to change up the gears prematurely (25-30mph on a hill the car will select 4th gear).

[HPsauce]

Originally Posted by Nuisanceneighbour

Just out of interest, what happens if you continue to press the accelerator during the sluggish periods?

In Auto mode it just changes up gears and accelerates gently until I get bored and "blip" the throttle.
It doesn't normally happen in a situation where I have the luxury of waiting to see what transpires.

I've ordered a genuine Bosch Fuel Pressure Regulator (from ECP) in desperation, should arrive tomorrow.

[Goran]

Originally Posted by HPsauce

I'm still confused by this, it's certainly a bit different since I changed the FPR. I was going to swap it back but have had problems with the other car (Focus - fuel pump fuse blew!) so I'm not touching the Audi at present as I am relying on it.

I now seem to have 3 modes of operation:
1. Normal (most of the time)
2. Sluggish, rare as reported above, resolved by a quick "blip" of the throttle
3. Scalded cat!

The latter is new. Just using middling throttle (definitely not flooring it) it will sometimes take off much faster than I'm expecting. And you can really feel the power kick in as the flaps open at higher revs.

Forgot to add, starting is still a bit "iffy"; I find that if it doesn't catch quickly I turn off and try again and it's usually OK 2nd (or 3rd) time.

All I can add is that this reminds me exactly how my old S8 behaved. Sometimes normal, sometimes sluggish, and on rare occasion accelerates faster than expected without much throttle.

My guess is its the Transmission software which can limit engine torque that's causing these varied behaviours. Certainly no such behaviour on the manual S8, doesn't matter if the weather is cold, warm, engine cold or warm, it just pulls the same with only the standard split-second delay every time (probably because of the electronic throttle). More like a jerk really rather than delay and could be because of the manual running gear, clutch, flywheel, etc.
So by deduction if you eliminate the transmission and these symptoms dissapear, the Auto symptoms must be transmission related in a lot of cases?

[HPsauce]

Originally Posted by Goran

So by deduction if you eliminate the transmission and these symptoms dissapear, the Auto symptoms must be transmission related in a lot of cases?

Any suggestions how to do that? I've tried in Tip mode and the symptoms were still there as noted a couple of posts previously.

[HPsauce]

Originally Posted by HPsauce

I've ordered a genuine Bosch Fuel Pressure Regulator (from ECP)

Arrived and fitted. Slight oops in that I forgot to tighten up the connection to the fuel rail that I'd loosened to release the pressure. Very smelly of V-power around the engine bay for a while until it evaporated.

Been for a drive round, lots of short segments, stopping and starting in between and choosing "challenging" junctions. All in Auto mode. No problems so far.