Please help if you can: F355 throttle body bypass screws | FerrariChat
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Please help if you can: F355 throttle body bypass screws

Discussion in '348/355' started by jm3, Aug 9, 2008.

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  1. jm3

    jm3 F1 Rookie

    Oct 3, 2002
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    JM3
    #1 jm3, Aug 9, 2008
    All 8 of my bypass (idle) screws are turned all the way in. I bought it that way, its been fine but I want it a little better. Where are your screws adjusted to?
    I am not talking about the stop screws for the throttle plates, I am referring to the tapered screws in the side of the throttle body.


    Thanks, Jay
     
  2. plugzit

    plugzit F1 Veteran
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    Dec 1, 2004
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    Bruce Bogart
    #2 plugzit, Aug 9, 2008
    First, make sure you're talking about the bypass screws and not the nipples that are there to test the vacuum, which should be all the way in unless you have a vacuum guage hooked up.
     
  3. jm3

    jm3 F1 Rookie

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    #3 jm3, Aug 9, 2008
    yes, guages hooked up, referring to the screws that are parallel to and above the pcv hoses that go up to the plenum. I have turned them about 3 turns and nothing much happens. My throttle plates are cracked about 1/2 turn on the plate setscrews. The idle with no Idle air controllers is about 750, and then the iac bumps it to 1020.

    I may crack the throttles a bit more, but I am seeing very little influence from the bypass screws which I have arbitrarily set at 1 turn out.

    The thing idles like crap all of a sudden, I'm about to start looking at electrical problems.

    Have you ever seen a coil pack go bad? new plugs, new wires, but it misses at idle.


    Jay
     
  4. Llenroc

    Llenroc F1 Veteran
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    Vern
    #4 Llenroc, Aug 10, 2008
    Last edited: Aug 10, 2008
    I responded to a post of yours a couple weeks where you told me a that you set your idle adjustment with a vacuum gauge. Your posts seem to be contradicting. If you did the adjustment before you would have had to used these ports and their corresponding screws. As I said then and repeat myself now there is absolutely no effective way to adjust the throttle balance or set those bypass screws using a vacuum guage, you may think you are but your vacuum gauge's needle will bounce all over the place and you will not get proper/accurate readings. Even if you could use a vacuum gauge you would need 8 of them but again irrelevant they are not accuate enough for this job. This is also why you are not seeing any change when you turned the bypass screw out one turn. Also, the bypass screws are the FINAL (fine tune)adjustment point, ie; not the place to start the place to end. If you want to do the throttle balancing and setting the bypass screws use a manometer, either a 4 tube or two 4 tube units but start with the mechanical balance of the throttle plates using screws on the linkage and the proper order of them and then fine tune each throat/choke with its bypass screw.
    To answer your OP; theoretically the bypass screws could be turned in all the way if your system was able to be adjusted by the linkage only but unlikely, whoever had done it in the past didn't know what they were doing or just said that's good enough and left it.
     
  5. jm3

    jm3 F1 Rookie

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    #5 jm3, Aug 10, 2008
    If somebody has an idea of approximately how far out their screws are, I would really appreciate a reply.

    The dealer who worked on the car for the previous owner, has "lost the rabbit" in terms of adjustments. I cannot tell if the throttle plates are too closed, the adjustment screws are set wrong or what, because I may not be at the baseline. The baseline is that the throttle plate clearance is 0.2mm -0.25mm, which is 8 to 10 thousandths of an inch clearance. I don't feel like taking the airboxes off to measure the distance if I don't have to, so the experience of other peoples adjustments will let me know how close I am to the ballpark where I hope to find the rabbit.

    Secondly, with regard to using a vacuum gauge (which I own) instead of a manometer (which I also own), I have used the vacuum gauge to set the 4 (not 8 as has been stated) throttle plate setscrews. The vacuum guage is desirable for this, because you can flick the throttle with a vacuum guage, but not with a manometer. My method has worked very well in the absence of using a $20K SD1 tool as the WSM procedure recommends. I don't know har far away I am from 0.008 clearance, but my manometer wouldn't tell me that either.

    Using the vacuum gauge and 4 hoses, plus a 7mm socket on a tune-up screwdriver allowed me to go for a drive in the evening because I didnt have to even take the expansion tank off after one of the screws lost its adjustment. This was my reason for the desire to "flick" the throttle, to be sure everything was settling consistently.

    Now I am past that point, and am looking to double-check / fine-tune everything to eliminate the idle settings as a cause for my (barely) skipping idle, which seems more and more likely to be electrical.

    Thanks, Jay
     

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