Peculiar problem with ITB's
Discussion
I'm having a weird problem with my Suzuki throttle bodies-equipped MX5 and I've run out of ideas on why it's happening and how to deal with it. First, some car details. It has 1.8 engine, skimmed head, aftermarket camshafts with more lift and duration (264 degrees, 10 mm), Suzuki GSXR throttle bodies on Danst intake adapter/manifold, aftermarket exhaust manifold.
When pressing the loud pedal, there's a fair bit of resistance in it at the first few millimetres of movement. It's especially pronounced when going on throttle after engine braking (as opposed to when going on throttle from idle when it's not really noticeable). It almost feels like engine is sucking the throttle butterflies shut and I have to overcome this by putting more pressure on the pedal before it finally starts moving. When the pedal goes past this point and opens the throttles a bit this weird resistance disappears and further pedal travel happens normally. To give you an example. I'm approaching a bend, I stay in gear and brake a little. When it's time to accelerate I move on the throttle and try to press ir a little, but the pedal resists. I have to increase pressure until the pedal finally yields, but the resistance abruptly disappears the moment the pedal has moved and of course it means the pedal jerks forward, adding too much throttle opening. Not ideal, especially on a wet road
It also seems to be most prominent in the middle of rev range but less pronounced when over 4.5 thousand revs and as I've said, not really felt when going from idle. This makes me think it might be related to the way the engine breathes, the torque curve lies low below 4000 rpm but at 4k the engine wakes up and goes like stink. It seems that the "throttle sucked in" effect is most noticeable in the rev range where volumetric efficiency is low. Could there be a connection? Has anyone experienced anything similar? Any idea why this might be happening? What could be done about it? I'm really at a loss here and everyone I've spoken with locally just shrugs.
When pressing the loud pedal, there's a fair bit of resistance in it at the first few millimetres of movement. It's especially pronounced when going on throttle after engine braking (as opposed to when going on throttle from idle when it's not really noticeable). It almost feels like engine is sucking the throttle butterflies shut and I have to overcome this by putting more pressure on the pedal before it finally starts moving. When the pedal goes past this point and opens the throttles a bit this weird resistance disappears and further pedal travel happens normally. To give you an example. I'm approaching a bend, I stay in gear and brake a little. When it's time to accelerate I move on the throttle and try to press ir a little, but the pedal resists. I have to increase pressure until the pedal finally yields, but the resistance abruptly disappears the moment the pedal has moved and of course it means the pedal jerks forward, adding too much throttle opening. Not ideal, especially on a wet road
It also seems to be most prominent in the middle of rev range but less pronounced when over 4.5 thousand revs and as I've said, not really felt when going from idle. This makes me think it might be related to the way the engine breathes, the torque curve lies low below 4000 rpm but at 4k the engine wakes up and goes like stink. It seems that the "throttle sucked in" effect is most noticeable in the rev range where volumetric efficiency is low. Could there be a connection? Has anyone experienced anything similar? Any idea why this might be happening? What could be done about it? I'm really at a loss here and everyone I've spoken with locally just shrugs.To add, it's an issue I've had from the start. I've made most of the changes to the engine (ITB's, cams, headwork, everything apart from exhaust manifold) in one go and the problem was there from that moment. I thought it might be to do with mapping and played with throttle tip-in enrichment for many times but it doesn't seem to affect the issue. I wouldn't fully dismiss it but it seems unlikely. If the enrichment was incorrect the pedal would move freely but the engine would buck with too much or not enough fuel. As it is, it's the pedal being a bit stuck at 0% position. Throttle cable or linkage is also unlikely as a culprit as the pedal moves freely with engine off or from idle.
I think it should be fairly obvious that butterfly throttle systems have been around long enough for anyone to know that air pressure doesn't stop them working properly. Your linkage is at fault. It sounds like it's going "over centre" in the closed position or something is just jamming against something else. No point asking anyone else. Pull it apart and find out why.
FordPrefect56 said:
I think it should be fairly obvious that butterfly throttle systems have been around long enough for anyone to know that air pressure doesn't stop them working properly. Your linkage is at fault. It sounds like it's going "over centre" in the closed position or something is just jamming against something else. No point asking anyone else. Pull it apart and find out why.
It is indeed known that butterflies aren't affected by air pressure, that's why I'm baffled. Had them off the car and disassembled / put back together numerous times, couldn't see anything wrong and they open freely with engine off. Anyway, thank you for the input.Depends on how knackered it all is! Many years ago on an off road racer with a tweaky rover V8 i found an issue where on high vacuum (ie overrun) the throttle plates got sucked towards the intakes and actually jammed the plates in the bore. On that car, the throttle cable kept breaking and the owner couldn't work out why! Being an off road racer and driven in steel tow capped boots, he hadn't noticed the sticky throttle really and just stamped on the accelerator! The shafts and bearings in the bodies were worn as a worn thing, probably from all the mud 'n crap flying around the place!
I would try setting a high tickover so that the butterflies don't actually shut. Then see if the symptoms change.
I've got GSXR bodies on my car. I fitted a larger diameter quadrant to replace the ludicrously small standard one. I also offset it to give a rising rate to the throttle progression.
Make sure they are well balanced too.
I've got GSXR bodies on my car. I fitted a larger diameter quadrant to replace the ludicrously small standard one. I also offset it to give a rising rate to the throttle progression.
Make sure they are well balanced too.
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