paul vixie Posted May 31, 2010 Share Posted May 31, 2010 in researching why my car (280zxt 1983) won't boost past ~2 like it used to, i noticed that the timing jumps forward immediately when i come off idle. that is, i'm watching the timing, it's right at 24btc where i set it, but then as soon as i touch the throttle -- just enough to budge it, not enough to change the rpm's at all -- the timing jumps up to ~10btc. then as i press on and the rpm's increase, it advances normally. by 4000rpm it's way past where i can see it. on a hunch that it might be seeing detonation and thus retarding timing to save my ass for me, i disco'd my detonation sensor. no change. i've seen this now on two ecu's so if it's an ecu problem it's a hell of a coincidence. i can't think of any reason the car would retard timing as soon as the tps indicates that i've come off idle. anybody got a checklist? is this normal? needless to say there is hesitation when i come off idle. i can feather the pedal to get around it, but it's not right, and it's gutless too. Quote Link to comment Share on other sites More sharing options...
rossman Posted May 31, 2010 Share Posted May 31, 2010 (edited) i noticed that the timing jumps forward immediately when i come off idle. that is, i'm watching the timing, it's right at 24btc where i set it, but then as soon as i touch the throttle -- just enough to budge it, not enough to change the rpm's at all -- the timing jumps up to ~10btc. then as i press on and the rpm's increase, it advances normally. by 4000rpm it's way past where i can see it. That is normal "vacuum advance" behavior. You need to set the timing with the vacuum advance disabled. On earlier models that is done by disconnecting the vacuum hose from the distributor. I'm not sure how that is done on a ZXT. Download the service manual. You should look elsewhere for your low boost problem. Have you checked that your wastegate is functioning properly? Edited May 31, 2010 by rossman Quote Link to comment Share on other sites More sharing options...
Ineptitude01 Posted June 1, 2010 Share Posted June 1, 2010 ZX Turbos have electronic distributors. You can't unplug the vacuum advance on a car that doesn't have it... Quote Link to comment Share on other sites More sharing options...
rossman Posted June 1, 2010 Share Posted June 1, 2010 ZX Turbos have electronic distributors. You can't unplug the vacuum advance on a car that doesn't have it... If you read my post carefully I state: "On earlier models that is done by disconnecting the vacuum hose from the distributor. I'm not sure how that is done on a ZXT." Quote Link to comment Share on other sites More sharing options...
paul vixie Posted June 1, 2010 Author Share Posted June 1, 2010 "On earlier models that is done by disconnecting the vacuum hose from the distributor. I'm not sure how that is done on a ZXT." so, on ecu models like the zxt, advance is electronic, and there's a throttle position sensor, and there's one advance map for "when you are at idle" which is when the TPS reports that there's no pressure on the accel pedal, and another advance map for "not at idle but not wide open" and yet a third advance map for "wide open". the timing on this car is set at idle. i could unplug the TPS at which point it would still think i was at idle. i could short the right pin to get it to think i wasn't at idle and then set timing from that but there should be no difference between the idle map and the bottom of the non-idle non-WOT map. if i set physical timing using the bottom of the non-idle map then i'll be advancing ~15 degrees, and then when it goes back to the idle map it'll be at ~40btc. i used to own and work on vacuum based cars. i liked them, and the points and the rest. the behaviour i am expecting is, begin to advance once the rpm's go up, and not just because my TPS reports a non-idle condition while the rpm's are still ~600. for this car to move from ~24btc to ~10btc just because i'm changing from the idle map to the nonidle map (based on TPS, not RPM) means it's retarding it on purpose. as i said in the base note here, i unplugged the detonation sensor and saw no change in behaviour. therefore i am mystified. no ecu car and no vacuum advance car i have ever worked on has retarded timing while coming off idle. Quote Link to comment Share on other sites More sharing options...
paul vixie Posted June 6, 2010 Author Share Posted June 6, 2010 i found this other thread where timing retard was discussed and followed up there with some information i found on another forum. i'll post followups there not here. this topic should not have been created, i hadn't searched carefully enough on the timing retard question, sorry all. Quote Link to comment Share on other sites More sharing options...
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