Erratic throttle response when cold

HPsauce · #1 14th February 2013, 05:01 PM

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"

(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.

Architex_mA8tey · #2 14th February 2013, 07:16 PM

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

HPsauce · #3 14th February 2013, 07:20 PM

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.

HPsauce · #4 22nd March 2013, 09:55 AM

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!

ainarssems · #5 22nd March 2013, 03:10 PM

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

HPsauce · #6 22nd March 2013, 03:13 PM

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?

MikkiJayne · #7 8th December 2016, 10:22 AM

Quote:

Originally Posted by HPsauce

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.

Interestingly, since having 'mended' my F125, mine is now doing this most of the time. The first 50% of the throttle does very little, but prod it a bit further and it takes off like a rocket. It happens mostly in 1st which is very annoying, but also randomly in other gears. If I'm in town pootling around at 30 and slow down for something it'll take a hefty prod to make it move, at which point it changes down to 2nd and goes a bit nuts. Its much better behaved in tiptronic, to the point I had to tow the FE home entirely in tip otherwise it would barely move. No codes either.

Time for a new switch methinks...

moltuae · #8 8th December 2016, 11:08 AM

Quote:

Originally Posted by MikkiJayne

The first 50% of the throttle does very little, but prod it a bit further and it takes off like a rocket. It happens mostly in 1st which is very annoying, but also randomly in other gears. If I'm in town pootling around at 30 and slow down for something it'll take a hefty prod to make it move, at which point it changes down to 2nd and goes a bit nuts.

Check your brake pedal switch too. Had a similar problem with mine a few years ago, turned out to be iffy brake pedal switch contacts. I noticed the fault would tend to occur more after firm braking and tapping/wiggling the brake pedal before setting off again seemed to alleviate it.

steamship · #9 8th December 2016, 01:06 PM

Quote:

Originally Posted by MikkiJayne

Interestingly, since having 'mended' my F125, mine is now doing this most of the time. The first 50% of the throttle does very little, but prod it a bit further and it takes off like a rocket. It happens mostly in 1st which is very annoying, but also randomly in other gears. If I'm in town pootling around at 30 and slow down for something it'll take a hefty prod to make it move, at which point it changes down to 2nd and goes a bit nuts. Its much better behaved in tiptronic, to the point I had to tow the FE home entirely in tip otherwise it would barely move. No codes either.

Time for a new switch methinks...

After I had the faulty F125 switch problem (dashboard lit up), I had it replaced with one from the forum sponsors. That has worked fine most of the time, but I too started experiencing the same issues as Andrew, and resolved it with the same quick stab on the pedal, although I only tend to experience it in the lower gears like moving off from junctions or having slowed at a roundabout.

Quote:

Originally Posted by moltuae

Check your brake pedal switch too. Had a similar problem with mine a few years ago, turned out to be iffy brake pedal switch contacts. I noticed the fault would tend to occur more after firm braking and tapping/wiggling the brake pedal before setting off again seemed to alleviate it.

Curious you mention this as I've had error codes about the brake switch before and had it replaced with a brand new one. However, I still get error codes relating to the brake switch. In fact, I get it in Engine (01) 16995, Transmission (02) 17087 and ABS (03) 00526. Is it possible that the connector on the loom could be loose and not giving me 100% contact? If so, can the connector be opened up and the crimps tightened, although from memory I think they are round in shape.

MikkiJayne · #10 8th December 2016, 01:54 PM

Quote:

Originally Posted by steamship

Curious you mention this as I've had error codes about the brake switch before and had it replaced with a brand new one. However, I still get error codes relating to the brake switch. In fact, I get it in Engine (01) 16995, Transmission (02) 17087 and ABS (03) 00526. Is it possible that the connector on the loom could be loose and not giving me 100% contact? If so, can the connector be opened up and the crimps tightened, although from memory I think they are round in shape.

Likewise. I've just changed the brake switch because all three modules were complaining about it. With the new switch only the ABS unit logs a fault. I wonder if that is still affecting the ECU?

Further diagnostics needed