elpete77 Posted December 24, 2015 Share Posted December 24, 2015 Search didn't yield much help. Figured I'd put this question out there. 05 lgt, stg 2, just replaced turbo inlet and installed aos, idle is solid, don't think there are any vacuum leaks. Car runs well, no stutter on acceleration, but on downhill sections with very light throttle I'm getting a kind of lurching sensation. Tach is solid, and if I add throttle or completely remove throttle, it goes away. It's especially noticeable if using cruise control. It almost feels like slack in the drivetrain. Any thoughts? Link to comment Share on other sites More sharing options...
xtea Posted December 24, 2015 Share Posted December 24, 2015 I soent monthes trying to figure out the same thing. Best advice I got was worn half shafts but I never for it confirmed. Do you find that its worse when the engine is cold? Link to comment Share on other sites More sharing options...
SBT Posted December 24, 2015 Share Posted December 24, 2015 Sounds like mismatched tires (more than 1/4" circumferential difference) between the front and rear, or incorrect pressure bias front to rear. When there is a circumferential difference, the wheels don't turn at the same speed, and the center differential loads up and releases with a jerk. Incorrect tire pressures will cause the same effect. Just remember 35 front/33 rear and keep the same bias (up/down) if adding/removing pressure. Even side-to-side pressure differences, particularly on the rear where the LSD loads and unloads into the center differential. Unless you have already checked these two areas, I'd tape the tires or check the pressures. - Pro amore Dei et patriam et populum - Link to comment Share on other sites More sharing options...
elpete77 Posted December 25, 2015 Author Share Posted December 25, 2015 Sounds like mismatched tires (more than 1/4" circumferential difference) between the front and rear, or incorrect pressure bias front to rear. When there is a circumferential difference, the wheels don't turn at the same speed, and the center differential loads up and releases with a jerk. Incorrect tire pressures will cause the same effect. Just remember 35 front/33 rear and keep the same bias (up/down) if adding/removing pressure. Even side-to-side pressure differences, particularly on the rear where the LSD loads and unloads into the center differential. Unless you have already checked these two areas, I'd tape the tires or check the pressures. Interesting thought. I am on my winters now, so pressures are higher, fronts are 50, rears are about 48, but I've been meticulous with rotations, and it was happening on my summer tires too, which have also been religiously rotated. I will try to measure the circumferences asap. But if that was the case, any idea why it would occur only when described? Link to comment Share on other sites More sharing options...
joeleodee Posted December 25, 2015 Share Posted December 25, 2015 If you have the means to log, monitor AF correction. If the correction figures swing wildly from positive to negative corresponding with the lurching, there is a fuel delivery issue. Link to comment Share on other sites More sharing options...
SBT Posted December 25, 2015 Share Posted December 25, 2015 When you are under load, the CD releases much faster and you have minor tire slippage when accelerating that discharges the bind - still not good though - but not likely to be noticed as much When you're not under load, i.e., the circumstances you've described, is when you'll notice it the most. Do check your circumferences though. I can also be worn splines or CVJs, and, IIRC, there's a protocol for checking that in FSM. - Pro amore Dei et patriam et populum - Link to comment Share on other sites More sharing options...
elpete77 Posted December 25, 2015 Author Share Posted December 25, 2015 I soent monthes trying to figure out the same thing. Best advice I got was worn half shafts but I never for it confirmed. Do you find that its worse when the engine is cold? This is occurring at normal operating temps. Link to comment Share on other sites More sharing options...
xtea Posted December 25, 2015 Share Posted December 25, 2015 This is occurring at normal operating temps. Mine is more magnified with a cold engine. Link to comment Share on other sites More sharing options...
SBT Posted December 25, 2015 Share Posted December 25, 2015 I'd almost bet you'd get the same symptoms idling along in first gear with no throttle. - Pro amore Dei et patriam et populum - Link to comment Share on other sites More sharing options...
RckyMtnGT Posted January 2, 2016 Share Posted January 2, 2016 I've experienced and wondered about this same sensation. I notice it most at low speed, like crawling through my apt complex in 2nd gear, at the very lowest-to-no throttle. I'll also feel it on the highway, especially with cruise. Seemingly connected to very light - no throttle, and transitioning up or down a hill. Down hill, where it cant quite coast and maintain speed, but then turns back on abruptly. No real insight as to what causes it. To me, it feels like poor fueling map at super low throttle. I notice the MPG gauge fluctuates wildly while its happening, which again makes me think fueling, or rather poor low-throttle mapping. FWIW, I'm running a Cobb stg2 OTS map, but I noticed it stock, as well. Also on brand-spanking new snow tires, no circumference differences, and no noticeable drivetrain lash anywhere. Hmm...I should pull up throttle position reading while its occurring, or Inj duty cycle, see if they're basically going on/off. Link to comment Share on other sites More sharing options...
xtea Posted January 2, 2016 Share Posted January 2, 2016 I've experienced and wondered about this same sensation. I notice it most at low speed, like crawling through my apt complex in 2nd gear, at the very lowest-to-no throttle. I'll also feel it on the highway, especially with cruise. Seemingly connected to very light - no throttle, and transitioning up or down a hill. Down hill, where it cant quite coast and maintain speed, but then turns back on abruptly. No real insight as to what causes it. To me, it feels like poor fueling map at super low throttle. I notice the MPG gauge fluctuates wildly while its happening, which again makes me think fueling, or rather poor low-throttle mapping. FWIW, I'm running a Cobb stg2 OTS map, but I noticed it stock, as well. Also on brand-spanking new snow tires, no circumference differences, and no noticeable drivetrain lash anywhere. Hmm...I should pull up throttle position reading while its occurring, or Inj duty cycle, see if they're basically going on/off. This is exactly what I have but its at a specific rpm range 2300-2700ish. 3 tuners including cryo didnt see anything wrong with the tune. For me its worse when the engine is cold. Ive learned to drive around it since then. Link to comment Share on other sites More sharing options...
RckyMtnGT Posted January 2, 2016 Share Posted January 2, 2016 This is exactly what I have but its at a specific rpm range 2300-2700ish. 3 tuners including cryo didnt see anything wrong with the tune. For me its worse when the engine is cold. Ive learned to drive around it since then. Yeah, I don't know if we'd see anything 'wrong' with the tune, it feels like a gap in the TPS programming or something. For example, my diesel truck has an idle verification switch in addition to the throttle position switch. Its a little bit out of adjustment on my truck, so you can press the throttle lightly - the TPS is reading a very small amount of throttle, but the idle verification switch is still closed, so the truck doesn't respond until later in the pedal stroke. However, when it does respond, the TPS is already at a higher value, say 10%. So it goes from zero to 10%, and you get a jump in torque. Obviously this has no bearing on our cars, but its just an example of feeling. Its like there's a gap in the super low portion of the throttle movement. I have no idea really, I'm just spit-balling here... Link to comment Share on other sites More sharing options...
elpete77 Posted January 15, 2016 Author Share Posted January 15, 2016 Finally got the tires checked, they are all w/in 1/8th inch circumference. So I don't think it's that. Feels like something somewhere just doesn't know how to ease in some fuel real slow-like. P.S. this all started about 1.5 mos ago, when I was running summer tyres (tires?), also measured and also within spec. Car has ~120K miles, and it was happening before and after replacing turbo inlet. No vac leaks to speak of. Link to comment Share on other sites More sharing options...
SBT Posted January 15, 2016 Share Posted January 15, 2016 Curious, were the tires all at proper inflation, when you checked the circumference? And, this was a "circumference" post that I made a while back, which lays out mathematically, how easy it is to have >1/4" of rotational difference. I didn't believe it myself, until my master tech proved to me that my "lurching" at idle and slowdown was caused by the tires. - Pro amore Dei et patriam et populum - Link to comment Share on other sites More sharing options...
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