Erratic throttle response when cold

[HPsauce]

Just a minor update on this still-unresolved issue.
Errant behaviour still happens every now and then, so I've now followed the lead of some others and order a replacement throttle pedal position sensor (potentiometer) from our sponsors.

The car is due in for a check up on a lift later this week; loud rattling when cold from under the car (stationary or moving) so I suspect that the iffy catalyst is on the way out and that the SAI pump is adding to the symptoms when cold. Might get the garage to fit the replacement potentiometer at the same time if it has arrived.

[RanjS8]

Have u consider the fuel pump relay or fuel pump?

[HPsauce]

Hi Ranj, yes I've considered it (and had the filter replaced) but the FACT that it ALWAYS reverts to normal good behaviour with a little off/on twitch of the throttle pedal suggests the problem is elsewhere.

[steamship]

Originally Posted by RanjS8

Have u consider the fuel pump relay or fuel pump?

Mine sort of confirms what HP says, as I have a similar problem, but not as pronounced, as mine runs on LPG, and the twitch on the throttle resolves it.

[HPsauce]

Originally Posted by HPsauce

I suspect that the iffy catalyst is on the way out

Definitely is, extremely rattly. But over £1000 to replace with a new one.

[HPsauce]

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

The 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 - would that be 2nd gear?.
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 it again whenever it happens and it's safe to do so. Anything worth noting at the time or changing?
(one thing I might try next time is pressing the cruise set button at a lower speed, below the normal activation limit which I think is 25mph.)

[Delboy]

Almost sounds like it's not reading the throttle pedal position correctly, cruise control makes its own calcs and therefore commands the throttle wax accordingly.

Did you ever check the wiring to the pedal sensor?

[HPsauce]

The intriguing thing is that it appears that the throttle pedal is being misread/under-read OR misinterpreted BUT just pressing the cruise set switch means that the pedal is read correctly (maybe it's reread?) and as it's already a fair way down it overrides the cruise setting until I lift my foot when it settles down.

It makes me think there's something weird going on in the engine ECU.

[brasiliangringo]

Originally Posted by HPsauce

Just a minor update on this still-unresolved issue.
Errant behaviour still happens every now and then, so I've now followed the lead of some others and order a replacement throttle pedal position sensor (potentiometer) from our sponsors.

The car is due in for a check up on a lift later this week; loud rattling when cold from under the car (stationary or moving) so I suspect that the iffy catalyst is on the way out and that the SAI pump is adding to the symptoms when cold. Might get the garage to fit the replacement potentiometer at the same time if it has arrived.

\

If you post up your short / long term fuel trims at idle i can advise if they are healthy, group 033 ad 034 screen grab.

Make sure you have some miles on the car since the last time you cleared codes in engine module group as is reset when this happens.

[Delboy]

Of course I mis read that, so it's possibly re reading the demand then noticing a different value then taking off.

How about as a test start out each journey and get the cruise control set at say 50mph and leave it active but not engaged, then if the car bogs down, don't throttle anymore but instead hit resume and see if the car starts to accelerate normally again.