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Sticky throttle issue on 355 - want to fix once and for all!

Discussion in 'Technical Q&A' started by BigHead, Jul 23, 2005.

  1. BigHead

    BigHead Formula Junior

    Oct 31, 2003
    992
    Outside of Boston
    Full Name:
    Dennis
    Yes, I've been stricken for some time with the classic "sticky throttle" problem on my 1995 355. Here's my analysis of the situation, and I'd greatly appreciate your comments.

    1. We've replaced/adjusted all possible "mechanical" issues. Cleaned up the gas pedal box. Replaced the throttle cable (twice). Made sure that it's not kinked or caught on anything, nor is any part of the cable caught on something hot enough to melt the sheath. Replaced the "quadrant" with the updated parts from Ferrari. I do believe everything mechanical has been ruled out as a potential cause, confirmed by the fact that the throttle is not sticky when in neutral, not even a little, and it's not sticky when the engine is COLD.

    2. That last little tidbit of info makes my curious. Why is the throttle sticky only when the engine is warm? Keep in mind that I have the mechanical aptitude of a retarded eggplant. The problem is especially noticeable when I lift entirely entering a corner, then have to reapply throttle again to accelerate out. It's REALLY sticky right there, as revs are up around 3k-3.5k, and the throttle is closed all the way, and "forcing" the throttle open make the pedal STICK, as if I was trying to pop the top on a car of new tennis balls. Ugh.

    Thinking that it's related the the throttle body, we OPENED UP the bypass valves, to allow air to flow AROUND the throttle butterflies. This made a small improvement, but as we opened up the bypass more and more, the idle kept creeping up. To the point where in neutral, it would idle at 2300 rpm - but then gradually drop to, say, 1200 rpm. Then we tried the opposite way, by opening up the throttle butterflies themselves, and closed the bypass valves, so that there would always be at least some small measure of air going through the butterflies themselves. This made a noticeable improvement, to the point where the car is at least driveable. :)

    This is really where I need some suggestions. Why is there SO MUCH "vacuum" pulling from underneath the butterflies? Is that unique to '95 models, or due to some other modification made to my car? Again, forgive my mechanical ignorance. Could this result from removing the bypass valve on the exhaust side? Tubi exhaust? Aftermarket chips?

    Is there anyway to reduce the amount of suction from under the butterflies? And why is this less noticeable when the car is COLD? Something to do with the emissions system making compensations when the car is cold?

    Basically, I think the answer really lies in equalizing the air pressure above and below the throttle butterflies. ANY and all suggestions on the best way to do that would be VASTLY appreciated.

    Thanks!

    vty,

    --Dennis
     
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  3. Aircon

    Aircon Eight Time F1 World Champ

    Jun 23, 2003
    88,830
    Melbourne, Australia
    Full Name:
    Peter
    I have no idea if this will help, but my car has Motec which has no cold start idle speed increase, as such. To help overcome this, i've opened the throttle at idle quite a bit and retarded the ignition to keep the idle at 1000rpm. When it's cold and introduce more advance until the engine coolant is at 60deg C to crank up the idle speed. Anywayyyyyy.......i have ZERO throttle stick...so I think your theory about vacuum holding the butterflies shut is a good one. Clearly, mine are open at idle WAY more than normal.
     
  4. fazzaz1

    fazzaz1 Karting

    Nov 4, 2003
    89
    Mississippi
    Full Name:
    Sam Scott
    My 98 had a ticking problem due to the throttle cable in the engine compartment. Just needed cleaning and lube. Are you sure you have eliminated that possibility?
     
  5. BigHead

    BigHead Formula Junior

    Oct 31, 2003
    992
    Outside of Boston
    Full Name:
    Dennis
    Yes, definitely - replaced the cable twice. As I noted, the problem exists only when in gear and with some revs. No binding at all when (a) engine is off, (b) engine is in neutral, or (c) in gear but engine is cold. Grrr...

    vty,

    --Dennis
     
  6. Steve B

    Steve B Formula Junior

    Dec 23, 2003
    521
    Naperville
    Full Name:
    Steven L. Biagini
    The service department of my dealership (Continental Autosport of Hinsdale, Illinois) told me that the "sticky throttle" problem is unique to the 1995s and results from the high vacuum level acting against the throttle plates. They told me that the problem can be minimized by adjusting the throttle plates (precise synchronization?). It is unrelated to the mechanical linkage.
     
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  8. Aircon

    Aircon Eight Time F1 World Champ

    Jun 23, 2003
    88,830
    Melbourne, Australia
    Full Name:
    Peter
    oh ...that's another thing. I DID do a butterfly balance (to within an inch of its life) and it was out by a lot, actually. Not sure that that helped stopped stickiness, though.
     
  9. BigHead

    BigHead Formula Junior

    Oct 31, 2003
    992
    Outside of Boston
    Full Name:
    Dennis
    Yeah - I'm convinced it's the high vacuum level.

    So here's the next question, coming from the mechanically impaired - what determines the level of vacuum under the throttle butterflies? Is it related to the exhaust? ECU programming? Why do some '95s have this problem, and not others? Anyone?

    thx,

    --Dennis
     
  10. Llenroc

    Llenroc F1 Rookie
    Rossa Subscribed Consultant

    Jun 9, 2004
    3,343
    Colorado
    Full Name:
    Vern
    Hi Dennis, I have the same problem with my 355C. Not so much a sticking throttle as a flat spot, somewhat like an out of adjustment accelerator pump on a carburator. In my experimenting with mine I have come up with this solution. The throttle plates need to be closed almost to the point of being completely shut and then adjust idle up to recommended 1100 rpm by opening the air bypass screws(8). I use the sync gauges that I have for weber carbs (I use 2) this requires the black plenum covers to be removed. Ferrari says to use mercury tubes to do this but I don't have access to these so I use the sync guages instead. I first sync the throttle plates and then open the bypass screws to bring up the idle to specs also syncing these to equal readings on the guages. This is a lot more work but that is the only way for me to measure the balance between each of 8 throttle plates, because I don't have the mercury columns. The bypass screws bypass air around the throttle plates inableling you to equilize the presure on either side of throttle plate. I hope this helps. Regards, Vern
     
  11. horseman

    horseman Rookie

    Jun 15, 2005
    2
    the variation from vac guages of any type to mercury manometer is very noticable if you are not using merc which is calibrated to open atmosphe and pulling against the other cylinders you are not getting a precise syncronization hence the slow return to idle throttle plates should all be synronized with bypass closed and confirmation that no butterfly contacts butterfly housing even under max manifold vac then proceed to sync bypass and set idle speed not a quick procedure not to be tampered with unless you have a manometer (mercury column) .before this is performed a confirmation of cable route near water lines should be checked possibly even changed the change from cold to hot ?air bypass into manifold during warmup. J.M.
     
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  13. BigHead

    BigHead Formula Junior

    Oct 31, 2003
    992
    Outside of Boston
    Full Name:
    Dennis
    Thanks for the suggestion, Vern. We tried exactly what you described (with mercury tubes). It helped a bit, but not enough. (my mechanic did do this to other 355s, and it helped a lot on other cars.) So we then tried the alternative, which was to close the bypass but open the throttle, thus allowing the throttle itself to "leak" air and allow some pressure equalization. This helped a bit more, but still not where I'd like it to be. So I'm still looking! Anyone have any other thoughts?

    thx,

    --Dennis
     
  14. BigHead

    BigHead Formula Junior

    Oct 31, 2003
    992
    Outside of Boston
    Full Name:
    Dennis
    Very interesting, thanks. How does one confirm that no butterfly contacts the buttefly housing? Visually?

    Also, what do you mean by "possibly even changed the change from cold to hot ?air bypass into manifold during warmup."

    Thanks!!

    vty,

    --Dennis
     
  15. F355Bob

    F355Bob Formula 3

    Are you sure this in only a vaccum problem. When the throttle linkage and butterflies are hot the metal will expand and could cause the binding. You say your problem is only when the car is hot. I don't know but I don't think the vaccum changes with heat.
     
  16. f355spider

    f355spider F1 World Champ
    Rossa Subscribed Owner

    May 29, 2001
    17,271
    USA
    Full Name:
    Hugh G. Rection
    D, have you contacted Dave Helms? I believe he had a fix for this.
     

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