Car intermitantly jerks at constant speed

Discussion in '8-valve' started by SiddyGTI, Jul 3, 2019.

  1. SiddyGTI New Member

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    Hello,

    I've got a 91 GTI 8v Digi, and today cruising at 70 on the motorway the car jerked forward and then went back to normal straight away. This happened twice with about a 30 second gap between, and then once or twice just before turning off later on. Not sure if this is a misfire, but I could replicate the sensation by bliping the throttle and lifting straight back off. Does anyone have any ideas where and what I should start checking to sort this?

    Thanks in advance
     
  2. costel1969 Paid Member Paid Member

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    I would start with the basics - confirm fuel filter and fuel pumps (wiring/condition) are OK. Presuming they are give confirm rotor, distributor cap, HT leads, coil and spark plugs and blue sensor (see RJ's guide) are all good. After that it's https://clubgti.com/forums/index.ph...ose-and-tune-your-mk2-digifant-gti-8v.124949/ as kindly provided by RJ.
    If that doesn't get her going like the clappers nothing will...
     
  3. H8V

    h8v New Member

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    After checking the above,

    1. If the rev counter drops down when this happens it might be the hall sensor in the distributor that is beginning to fail.

    2. Have someone blip the throttle to replicate the problem while you have a look at the engine to see which cables are flexed when the engine rocks. Look especially at the plug and two wires that operate the injectors.

    I've just suffered a problem where the engine would rev fine when stationary, but stopped whenever I tried to move off. One of the wires into the injector plug had fractured all its copper strands and was just hanging on by part of the plastic insulation. Any engine movement broke the connection, meaning no fuel injection and a stalled engine. Had me puzzled for an hour or two.
     
  4. pigbladder Forum Addict

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    hall sender isnt a bad shout ...often the rev counter goes mad with this

    other thoughts ecu relay , check battery terminals and earth cables are clean and tight
     
  5. rubjonny

    rubjonny Administrator Admin

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    have a close look at the clocks if it happens again, if it looks like they reset at the same time look at the ignition switch and wiring. Check the ecu and fuel relays for corrosion and water damage, wiggle to ensure seated. Slots 1 and 2 if CE1, 3 and 12 if CE2.

    Couple times I've had this, first it was CO set a little weak and 2nd time it was burnt out fuel pump fuse due to fusebox damage. fuse 5 is CE1, 18 if CE2 :)
     
  6. SiddyGTI New Member

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    Not had chance to properly check yet as I'm waiting for a new fan and in this weather dont want to risk over heating, but followed the set up guide and found the allen screw on the MAF took quite a few turns clockwise before a smooth idle was achieved so crossing my fingers this was the problem and it was running lean (I believe that is the correct way round?)

    Will confirm if this was the issue after more use and will work through the suggestions if not and update. Thanks.
     
  7. SiddyGTI New Member

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    Took her along some b roads and a bit round town last night with no issues, and have done the commute from Nottingham to Sheffield this morning without any jerking. Think running rich has been the problem, had to turn the allen screw on the MAF clockwise (think this is the correct way round and me saying lean in the previous post was incorrect). Thanks for the help.
     
  8. rubjonny

    rubjonny Administrator Admin

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    clockwise richens the mixture, anti-clockwise leans it :)
     
  9. SiddyGTI New Member

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    Revisiting this as my previous thoughts that it had been sorted have turned out not to be true.

    Still have the 'bucking' problem intermitently. Could also add to my previous explanation that it can happen at any speed, I think also when hot or cold as I seem remember it happening after recently being started once. The sensation is a brief complete loss of power where the revs displayed on the dash completely drop (as does actual engine rpm) then violently the power comes back bucking the car, also the digital display doesn't change while this is happening. It has also happened where it has done it and the car has stalled and refused to start once, however when this happened I towed it home and it started again but developed a misfire when warmed up, figured it was HT lead on cylinder 3 so swapped these and the misfire has stopped (cant even be sure this is related to the bucking problem, but just explaining the order of events). So with it seemingly running fine I have: checked earths, checked the fusebox and to be honest I'm not entirely sure what I'm looking at when it comes to the fusebox but as far as I can tell nothing seems damp, burnt out or loose so I can't see a problem there, i have also while it's running wiggled the wires going to the distributer/ hall sender, and to the injectors without anything happening, I have taken the dizzy off and confirmed it gives a spark on ever cylinder and I can hear the injectors working for every cylinder while rotating it by hand so nothing strange there either, finally I have changed the TCI spark ignition unit for the one available from VW heritage, again I have felt the car jerk since. The next things I can think to do are swap the dizzy/ hall sender or maybe the ECU but I dont think it is ECU to be honest. So before I buy a complete dizzy with hall sender does anyone have any further ideas because they arent the cheapest if it turns out not to solve the issue? Thanks.
     
  10. rubjonny

    rubjonny Administrator Admin

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    have a good look at the wiring harnes in the engien bay for any loose or dirty contacts, and check plugs for loose, pushed in or dirty pins. in particular check over the ecu and spark module plugs, hall sensor and the earths to the head and battery. next time it happens and particularly if it wont start listen carefully for the main pump cutting in and out, though granted not so easy to hear at speed...
     

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