Hesitation under light throttle, has anyone experienced this?

[Jaymz]

Guys the GT has been running the standard map for awhile, thanks to the DPF getting blocked and going into limp mode. The tech didnt read the worksheet where it said dont remap this has been chipped

All good though, they came to the party and Peter James from Superchips has given A1 customer service aswell

I am on my second tank of BP diesel now with the standard map and it has given me a good opportunity to check figures like fuel consumption and just generally how the car performs compared to the Superchips tune.

Fuel figures are really not all that different which is surprising, maybe 50k difference out of a full tank.

My car now has 11,000k's and it recieved it's first service at 7,500 k's.
For those approaching the 7k mark i cant recommend strongly enough doing your servicing at 7,500k's instead of the service book 15k's.
The car is so much smoother after the service, i was pleasently impressed.
The gearbox is slicker, the engine revs easier, just alround a nicer thing to drive.

Now over the last month the car has had a hesitation at very light throttle, almost feels like a flat spot around the 1100-1500rpm point.
I asked Peter to speak to the guys at Superchips HQ about this and they said they don't touch any of the map under 1500rpm so that counts them out.
Now the car has the standard tune it still feels the same.

I get this mainly in the lower gears, off boost, slowly working up to 60. I can also feel a hesitation when pulling up a driveway slowly in first, which is on a incline and also the same if i reverse up.
It's definately there and i'm sure it wasn't there before.

Has anyone else experinced this? It almost feels its starving for fuel at very light throttle.
Blocked fuel filter? Any thoughts anyone?

FYI this car has lived on BP diesel since day dot, never used anything else

[mikinoz]

I made a comment on this about a year and a half a go.

It is only under similar circumstances, light acceleration on slight uphill gradient.

I asked the dealer and no codes were thrown - I think it is something to do with the turbo variable nature, and such a slight acceleration is not enough to identify where it needs to open and as such the hesitation.

I can make it do it if the conditions are right. Doubt that there is much that can get fixed to be honest...

[Jaymz]

Cheers mikinoz.
You could well be on to something. Maybe the limited exhaust gas flow at low revs is the cause. And it could also be related to the amount of fuel that is being delivered at light throttle. It doesn't struggle at all when on boost.
If its common to diesels, no problem i'll move on.

If only we could have separate maps for fuel and timing and then ad a EBC, we would be in business then

[Pumpe-Düse]

My Polo does it on the odd occasion... and it hasn't got a DPF so...

[mikinoz]

I have no DPF either.

[gldgti]

referring to a part of the rev range that i would say isnt included in normal driving - i would make light of the fact you have yourself identified that it is a very light "throttle" - a setting where you could be slowing down, easing off, etc.

remember that the ECU doesnt know what you are going to do next - the only information it gets is your input and current curscumstances - ie accellerator position, and all the things it measures that the engine/drivetrain is doing at that instant.

i am reminded of driving mischa's GT, and stalling it - being used to my old IDI, no computer diesel. because the mk3 is governed by linear springs and flyweight mechanisms, there is a combination of linear control responses to any accelerator position - but all of these equate to the injector pump wanting to put more fuel in the cylinder - and hence, at idle, there is useable torque, and its a seamless progression as revs go up.

the digital equivalent system that your ECU replicates is not bounded by springs and weights, and so can be non-linear (read, piece-meal) in its control response. as a result, it is probably programmed to give little to no fuel at very light accelerator settings, so as to minimise fuel usage, for example, during gear changes, and when transitioning between cruiseing and braking.

i would very much rule out anyhting related to boost pressure - even the VNT on your TDI wont make useable boost until you're adding a reasonable amount of fuel to the mixture ESPECIALLY under 1500rpm, and so does not apply for "light throttle settings"

[Jaymz]

Thanks gldgti. So what you're saying is i should stop babying the thing and get on the loud paddle some more

[gldgti]

Thanks gldgti. So what you're saying is i should stop babying the thing and get on the loud paddle some more

roger that.

[Russ59]

Hi Jaymz,

I have had a similar experience with mine trying to accelerate at low revs in 3rd gear on a slight hill and until the turbo kicks in it is pretty sluggish. I guess it would be more a case of a bit of turbo lag, I just have to get into the habit of changing back a gear and keeping up the revs. I'm so used to highway cruising and overtaking in 6th like it's in 3rd or 4th gear (jeez I love the torque). Interesting to hear what you said about your car running heaps better after the 7000 k service, I've always been a strong believer in regular oil changes on all my cars over the years and they always seem to run a lot better once done.

Russ

[mikinoz]

I agree that this is a miss and is probably as mentioned the ECU guessing what is going to happen and getting it wrong.

Bloody electrics, give me mechanics any day...

[bluey]

Couldn't it be sticky vanes on the turbo? (In which case if you regularly rev it a s per the guidelines, it should go away.)

Why would a 7500km service make any difference? wouldnt it be an oil change and flog around the block?

[gldgti]

Couldn't it be sticky vanes on the turbo? (In which case if you regularly rev it a s per the guidelines, it should go away.)

As stated earlier - it is highly unlikely the turbo has any real effect on engine performance below 1500rpm.

[Greg Roles]

Mine feels eggsactly the same. To me it's most obvious as being "unhappy" at partial throttle at about 1700rpms, just before boost, where it tends to surge a little if driven quietly in this off boost range. Only find this on partial throttle situations, and I figure it's just the "turbo lag" thing and no big deal. But then again I manned up a while back, so perhaps that's the answer?

[bluey]

Can I presume this is happening crawling along in peak hour traffic? No sign of it yet for me. I can't say I can detect any turbo lag at all, but that might be the difference between 8 valves and 16 since 16 should have less torque at low revs.

[mikinoz]

It happens for me at any speed above 50km/h under light acceleration, most evident around 65-70 in 4th.