Who's in control of my TDI?

[rory]

When cruising along with my foot holding the accelerator " cracked open " and firm so as not to change my foot position; power is reduced when starting down slopes and alternatively increased when starting up hills. (Feel it in my seat and confirmed by the fuel consumption indicator) It does this in a "step" or "steps" according to the slope I guess. It does not do this as a smooth progression.
My foot definitely does not move.
My TDI is a 77kw 2009 Jetta one (with 7 speed DSG); the 1.6 litre diesel one.

I'm wondering is this a feature of just my model? Or is it in Golfs as well?
This morning I chatted to a Golf TDI (125 Kw with DSG) owner (an earlier model as he'd had it for 3 years) at a servo and described my experience and asked if his did this too.
He said it did not and suggested there may be something wrong with my lovely near new Jetta. (Cruise Control is off for all this of course)
Anyone care to comment? Please?

[wokwn]

No I haven't noticed that. You could use VCDS to log pedal position sensor against some other things like speed, manifold air pressure (boost), torque requested and calculated torque output if you wanted.

Some other points to consider:

- Your turbo is a variable vane type and is independant of the acellerator pedal; it can dial up any amount of boost that the ECU thinks it needs and the amount of boost is not dependant on the engine RPM (within boundaries)

- I'm not actually sure what the acellerator pedal does. It doesn't have a direct relationship to fuel injection quantities - perhaps it's actually a torque-demand control. There may be some sort of long-order algorithm in play here to smooth the driver inputs in order to present a smoother-feeling ride, in the same manner as the power steering makes subtle suggestions to the driver (by applying pressure to the steering wheel) to get the driver to counter-steer in cross winds. The new ESP (not sure if it's in the MKV/VI) actually makes non-subtle suggestions to the driver in order to reduce or prevent skids.

- There may be some psycho-somatic thing going on here.

- The instantaneous fuel economy is vague at best and often disagrees with other devices (such as a scangauge)

- If you have a DPF the ECU will play with the exhaust temperatures (particularly on freeways) in order to burn off the deposits in the DPF. This is achieved by adjusting mixture which may manifest as changing power levels.

[ian]

Are you sure that your cruise control is not engaging, because thats exactly how it works?

[rory]

I thought it was a side effect (feature) of the cruise control (which I think is absolutely brilliant) which I would have to grin and bear when not using it. The cruise control is definitely OFF when this is happening.

When I'm travelling at a steady 80 ks and the instant fuel consumption reads "say" 4.2 litres/100ks; I have my foot frozen to the accelerator; and I start down a hill; as the car speed increases;, I feel "by the seat of my pants" a deceleration force ,( not unlike as if I'd taken my foot off -and I hadn't) and a reduction of the instant fuel consumption to 0.0 litres/100ks.
My question is if this is "normal"

[benough]

I'm not crystal clear on your description but I have experienced something where if you have your foot on the throttle ever so slightly, enough to get the injectors going, the car will behave similarly. i.e it's enough throttle to keep it moving but not enough to match the acceleration of the car's own weight down a hill.

I highly doubt it's wrong/broken just a part of the "drive-by-wire" throttle.

The point about the VVT is good also. It's like a second throttle. Try and find someone with a boost gauge in a TDI, or buy mine and then your brain will understand how the motor/turbo behaves.

[rory]

I'm putting the foot down in anticipation as I go down hills to achieve a smooth drive.
Also being a fuel miser ( a reason I got the 1.6 version) I like to think I'm can avoid unecessary engine braking as well as normal type braking.
Maybe I'm jealous that I can never aspire to drive my car as smoothly as the cruise control does?
Thanks for the boost guage offer; it might help me think my car is "normal" but won't really change things.

[maca]

I'm putting the foot down in anticipation as I go down hills to achieve a smooth drive.

See, I've never noticed this issue because I always do exactly what you've said. I don't floor it up hills, I just compensate for what the car will need to get up the hill without reducing speed.

[rory]

My Jetta TDI is the only car I've ever owned where I find myself putting the foot down to get down hills.

[rory]

YES!
Had my 30,000 k service today and on the test drive the mechanic noticed something about the DSG. Went back to check if there were any "updates" for the computer involved. Yes there was; so he applied it. Problem solved. I now, for the first time since I bought my brand new Jetta last April, have 100% control of the power (except when cruise control is engaged) and am ecstatic .
No more putting the foot down heading down hills so as to get a smooth power delivery.
My VW experience just got better!!!

[Mickey Black]

Glad to hear you got a result, it sounded totally muffed. Definitely not normal!

[Greg Roles]

I WAS going to say DSG, but glad you got it sorted at the dealer. For the record, in a diesel, the accelerator pedals does indeed control fuel injected, as a diesel is throttled by fuel not air as in a petrol, but it would do so via a fuel map and consider all sorts of variables, so it won't feel like a direct link at all times. That's common to any drive by wire car built for the masses.