Stutter or hesitation at about 5500 rpm

Discussion in 'Subaru Impreza WRX Help & advice' started by longsh07, Oct 3, 2018.

  1. longsh07

    longsh07 V-Limited Member

    Location:
    Bedfordshire
    Morning all, after a bit of advice from the experts :thumb

    99 version 5 JDM WRX Impreza, no engine modifications/remaps to my knowledge other than what looks like some aftermarket blue ignition leads and a prodrive back box. I've done regular oil / filter changes, but no record of spark plugs, fuel filter, or air filter change and I've not done these since I bought it about a year ago. Timing belt was done just before I got it. Car is using oil, no smoke from exhaust, believe it's misting somewhere as no puddles/drips, possibly front of block as there is some staining on the manifold.

    The car was sat not driven for around three weeks, maybe more, maybe less. Since getting it back out it seems to have developed a bit of a stutter or hesitation around 5500 rpm. This started off pretty bad but has started to improve a little with normal day to day driving and a couple of full revs, full boost sprints.

    Initial thought was a boost issue as the car rev's fine all the way to the red line in neutral on the drive. Fitted a boost gauge but this shows consistent boost pressure, peaking about 1.1/1.2 before easing back to about 1 bar for the remaining rev range.

    The stutter appears around the 5.5K rpm range only when on full throttle, building revs more gradually, or stamping on it from higher in the rev range to being with and the car seems fine.

    What do we reckon the issue could be? I guess it could be as simple as some slightly iffy fuel.
    Plan of action at the moment is a bit of general maintenance and go from there but open to suggestions.
    • New spark plugs, thinking grade 7 NGK iridium as I don't know if the car is mapped, plus this should help with running UK 99 RON fuel right?
    • Replace air filter
    • Replace fuel filter
    • General inspection for any cracked or leaking hoses
     
  2. Burt2000

    Burt2000 V-Limited Member

    Location:
    Belfast
    yep new fuel first, then check plugs and leads
     
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  3. Kev_STI

    Kev_STI V-Limited Member

    Location:
    Nottingham
    Agree with this above. My twin scroll was generating a misfire (like an woofly ignition cut) around a similar revs at Curborough, I’ve since had the NGK 7’s gapped up properly and a downpipe blow at the joint sorted. Runs spot on now.

    They can be very fickle.
     
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  4. Topper Harley

    Topper Harley V-Limited Member

    Location:
    Westmeath Ireland
    Start with the basics .

    Spark plugs.
     
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  5. longsh07

    longsh07 V-Limited Member

    Location:
    Bedfordshire
    Cheers all, will update with results when its done.
    If anyone has any other input I'm happy to have a list of things to tick off.
     
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  6. Fatharrydog

    Fatharrydog MY98 WRX Type R V-Limited 176/1000

    Location:
    Midlands
    If the above doesn't work then consider the fuel pump.
     
  7. Pete1968

    Pete1968 V-Limited Member

    Location:
    Warwickshire
    Well I know what I'd be doing for sure and that's get the fueling checked for sure by a professional tuner and this will also see if it's stock power too especially if you can get it on to a rolling road. Last thing you want is DETTING.

    Keep us updated.
     
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  8. Pete1968

    Pete1968 V-Limited Member

    Location:
    Warwickshire
    And remember, until it’s had a health check, or problem solved drive off boost is my advice
     
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  9. longsh07

    longsh07 V-Limited Member

    Location:
    Bedfordshire
    Cheers all, not had chance to order any parts yet so afraid I've not really got a very useful update.

    @Pete1968 - I'd love to get it checked/mapped by a pro tuner but that's out of the question cost wise for now. Do you know of any other way to check for a remap? My 350Z for example has multiple maps that can be changed from the cruise control. Obviously being an older car I expect the ECU is not quite so flexible. Anyway, I have no reason to believe it has been mapped, there are no engine mods to justify a remap, its still has the standard exhaust (except for a prodive backbox off a later model fitted by the previous owner) and a paper panel filter. As I say, the only thing I can see is the HT leads which look like they've been replaced as maintenance rather than for performance.

    On the subject of the exhaust, I have just discovered a leak. Its in the down pipe, after the down pipe cat but before the join to the rear cat. This shouldn't upset it should it? Its after the last O2 sensor so shouldnt affect fuelling.
    (Anyone got a diagram for a 99 WRX exhaust system?)
     
  10. gus

    gus V-Limited Member

    Location:
    whitehaven
    .................wrongo_O STELLA:rofl:
     

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