Revs falling below idle after coming off throttle

Discussion in 'Mk2' started by SiddyGTI, Dec 30, 2019.

  1. SiddyGTI New Member

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    Not seen anything conclusive on this so thought I'd see if anyone else has had something similar.

    The car is a 1991 Golf GTI 8v (CE2), after coming off the throttle the revs drop well below idle (around 500 rpm I'd say off the top if my head) and could be described to sound like it's going to stall, then bounce back up sometimes going a bit over 900 then settling straight back down to a normal smooth idle at 900 rpm. Mainly notice this when crawling in traffic or just after reving the engine for whatever reason. Cant say it's a massive inconvenience or concern but thought I'd post here to see if anyone else has experienced this and it can be fixed or if it's normal.

    For reference I've recently replaced the blue temp sensor (good quality replacement), followed RJs tuning guide (however dont have access to a CO metre so cant definitely say that's set right but it's as close as I can get by ear) and recently replaced idle control valve.

    Anyone know why this might be happening and how to sort it or if it's nothing really to worry about?
     
  2. Tristan Forum Junkie

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    Is it when the idle switch is released?
     
  3. SiddyGTI New Member

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    It happens when I've completely come off the throttle, so no foot on the pedal at all, and engine speed is declining after throttle being open. So I would imagine the idle switch is is closed at the point its happening.
     
  4. Tristan Forum Junkie

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    Worth checking the switch is working.
     
  5. costel1969 Paid Member Paid Member

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    If there is one bugbear I have with the Mk2 GTI/16V, it's that everyone of them in my experience (my Dad had 2 and I've owned 2) played up with the idle at some stage or another - My current 8v is also misbehaving but slightly differently from yours.
    Tristan is right about those switches - they and their setup (or lack of it) can send the idle way out of whack.

    I know you replaced the ISV - was it with a genuine unit and did you also try cleaning it with petrol prior to installing it? - worth doing that if nothing else just to see the crud that can come out of them.

    According to RJ and he is the sage on VW's, the blue sensor should only be replaced with a genuine VW one - from what I have read, the non-genuine one's just don't do the business.

    After that - what is the service situation like with your Mk2?
    In particular, spark plugs, fuel filter, HT leads, rotor, distributor cap and if it comes to it coil? Others may disagree but I only ever use Bosch - RJ recommends Beru but I haven't had much experience with that brand.

    Another contender - is the hall senders within the distributor, they can also cause poor running/cutting out, bad idle etc. - I'm taking out the entire distributor when I get back to Ireland and will install a full Bosch replacement.

    Per previous advice from Tristan, check your ECU connections (no corrosion/burning, poor wiring etc.) and also your vacuum hoses (the current one's are also getting binned when I get back and are being replaced with new genuine one's)

    Finally give your general wiring and good check over, poor/loose connections, corrosion, bad earths etc. - to be fair I found some of my wiring to be poor and I repaired/replaced it but my GTI is still misbehaving - it starts on the button even from cold but once it warms up there is distinctive and periodic drop in the idle revs from 900 to about 500 and even when driving I do find the occasional judder/miss which is what is really driving me simple.

    I have sourced a used VW ISV - I cleaned the existing one (no difference) so that's another point of attack for later this year as well.

    I hope this helps and if/when I do find what is causing my issue, I will post it up - Tristan do you fancy a bit of consultancy work around November/December 2020 - if I don't get to the bottom of this?
     
  6. Tristan Forum Junkie

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    My own GTI was idling erratically, and I found I'd a faulty ISV, a vac leak, a frayed throttle cable, and corrosion on the plug on the ecu! And it had been running fine up to that!
     
  7. costel1969 Paid Member Paid Member

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    It's probably an accumulation of different faults and then finally the car has enough - I know in my Dad's 16V it was a throttle switch that broke and had to be replaced, in my old 16V, it was the cable/connector on the idle switch and coupled with that, she insisted on a new fuel filter and distributor cap before sanity was returned
     
  8. Tristan Forum Junkie

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    I'd love to, are you coming to Tipperary? Haha.

    Another Mk2 gti I (re)built had an intermittent weird hesitation, I tore it apart trying to fix it, finding a miriad of possible issues. Blue sender, coolant too cold due to corrosion around the thermostat seat, dodgy wiring to senders and idle/WOT switches, dodgy earths, perished vac lines, dodgy plugleads even the hardline into the MFA needed to be changed. I even tried an ecu, MAF and fuel pump. All improved the car slightly but didn't solve it.
    I even changed the "flying saucer" on the rocker cover breather as the diaphragm inside fails.

    Eventually I found corrosion on the bottom of the MAF pins, and in the plug, Cleaned that up, remade the pins and the car was perfect.
     
  9. Tristan Forum Junkie

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    We had my mates one come to a barely driving halt in France during the summer, a lot of side of the road digging ensued before we eventually found a burnt/corroded connection on the black/yellow wire on the 5 or 6 way plug beside the ECU. which happens to be the ecu power supply. It was only giving the ecu about 9 or 10 volts.
     
  10. costel1969 Paid Member Paid Member

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    The car will be declared off the road at that stage and NCT will be out as well - I wouldn't get 10 yards up the road with the cops - guaranteed
    I'll look after you if you can come down and you'll get fed as well and sure while you're there you can check out the '86 E28 too.
     
  11. costel1969 Paid Member Paid Member

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    I love Mk2 Golfs - I always have since I first saw them and not just GTI's etc. but they do have their weak spots and head scratchers plus a lot depends on prior owners too.
     
  12. SiddyGTI New Member

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    Thanks for the advice, I've also had a problem with some hesitstion/ bucking, this is the link to the thread on that https://www.clubgti.com/forums/inde...-jerks-at-constant-speed.288929/#post-2482368 Maybe both of these problems are linked. Looks like I need to have a very thorough look over of wires and vac lines. The ISV was a genuine VW from a mk2 breakers (I can crawl along at idle which I read is a good test these are working properly?) I have recently replaced HT leads, distributer cap is clean and all looks fine, coil has been replaced (done by previous owner but I think is bosch), blue temp sensor is from vw heritage part number 025906041A so maybe not up to scratch, and I have new spark plugs to go in when I get round to these checks. Hall sender keeps cropping up aswell which I'm rather hoping it isnt, but I might be trying that if I have no joy with anything else. Cheers.
     
  13. costel1969 Paid Member Paid Member

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    Cheers for the link, it’s a compete bast&&d when they behave like that, I will check the rest of the engine bay wiring when I HSE back.
    I didn’t want to cut the tape until I had enough of the proper loom tape and I have that now. I’m hoping ISV and or distributor will sort it and if it doesn’t I will go full “Basil Fawlty and give her a damn good thrashing” and failing that I will beg Tristan to come down
     
  14. costel1969 Paid Member Paid Member

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    No idea what that HSE was about - was trying to say head back...
     
  15. Bl4ckGryph0n New Member

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    My daughters 1987 GTI 8v had that, it started to happen during a driving lesson for her but it was so bad that she couldn't keep it going. So lucky me got to drive it back :)

    When cold it idled 'fine', a little bit lumpy or character but ok. I thought her lack of clutch control and learner driver abuse 'killed' it and that it needed an Italian tune-up. So any excuse to take it for a blast :)

    But nope, it really wanted to cut out and I had to navigate back a route where I could avoid having to come to a halt

    Anyway, I noticed a few things in the engine bay;
    - Valve cover breather was wobbly, ha the seal was split. Also noticed that the previous joker (owner for 23 years) put one from diesel on there that sits higher, so I bet it was touching the bonnet. Anyway cheap and easy repair.
    - The second issue was that the earth line from the head had barely any connection. Whilst I was at it I replaced the other earth lines as well.

    The result, very stable idle and super smooth again.
     
    Simon Peter Dodgson likes this.
  16. costel1969 Paid Member Paid Member

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    Cheers for that - I did find a couple of poor earths and sorted them but still not the solution, as mentioned above I'm hoping the replacement ISV will sort it out and or the distributor. The wiring loom by the bulkhead was 'tidied up' by someone prior to me, that loom is covered in standard insulation tape but I didn't want to open it up (I'm hoping it won't look like a dogs dinner) until I had the higher temperature loom tape which I have now - if there is anything dodgy in there, I will sort it.
    I inspected and repaired and re-taped everything else in the bay that's north of that particular loom so it's just that bit for later this year - I checked that breather ad it looked ok but I will check it again plus replace the vacuum hoses - they look ok but you never know and I have them anyway.
     
  17. Tristan Forum Junkie

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    The flying saucer is dirt cheap , I'd just replace it anyway.
     
  18. costel1969 Paid Member Paid Member

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    Isn't there a nice bit of setting up with that saucer Tristan if you were to replace it?
     
  19. Tristan Forum Junkie

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    No? Unless you mean setting the CO at idle?
    There's no adjustments on the saucer.
    I took a couple apart and they were in ribbons inside.
     
  20. costel1969 Paid Member Paid Member

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    You'll be crossing the border into West Limerick Tristan, I can feel it, we'll give you a temporary emergency visa (Mk2 GTI not running well..) :thumbup:
     
    Tristan likes this.

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