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Slow return to idle on ‘81 308 GTSi — Tech Help Needed

Discussion in 'Technical Q&A' started by Smiles, Jun 14, 2019 at 6:23 PM.

  1. Smiles

    Smiles Moderator
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    Nov 20, 2003
    12,254
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    Matt F
    Hi. I already posted this in the 308/328 section, but I thought I’d get more traction here.

    When I lift off the throttle, my 308 GTSi returns to idle very slowly.

    If I’m cruising and I lift off the accelerator the car doesn’t slow down at all.

    What could be causing this? In my simple mind it feels like a weak throttle return spring.

    I looked at the spring when my son hit the throttle and it was fine. There’s nothing blocking the accelerator pedal.

    Matt
     
  2. Smiles

    Smiles Moderator
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    Nov 20, 2003
    12,254
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    Matt F
    Return to idle speed is about 1,000 RPM per second.

    Matt
     
  3. finnerty

    finnerty F1 World Champ
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    #3 finnerty, Jun 14, 2019 at 7:33 PM
    Last edited: Jun 14, 2019 at 7:48 PM
    Try it in the engine bay by disconnecting the throttle cable and manually opening and releasing the throttle valve directly on the throttle body (TB). This will eliminate all of the linkage from the loop, and you will be able to observe how fast the butterfly plate snaps back on its own ---- if it's slow, it could be a weak TB spring or sticky / binding valve mechanism or an obstruction.

    If it snaps back quickly (which it should), I'd start looking for vacuum leaks in everything that communicates --- downstream of the butterfly valve --- with the air intake system & the throttle / idle air bypass circuit.

    Also be sure to check the small diameter vacuum hose that runs from the intake plenum to the DigiPlex boxes (ECUs) in the trunk ---- for leaks / cracks. This hose commonly goes bad and not only bleeds in un-metered air, but also screws with the ignition advance curves ---- both will make the engine run faster than it should at various RPMs.
     
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  4. finnerty

    finnerty F1 World Champ
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    #4 finnerty, Jun 14, 2019 at 7:54 PM
    Last edited: Jun 14, 2019 at 8:01 PM
    Last but not least, if all the above checks out OK, there is a "Deceleration Cut-off (Overrun) Air Valve" on the the TB that connects both upstream and downstream (to by-pass) the throttle valve. It is a simple vacuum diaphragm / spring valve in a small metal canister. Check its hoses and connections thoroughly for leaks. The diaphragm inside can also fail, of course. It's rare, but it happens ---- you can check it by disconnecting both ends and doing a standard "push / pull" forced air test on it.

    But, again, do all the other stuff listed above first ---- 99 / 100 times you will find the problem before you get to having to check the Cut-off Valve.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Good luck and have fun !

    BTW, if you don't already have one, go to the parts store and buy a hand-held vacuum test pump kit........ it will save you a helluva lot of time on this ! :)
     
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