Delayed Throttle Response on a Turbo Diesel
Discussion
I've got myself a new pickup truck - had an Isuzu before, this time I went for a Navara. So far I'm having issues getting to grips with the throttle response on it, I suspect (hope) that there is some kind of technical issue, because it's a pig to drive. Basically, when I lift off the accelerator pedal, it seems to ignore my input, and carries on accelerating. This happens for about 0.5 to 1s after lifting off. As you can probably imagine, it's a b
h trying to drive it. To get any kind of smooth gear change I have to lift off the accelerator and wait for a second before pressing on the clutch, otherwise the revs rise even more rapidly as though you haven't lifted at all, and you end up with jumpy gear change. Adding an extra second of delay to every upshift has a dramatic effect on acceleration!
I can't think of another vehicle that I've driven that responds with any noticeable delay to my accelerator inputs - I expect the engine to stop accelerating with no discernible delay when I lift off the pedal. My previous Isuzu pickup (with a very similar size twin turbo diesel engine) certainly didn't exhibit this kind of behaviour.
I find it difficult to believe that this is normal behaviour, but at the same time I'd expect the ECU to throw a fault pretty quick if it has an inkling of an issue related to drive by wire accelerator input. So before I go back to the dealer and report it as an issue (which I'm half expecting to be fobbed off with an excuse of it being 'normal' behaviour), I've plugged into the OBD2 port and logged what the ECU is reporting.
There appear to be two accelerator pedal positions reported (Pid 0x49 and 0x4A, Accelerator pedal positions D & E), which I think is pretty normal for safety critical systems like drive by wire? They both report almost identical values throughout all of my logging, so I'm presuming they are working correctly, and I've just worked with one of them for the below plots. The range of accelerator position readings goes from 16% (no pedal input, to 82%, full pedal input, which again I think is normal to have some 'dead space' at either end of the range?
So I've logged engine RPM, accelerator pedal position, and vehicle speed (to give a record of what gear I was in at the time. This first plot below, I'm accelerating up to around 2.5k to 3k rpm and lift off the pedal completely, waiting for the vehicle to slow down in over run and then repeating a couple of times. The first 3 goes are in first gear, last two are in second. As you'd expect, it's more pronounced in lower gears, but happens to an extent in all of them. As you can see, the revs fall for a very brief moment, then revert back to rising again for a longer moment (just under a second) after I've started to lift off the accelerator. I'm lifting off pretty much instantaneously, and the pedal position reflects this. Even after I'm totally off the accelerator pedal the vehicle continues to accelerate for a short while, it must be deliberately choosing to inject fuel at this stage for that to happen?
There also appears to be a similar reverse sort of delay/spike when pressing the accelerator pedal - the revs start to rise, then they drop momentarily, before starting to rise again. But it's much less noticeable, and less of an issue than when lifting off. The second plot is what happens in a typical acceleration from standstill through the gears. Same thing is happening - it ends up behaving like you've pressed the clutch before lifting off - the revs shoot up between gear changes.
One thing that I've noticed by ear, is that the blow off valve doesn't open immediately when I release the accelerator, but more in time with the end of the delay when the revs actually start falling. Could it be caused by a sticking dump valve? Although I can't see how air alone would cause the revs to rise when under load - that would need fuel to be injected to create power?
h trying to drive it. To get any kind of smooth gear change I have to lift off the accelerator and wait for a second before pressing on the clutch, otherwise the revs rise even more rapidly as though you haven't lifted at all, and you end up with jumpy gear change. Adding an extra second of delay to every upshift has a dramatic effect on acceleration!I can't think of another vehicle that I've driven that responds with any noticeable delay to my accelerator inputs - I expect the engine to stop accelerating with no discernible delay when I lift off the pedal. My previous Isuzu pickup (with a very similar size twin turbo diesel engine) certainly didn't exhibit this kind of behaviour.
I find it difficult to believe that this is normal behaviour, but at the same time I'd expect the ECU to throw a fault pretty quick if it has an inkling of an issue related to drive by wire accelerator input. So before I go back to the dealer and report it as an issue (which I'm half expecting to be fobbed off with an excuse of it being 'normal' behaviour), I've plugged into the OBD2 port and logged what the ECU is reporting.
There appear to be two accelerator pedal positions reported (Pid 0x49 and 0x4A, Accelerator pedal positions D & E), which I think is pretty normal for safety critical systems like drive by wire? They both report almost identical values throughout all of my logging, so I'm presuming they are working correctly, and I've just worked with one of them for the below plots. The range of accelerator position readings goes from 16% (no pedal input, to 82%, full pedal input, which again I think is normal to have some 'dead space' at either end of the range?
So I've logged engine RPM, accelerator pedal position, and vehicle speed (to give a record of what gear I was in at the time. This first plot below, I'm accelerating up to around 2.5k to 3k rpm and lift off the pedal completely, waiting for the vehicle to slow down in over run and then repeating a couple of times. The first 3 goes are in first gear, last two are in second. As you'd expect, it's more pronounced in lower gears, but happens to an extent in all of them. As you can see, the revs fall for a very brief moment, then revert back to rising again for a longer moment (just under a second) after I've started to lift off the accelerator. I'm lifting off pretty much instantaneously, and the pedal position reflects this. Even after I'm totally off the accelerator pedal the vehicle continues to accelerate for a short while, it must be deliberately choosing to inject fuel at this stage for that to happen?
There also appears to be a similar reverse sort of delay/spike when pressing the accelerator pedal - the revs start to rise, then they drop momentarily, before starting to rise again. But it's much less noticeable, and less of an issue than when lifting off. The second plot is what happens in a typical acceleration from standstill through the gears. Same thing is happening - it ends up behaving like you've pressed the clutch before lifting off - the revs shoot up between gear changes.
One thing that I've noticed by ear, is that the blow off valve doesn't open immediately when I release the accelerator, but more in time with the end of the delay when the revs actually start falling. Could it be caused by a sticking dump valve? Although I can't see how air alone would cause the revs to rise when under load - that would need fuel to be injected to create power?
chammyman said:
What Navara do you actually have?
You don't have a dump valve either.
The usual faults I find with Navaras other than snapped cranks and snapping chassis are sticking vnt's, dodgy boost solenoids and EGR issues.
It's the current one, a D23 twin turbo manual.You don't have a dump valve either.
The usual faults I find with Navaras other than snapped cranks and snapping chassis are sticking vnt's, dodgy boost solenoids and EGR issues.
If it's not a dump valve, what makes the noise when I lift off the accelerator, that sounds like a dump valve? My Isuzu made the same noise as well.
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