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

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

 

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

 

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

 

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

 

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