Mk2 '89 1.8 8v Digi starts but won't run at idle

Discussion in '8-valve' started by Rolf 8v, May 16, 2021.

  1. Rolf 8v Forum Member

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    I've searched as much as I could both on here and other threads, but haven't yet been able to identify the issue I'm having.

    After a 2 year hiatus (renewing brake lines, brake components and upgrading to poly bushes along with refurbing chassis components), I have finally managed to get my Mk2 back on its wheels ready to ship out to have some welding done.

    Car was running fine prior to the above, so I connected up the battery, turn the ignition and it fired into life for a second or two, only for it to die. Fine, I thought, hasn't run in a couple of years, just needs a bit of throttle and all will be fine.

    Nope, try as I might, it will start on the button every time (as if all is well), but won't idle for toffee. It will however, stay running if I apply the throttle to about 3000rpm+ and rev freely from this point, but it will have a consistent 'rough patch' every couple of seconds when the throttle is held at ~3000rpm. As soon as you let go of the throttle, the rpm drops and it cuts out.

    I have checked the following:

    • All vac connections to the TB
    • Throttle switches etc on TB
    • Brake servo vac hose
    • MAF flap opens freely
    • Lift pump works (disconnected the main pump to check)
    • ISV buzzes with ignition on, has a notable effect on the engine running when disconnected and blasted some cleaner in it
    • Disconnected blue temp sender and confirmed that it has a notable effect on the engine running when disconnected
    • Checked the cam cover breather system and all appears fine

    I haven't changed the fuel (about a third of a tank in it still) or fuel filter and I haven't assessed the condition of the injectors, but on the basis that with some throttle applied it runs 'of sorts', I would expect that any issue with fuel or fuel starvation would result in no starting/running of any sort.

    I haven't done any electrical checks as such, but I have checked as many connectors as I can reach to see if there are any corrosion gremlins lurking somewhere, but nothing found.

    My next plan is:

    Chuck some fresh fuel in the tank
    Change the fuel filter
    Change the plugs
    New timing belt etc.
    Set the timing up (as far as possible on a static engine)

    Then see what happens. Other than that, if anyone has any other ideas, i'd be very grateful to hear them.

    Cheers

    Karl
     
  2. Tristan

    Tristan Paid Member Paid Member

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    Is the metering flap moving freely?
    The vac line connection at the airbox, at the rear of the TB, and onto the clocks?
    Condition of the pins/terminals onto the MAF and ecu?
    It almost sounds like a bad connection, where you have low voltage, and when you rev it up, you're getting a bit more of a charge.
     
  3. rubjonny

    rubjonny Administrator Staff Member Admin

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  4. Rolf 8v Forum Member

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    Thank rubjonny, I’ve read through this (which is fantastic) many times over the years and checked what I can. I just can’t help feeling like I’ve missed something really obvious.

    However, I think it’s going to be a case of digging the multimeter out and just working through every element bit by bit.
     
  5. Rolf 8v Forum Member

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    Yeah, all seems to check out. Like I said, I think I need to get the multimeter out and just dedicate several hours to tick things off the list.
     
  6. rubjonny

    rubjonny Administrator Staff Member Admin

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    idle switch always high on the list, without that working the base settings dont apply properly either afaik (not sure if ecu looks at idle switch or not when in setup mode)
     
  7. Rolf 8v Forum Member

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    Problem solved!

    Note to self, remove pointless period ‘tuning’ parts and replace with OEM prior to diagnosing running issues!

    Turned out be a perished rubber bellow joining my Bonrath Power Rohr onto my throttle body. Had a big split in it that I couldn’t see. Chucked the OEM rubber pipe on and car ran like a dream!!
     
    Tristan likes this.

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