Acceleration and stutter? help guys :)

Discussion in 'All Things Technical' started by Scutch, Feb 12, 2007.

  1. Scutch

    Scutch New Member

    Well, I've been having fun in the car since I got it back from remap with Andy F. Nothing extreme, just pottering around and listening to the funky sounds. But its about time I get to a little problem I think.

    I have one now, which unfortunatey could mean a number of possible things which you guys might know more about than me.

    I noticed it the other day first, when I took my mate out. I turned off a roundabout onto a 60mph zone and put my foot down in 2nd gear. Within a few moments it felt like the car had hit the limiter and kicked me back, but I knew I was no where near the rev limit. Nevertheless, I changed up a gear and foot down, all was fine. 4th gear, spot on, 5th, same. Amazing speed :)

    Again, with the car fully warmed up again, I pulled away from a junction and put my foot down and it did the same again, seemed to boost then hit the invisible limiter! Changed up a gear and it was ok. Slightly sluggish for a second but then fine.

    Went for some food a day or so ago with the missus, and although I wasnt giving it some welly, it felt like there was too much air trying to get through the exhaust and it was hesitating, and when I did put my foot down it was like I was forcing the air through so I could get some speed up. The exhaust pops when it does it too.

    The latter (lag I believe) is like how it is on some mornings in her Evo when its cold. Foot to the floor and its slowly building up revs before spinning into life.

    Now again earlier today, just less than 1/4 tank again and I turned left at a junction, put my foot down in 2nd and got to about 1.3bar boost before it shuddered again. I quickly changed to 3rd and through to 5th accelerating hard, was spot on boost and pull.

    Its random and doesnt happen all the time, but like most things, its annoying.

    Now, I've read a lot of posts on various forums and come up with a few things :

    MAF failing - dunno if its the right symptoms to be honest.
    Overboosting - again, could be? I think Andy set it to 1.3bar and its peaking at just over 1.5 at the moment due to the weather.
    Spark plugs - also changed them recently to top NGKs
    Boost solenoid?
    Coil packs?

    ...and the main problem, which I think it is : Fuel Starvation - as the car has less than 1/4 tank petrol in whenever this situation arises. However, is this a myth or does it actually happen? I've got an uprated fuel pump on and before the remap it never hesitated, even with the petrol light on. Will the remap, mods including Apexi ECU and fuel pump contribute to the starvation problem?

    Could be something else?

    Any tech bods have any handy tips or advice? Might be nothing, but I wouldnt want to risk anything :)

    Cheers guys!

    Its an Impreza P1 by the way. Front Mount intercooler, decat exhaust system and Apexi mapped, AP clutch, Walbro fuel pump, K&N 57i etc.
  2. jim litten

    jim litten New Member

    Nope fuel surge, as you come out of corners or boot it hard, when the tank is that low the fuel runs away from the pick up and so you loose fuel to the pump. Did you use the original sock filter on the pump, or the one that comes with the pump??
  3. Scutch

    Scutch New Member

    I really dont know mate, as it wasnt me who fitted it. Will that make a difference?

    I did think it was the fuel starvation, but just got MAF into my head as from searching a lot of people have diagnosed the MAF as the culprit. Just doesnt seem to have any other symptoms of MAF failing.

    Just odd how it didnt do this before the pump and map etc and now does with less than 1/4 tank.

    Thanks for the reply, your help is much appreciated

  4. jim litten

    jim litten New Member

    The sock filter that comes with walbro pumps is smaller than the standard one and as such won't sit in the fuel as much as the standard one, and with the car being mapped more aggressively than standard when you boot it the fuel will move around a lot more.
  5. Scutch

    Scutch New Member

    Thankyou Jim. So do you suggest I fit the original sock or one of a similar size, or will continuing to drive the car with more than 1/4 tank of fuel resolve the problem too? I'll find out tomorrow if this does indeed cure the problem, or at least erase the possiblity of it being fuel surge/starvation.

    You dont think it could be anything else?

    Thanks :)
  6. Neil

    Neil Active Member

    I too have this problem, and it is indeed fuel surge.

    Just try running over half tank to see if it cures it for the time being......
  7. Scutch

    Scutch New Member

    Going to do so at lunch time, dude, as I'm filling her up :twisted:

    Thing is though, it NEVER did it before the mods and map. So if it is indeed fuel surge, I would like to rectify the problem so it doesnt do it if I do run under 1/4 tank. If it didnt do it before, surely there is a way of preventing it from happening in the future, too?
  8. Neil

    Neil Active Member

    Oh the joys of modifying cars... :roll: :lol:

    You've got to remember all that extra fuel the engine is trying to consume, along with the extra acceleration you are enjoying out of corners.... :D
  9. AndrewC

    AndrewC Active Member

    Might be worth asking Andy what he set the boost limit at, if it's mapped for 1.3bar but pulling 1.5 then you may be close to the fuel cut.

    Also if you're still on the VF29 then 1.5bar is too much, the compressor blades won't take too much of that.

  10. Scutch

    Scutch New Member


    The thing is though, its not always on corners or coming from them when it does it :?
    On some occassions its straight line sprinting. Well, saying that, I'll have probably gone round a corner a at most a couple of hundred yards back. Will this still cause it? Cant actually wait to fill up with petrol now, just to see if it does it again! lol
  11. Scutch

    Scutch New Member

    Hi Andrew.

    In my Apexi controller, he's inputted the boost maps to 0.8, 1.0, 1.1 and 1.3 iirc. Its set to 1.3bar at the moment as thats what he said to leave it on for time being. Even so, its peaking at 1.55bar, I believe due to the dense air. Might it be worth changing the map to 1.1bar and see how that goes? I thought with it being set at 1.3bar it would limit itself on that and forbid it to go any higher. If thats not the case, what is the point of it being such a figure?

    Saying that, I was watching the defi's yesterday when accelerating in 2nd, and I'm pretty sure it boosted to 1.2 or 1.3 bar before it hit the 'invisible limiter' so it was nothing over the top. I put it into 3rd and it boosted to 1.4/1.5bar with no worries, 4th and 5th the same again. Strong.

    Also, sometimes the warning light will appear on the defi's, and other times it wont. Like the other day I was accelerating in 5th, it boosted to 1.5bar then held it at 1.4bar. Every few seconds the warning light would come on, then go off as if it was dropping boost and gaining it etc with my foot planted.

    Indeed, the joys of modifying and P1/Impreza ownership! :lol:
  12. AndrewC

    AndrewC Active Member

    In my first reply I missed that it was a PowerFC, I was presuming it was Ecutek'd. The boost cut in the Apexi is fixed at 0.25bar over target, so if you are hitting 1.55 with a 1.3bar target it will be fuel cutting.

    The boost levels you see in the commander are only targets, the number next to the boost target is the initial WG duty cycle to be used, if this is too high then you will get overboost. One of the problems with the limited boost control in the Apexi is you can't compensate boost control by gear so you have to give it a higher duty to get somewhere near target in lower gears (lower load on engine so less exhaust energy/time in gear to make boost) but not too high to provoke overboost in higher gears/cold weather. It's a balancing act.

    Make a note of the boost targets and duty cycles, and then take 2% off the duty next to the 1.3bar line and see what effect that has.

    You also mention a stutter when pushing on and the boost is around 1.3bar, what revs does this occur at and does the CEL flash with no obvious limits exceeded in the commander monitor (knock/IDC/MAF/revs/boost)?

  13. Neil

    Neil Active Member

    :roll: Why can't we just leave these cars standard...?? :lol:

    Now I'm confused... :?
  14. Scutch

    Scutch New Member

    PMSL :lol: Modifying is a bug!

    When its in full flight there's no stutter as such, its strong through the gears and the boost is good. Its just that lower gear hesitation/stutter which is the annoyance.
    Its running on the original VF28 turbo. I will check what it is boosting to at lunch time, but I know that its definitely peaking at 1.55bar, according to the defi controller. Whether it is hitting this at regular intervals is another thing. Possibly not, but 1.5bar certainly, and holding boost at 1.4bar.

    Are you suggesting I physically alter the settings in the boost by 2%?

    I've just looked at the boost maps with the engine off and ignition on and they are for 0.8, 1.0, 1.2 and 1.3 and have a duty of 30, 36, 44 and 49 respectively. The fuelling thing after that (I think) is set to 255, 255, 255 and 200 respectively. Why is the 1.3bar boost set to 200 do you know?

    Example picture I found (NOT my settings, mine are above) :

  15. AndrewC

    AndrewC Active Member

    Yes I suggest you scroll down to the 1.3 bar setting and across to highlight "49" and press the down arrow to reduce it by 2%.

    The last column is not something you can set on the commander, its a learned feedback setting in the ECU.

  16. Scutch

    Scutch New Member

    Thanks Andy. Excellent info.

    Not being a technically minded person in this respect, is there any reason why the settings on the picture above are different to mine, duty and fuelling wise?
  17. AndrewC

    AndrewC Active Member

    That picture is the boost control settings, nothing to do with fuelling whatsover.

    Different cars/setups require difference wastgate duty cycles to make the same boost, in my experience, 49 is on the high side for the std turbo to make 1.3 bar. I'd need to look at some STi5/6/P1 maps but IIRC more like 45-47 is the norm for 1.35bar which is usually what I set them at. BUT like I said different cars need different duty.

    The last column dropping to 200 does imply that the ECU is adjusting it's control algorithm to try to make the desired boost.

    None of this discussion has resolved whether the pulling out of junctions is caused by fuel starvation though, normally on later classics (97-00) there is no problem running down to the 'E' on the gauge, unless the pickup is not working properly. Early cars are completely different.

  18. Scutch

    Scutch New Member

    Indeed. Prior to the map and modifications, the car had used the petrol light on more than one occassion and had no hesitation problems whatsoever. Like I said, I'll try the tank of petrol and then reduce the duty by 2%. Great info, mate :)
  19. Scutch

    Scutch New Member

    Also as an update, when the Walbro fuel pump was fitted, they used the sock filter that came with it, not the original filter.

    Any chance of this causing problems?
  20. AndrewC

    AndrewC Active Member

    Yes, it is a problem as Jim said above.

    I think you are seeing a mixture of 2 problems, fuel starvation and overboost.


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