silent running

I've spammed this problem around on old threads about a similar hesitation phenomenon and don't seem to have got an answer so I'm asking it myself...

The situation is that I'm driving along on closed throttle or barely touching the accelerator. If I then sharply prod the accelerator, either keeping it down or letting off straight away, the car hesitates badly for less than half a second, then suddenly the drive comes in. I've done quite a lot of testing and trying stuff out and still haven't solved it. This is what I've worked out so far.

1. It's not the ECU - it hesitates on both a standard ECU and on a Power FC
2. It's not turbo lag of any kind. Flooring the pedal shows an instant vacuum-to-boost transition but the revs don't move at all whilst it hesitates
3. Don't think it's the pedal or cable, as small pedal movements do open the throttle and show changing voltage on the throttle position sensor.
4. TPS shows OK voltage range and response time, although when I provoke a hesitation, the voltage does occasionally seem to stick at its bottom value (0.7 at the moment, which I adjusted it to) and it seems 'slow' to react sometimes.
5. TPS baseline position doesn't really alter the hesitation (i.e. adjustment of bottom value)
6. The hesitation is totally independent of engine temperature or revs. It is entirely repeatable, time after time. Simply go from a closed or very slightly open throttle and suddenly prod the accelerator and it will hesitate.
7. At high road speed the effect is still there but not as marked because a sudden lump of sharp acceleration after a hesitation is obviously a lot less violent than at low road speed.
8. Injector duty looks like it rises instantly in line with boost, although this is harder to see on my Power FC display.
9. Mapping logs showed no leanness on sudden acceleration.
10. It is not down to acceleration enrichment - I tried various settings and none of them made any difference to the hesitation problem.
11. Once into the main pedal travel, the problem disappears completely. A sudden prod from say, a 1/4 throttle, produces an equally sudden leap forward and rise in revs. There doesn't appear to be any play in the the accelerator pedal travel. I did always wonder why there are two different throttle quadrants though - could this be the problem?
12. It's not the lambda sensor. I just changed it and the problem is still there just as bad although the car in general feels smoother.
13. Doesn't feel like the clutch as this is working fine at the moment.
14. MAF sensor (blue label F model) is showing nothing unusual - idles at 1000-1100 mv depending on revs, full boost/revs shows a fraction under 5v. Injector duty maxes out at around 96%, shows 0 on overrun and seems to pickup in line with the boost.

So, any ideas? It's not a major thing - I learnt to 'drive around it' by slightly slipping the clutch on each up-change but I don't like driving like that if I can help it and it's bloody annoying. If I AM wanting to bang through the gears all the way into 5th it's not exactly confidence inspiring having the thing kangarooing on every gearchange with the transmission thumping each time as if it was a dodgy auto box. Logic suggests the problem is tied up with the transition from closed to open throttle but whether it is to do with fuel, air, spark, drivetrain, throttle mechanism, I just don't know. Next thing I'm definitely trying is a change of TPS, although new ones are far too much, I'll pick one up second hand. HELP!

Scoob+Bike=Fun

I would definatly start with the TPS like you say.

have you tried the readings with long meter leads from sat in the drivers seat or on the bench? Maybe try altering the position of it and see if the hesitation takes place with the throttle in the same position or if it moves. AFAIK you will have to re-set the ecu to tell it where rest is.

have you checked the plugs or changed them?

anything else happened since the problem started?

Adam

silent running

I've tried reading the voltages two ways - first is by reading it off directly from my Power FC Commander's display, as this shows me what the Power FC sees. The only problem with this is that even though I presume the Power FC itself reads throttle position super fast, the display only updates a few times a second. The other way of reading it was when I had it out and on the bench. It was one of those three hand jobs - trying to hold two probes into the socket to get a resistance reading whilst carefully turning the spring loaded potentiometer...as far as I could see though it seemed to work OK. And of course, as its a digital multimeter, it still updates several times a second. Really I need something analogue. With the TPS fitted and reading off the Power FC, if I move the pedal slowly and steadily, I get a slow and steady rising voltage off the TPS. Of course the problem is that slow and steady accelerator movements aren't what I'm worried about!

I've never even seen the plugs let alone change them and I couldn't say when they were last done. I've had it since I bought it last year on 105k miles but it was serviced regularly beforehand.

silent running

Anyone else? Someone must have experienced this themselves?

ZEN Performance

remember you can graph with the PFC aswell as show numerical values.

Honestly, I don't think anything is wrong.

thedeester1

Normal! i can get 2 different responces from my scoob depending on weather. Remember japan is very humid. Since we dont get humid we have to settle for fog. My car goes like stink when its foggy or misty. Try a re-map before you start swapping parts. Even a car with an electronic brain will choke if you hammer the throttle.

silent running

Yes I've tried watching the traces but the problem is the horizontal scale is 10 seconds so if there is a momentary lag it doesn't show up on the screen. I've tried doing a through the gears run then freezing the display.

I know it sounds like I'm making a fuss over nothing, but the more I think about it the more weird it seems...the hesitation's always been there since I had the car - I'm sure of that now, but I've never known any other fuel-injected car to do this, whether fuel injected or not.

thedeester1

Id figure its not fuel injection. There is turbo lag. It is lesser in dense air. Like humidity or fog. I have to say however that flooring the throttle in ANY car under a driving load will see some lack of acceleration. There are 2 ways to use the throttle. The first is the gradual feed where you push the pedal to equal the power. The second is using the throttle like a switch. You will bump the throttle to the floor then lift it then bump it again. I have to admit im the switch kind of person. Once you get used to the method all will be good. A good deal of racing drivers use the switch method! its not wrong its just easier than trying to find the position on the throttle that will drive you.

silent running

Mmm I can see what you're saying but this is definitely not turbo lag or lack of acceleration...there is HUGE acceleration available when you floor the pedal from any other position, however gradually or sharply. The only problem is that if the throttle is closed or almost closed, you have to wait for a fraction of a second while absolutely nothing happens then it suddenly comes in with a thump.

Jolly Green Monster

iTrader: (4)

is it still on the standard fuel pump and reg and injectors?

silent running

No I swapped the pump for a Walbro 255 but it's still on the standard regulator and grey injectors which IIRC are 380cc. From watching the injector duties they don't seem to be out of range and response seems fine.

I did briefly wonder if it was the regulator not 'keeping up' with sudden changes in throttle position, but the more I drive it and think about the problem the more convinced I am that is is solely restricted to the transition from closed throttle to open. I can't see why the fuel system will be perfectly fine under 95% of conditions, yet in the least 'stressful' position - a slight blip from closed throttle - they give up?!