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Revs Dropping When Slowing Down

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  • #16
    To be honest I'd probably go with a good second hand Bosch one over an unbranded almost certainly Chinese replacement, some of the stuff they churn out is really poor quality, though I guess if it's rubbish you can send it back.

    An air leak could be effecting the idle, if your down on boost then I suspect you've got a small leak somewhere... At least leak testing these cars is really straight forward, there's various threads on here devoted to the subject. AndyP use to sell a test kit that works a treat for very reasonable money, I don't know if he still makes them but worth a try.

    If he doesn't then you can knock one up out a food tin and tubeless tyre valve, drill a hole in the end for the valve and wrap the body of the tin in insulation tape so it's a snug fit in the MAF hose. You'll also need to find something like a thick bolt to blank off the PCV connection on the MAF hose. You want to get the intake system air tight enough to hold one Bar for at least 15 seconds before it starts to drop off slowly, the MAF hose isn't designed for positive pressure so don't exceed one bar when testing. Use a spray bottle of soapy water on the joints as often the leak will be silent.
    1989 B3 2.0 3A 80 quattro. Long term project.
    1992 C4 100 2.8 Avant quattro, daily driver.
    1995 RS2, MTM K26/7, 380 BHP conversion.
    1997 C4 2.8 30V quattro, currently breaking for spares.
    1991 MK2 Golf G60. For sale.
    1990 Corrado G60. Looking sad.


    • #17
      Had this exact issue but the revs dropped so low the car stalled at every junction! Turned out the ISV was stuck almost closed and couldn't even be opened manually. I just launched it in the bin and replaced. Works perfectly now .


      • #18
        I've replaced the ISV and done thr coilpack conversion and it behaves so much better now, but I do also believe I've a boost or vac leak as I'm only getting around 10psi on wot and around 13psi overboost, so my next step is I'm just going to overhaul all the pipes for silicone.