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Thread: Strange hesitation/stumbling on throttle tip-in when cold

  1. #1
    Join Date
    Sep 2008
    Location
    Kodiak, Alaska
    Posts
    15

    Strange hesitation/stumbling on throttle tip-in when cold

    Hello everyone,

    Just thought I'd post here about a problem I've been having w/ my '95 Disco.

    The engine has this strange hesitation on initial throttle tip-in, but this only happens when you start the engine after sitting overnight. There have been a couple times pulling out of the driveway where the engine hesitated/stumbled enough to stall. If you keep your foot on the throttle, it will stumble/hesitate for a second then 'clear up' and you're on your way. And it only does it pulling out of my driveway, and for the first few stop signs in my neighborhood.

    Once the engine has run for a few minutes, the hesitation disappears and the Disco runs just fine and dandy. Subsequent restarts during the day are not affected by this strange hesitation - it only happens after the vehicle has been sitting overnight.

    So far, I've checked the timing and vacuum advance, changed the spark plugs, wires, cap and rotor, fuel filter, replaced some leaky vacuum lines, and checked the coolant temperature sensor resistance. All seems ok there.

    I've got a feeling that I might have a problem with the IAC (idle air controller) valve. If you rev the engine slightly on initial startup, the idle speed will drop too low and will take a bit before settling on high idle. As it warms up, the idle speed starts to come down - settling at a fairly normal 750ish once fully warmed up. I'll probably pull it out and see if it needs cleaning...

    What do you guys think?
    --------------------------------------------------
    Darren
    1995 Land Rover Discovery
    2004 MINI Cooper S
    1994 Jeep Cherokee Sport

  2. #2
    Join Date
    Oct 2006
    Location
    Oak Park, CA
    Posts
    2,122
    Do it!
    Cleaning the MAF and the AIC on mine, did wonders, it runs so much smoother now.
    Clean the whole intake while you are it, can't hurt...

    David
    <DCProven >
    <1977 Land Rover Santana, Series III, 2.25 Diesel>
    <1995 RRC, LWB, 4.2 V8>

    "Information is not knowledge,..." Frank Zappa

  3. #3
    Join Date
    Sep 2008
    Location
    Kodiak, Alaska
    Posts
    15
    Well, I removed and cleaned the IAC yesterday. It was pretty nasty!

    This morning, the hesitation/stumbling was still there - but it's definitely considerably better...at least it doesn't feel like its going to stall on me taking off from a stop sign. And it doesn't last nearly as long either. So that's definitely an improvement!

    I also checked the TPS this morning w/ a borrowed digital scope, and it seemed to have a good, clean sweep - no erratic behavior from it.

    I'm thinking that the IAC has become slow in its age (appears to be the original 135k 1995 unit), so I may just replace it anyway.

    Thanks for the tips!
    --------------------------------------------------
    Darren
    1995 Land Rover Discovery
    2004 MINI Cooper S
    1994 Jeep Cherokee Sport

  4. #4
    Join Date
    Jun 2007
    Location
    Millstadt, IL
    Posts
    662
    Have you cleaned the throttle body? Build-up can create havoc on start up.
    Larry K.
    Millstadt ,IL
    2011 LR4 HSE LUX
    Prev. owns: 87 RR Classic, 96 D1 4.0, 96 P38 4.6

  5. #5
    Join Date
    Sep 2008
    Location
    Kodiak, Alaska
    Posts
    15
    I forgot to mention - yes, I cleaned the throttle body, the MAF, and the inside of the intake as well. I had it apart when I first got it, so I could replace the injectors. Somehow the IAC got overlooked in the process.

    Thanks though!
    --------------------------------------------------
    Darren
    1995 Land Rover Discovery
    2004 MINI Cooper S
    1994 Jeep Cherokee Sport

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