daeron Posted January 8, 2007 Share Posted January 8, 2007 Okay.. short version of the background story, I am broke. Digi dash fuel gauge is showing no bars, driving home tonight, I have reason to think that my car, due to a malfunction, is getting horrible gas mileage right now.. so I am driving in a manner to conserve fuel to the utmost. I know when I let off the gas and coast to a redlight, it obviously burns less fuel then it would if I kept it stoich until I had to slow down.. My question is, am I correct in believing that popping the auto into neutral, and allowing the RPMS to drop, will burn less fuel than coasting in gear with no pressure on the throttle at all? I know that the no-gas-pedal coast is gonna burn LESS fuel, but does the air flowing into the motor cause the ecu to pump more fuel than it would take to keep it idling in neutral? I can see reasons why it might, and why it might not.. but without knowing the technical details of this EFI system, I don't know which side of my schizophrenic inner engineer to believe.. I mean, no throttle = no gas.. but i cant believe that as its coasting down from 70MPH in drive, it is pumping no more fuel than it would be at idle RPMs in neutral. Am I right? popping it into neutral saves the most fuel, right? EDIT: Oh, the irony of misspelling the word "advanced" in a phrase like "question for those with relatively advanced knowledge..." (i spelled it advnaced before I came back and edited it... ) Link to comment Share on other sites More sharing options...
Phizinza Posted January 8, 2007 Share Posted January 8, 2007 I have heard it is a bad idea to coast in neutral in a auto. As for the other stuff. I may be able to help, but I am not sure so I'll keep my mouth shut until someone who knows there stuff comes along. Link to comment Share on other sites More sharing options...
GeneralDisorder Posted January 8, 2007 Share Posted January 8, 2007 I mean, no throttle = no gas.. but i cant believe that as its coasting down from 70MPH in drive, it is pumping no more fuel than it would be at idle RPMs in neutral. Am I right? popping it into neutral saves the most fuel, right? Nope. On the SPFI, no throttle = no fuel if you are coasting. Always, 100% of the time. That's one of their big wins over carbs for enhanced mileage. Carbs still pull fuel through the idle circuit even under closed throttle coasting. There were various systems employed to try to prevent it - the Anti-Afterburn-Valve (AAV), but it's not 100% effective, and it's complex. On the SPFI, it shuts off fuel flow if the engine is turning more than idle speed (which it considers to be about 1200 RPM in closed loop near as I can tell), and thus you will use zero fuel coasting down a hill in gear. If you go to neutral you will actually use MORE fuel as the ECU then has to turn on the injector to maintain idle speed. Ever had the engine do it's strange surging, bouncy idle thing? Idle bounces between like 1500 and 1000 RPM? That's the idle fuel cut in action. Something is amis with the IAC adjustment or the MAF, and the engine wants to idle higher than the fuel cut, so it bounces back and forth - idle increases, fuel cuts out, idle decreases, fuel cuts back on, idle increases....ad infinitum. ECU can't even tell what gear you are in - only that you aren't in park. Auto's don't have the neutral switch, just the park switch. Crank inhibitor is part of the auto shifter mechanism - not controlled by the ECU at all. GD Link to comment Share on other sites More sharing options...
daeron Posted January 8, 2007 Author Share Posted January 8, 2007 Nope. On the SPFI, no throttle = no fuel if you are coasting. Always, 100% of the time. That's one of their big wins over carbs for enhanced mileage. Carbs still pull fuel through the idle circuit even under closed throttle coasting. There were various systems employed to try to prevent it - the Anti-Afterburn-Valve (AAV), but it's not 100% effective, and it's complex. On the SPFI, it shuts off fuel flow if the engine is turning more than idle speed (which it considers to be about 1200 RPM in closed loop near as I can tell), and thus you will use zero fuel coasting down a hill in gear. If you go to neutral you will actually use MORE fuel as the ECU then has to turn on the injector to maintain idle speed. Ever had the engine do it's strange surging, bouncy idle thing? Idle bounces between like 1500 and 1000 RPM? That's the idle fuel cut in action. Something is amis with the IAC adjustment or the MAF, and the engine wants to idle higher than the fuel cut, so it bounces back and forth - idle increases, fuel cuts out, idle decreases, fuel cuts back on, idle increases....ad infinitum. ECU can't even tell what gear you are in - only that you aren't in park. Auto's don't have the neutral switch, just the park switch. Crank inhibitor is part of the auto shifter mechanism - not controlled by the ECU at all. GD Excellent answer, McFly!!!! thanks GD. as I said, I saw a perfectly good reason for either case to be truth, but I lacked the knowledge. Phiz, what have you to say against coasting in neutral with an auto?? I have browbeaten some of my friends for upshifting and downshifting their autos, then wondering why they blew their tranny.. I hate the things, give me a five speed or give me death.. or give me a free car that I can't complain about.. Its been over two years since I had to switch from my Zcar to the soob, and i STILL get antsy and start pounding my left foot against the floorboard when I can't get enough power to pass someone.. I know a 4-speed auto would be less godawful than my 3AT, but it still wouldn't be the same. Three speed transmissions were not meant to be coupled to four cylinder engines, if you ask me. Link to comment Share on other sites More sharing options...
Frank B Posted January 8, 2007 Share Posted January 8, 2007 I have read that simply shifting into nuetral while at a stop light will increase mileage as well. It takes that extra drag of turning the tranny off the idling engine. Also helps to reduce tranny temps, which is good. I have only one auto, a 97 Lumina, but I don't drive it enough to notice those things. Link to comment Share on other sites More sharing options...
idosubaru Posted January 8, 2007 Share Posted January 8, 2007 one way to sort of see this visually is just to remove the intake hose and watch the butterfly plate. when you let off the foot pedal/throttle cable, it closes the throttle plate completely and the engine is forced to run on the IAC or whatever your idle air control system is composed of. less air = less fuel. Link to comment Share on other sites More sharing options...
NorthWet Posted January 8, 2007 Share Posted January 8, 2007 I totally agree with GD as far as how the SPFI should handle this. Note that there is a certain amount of drag from the tranny/drivetrain, depending on whether it is usuing an overrun clutch or not (and whether you want engine braking). Link to comment Share on other sites More sharing options...
Skip Posted January 8, 2007 Share Posted January 8, 2007 Also as GD alludes to in all EFI systems when the VSS indicates a speed of over ~ 10 mph in gear (thus the reason for the neutral switch on manual shift boxes) and the TPS 's idle switch is closed the ECU turns off the signal to the injector(s) and thus zero gas is used. Link to comment Share on other sites More sharing options...
nipper Posted January 8, 2007 Share Posted January 8, 2007 Also as GD alludes toin all EFI systems when the VSS indicates a speed of over ~ 10 mph in gear (thus the reason for the neutral switch on manual shift boxes) and the TPS 's idle switch is closed the ECU turns off the signal to the injector(s) and thus zero gas is used. Automatics have a NSS that indictaes neutral and park. SO thats just something to look into. The reason for not coasting to light or stop in neutral is incase you have to take evasive action (the little pedal works as well as the big pedal). There may be a minute amount of increased wear on the tranny, but otherwise its not a big deal. In general you use less gas in neutral at a light then in dirve, but it really is a small differnce, unless you are in a city and get a lot of red lights. Link to comment Share on other sites More sharing options...
GeneralDisorder Posted January 8, 2007 Share Posted January 8, 2007 And for anyone listening - this is exactly WHY I advocate hooking up all sensors and switches when doing the SPFI conversion. You could adversely affect your mileage. GD Link to comment Share on other sites More sharing options...
daeron Posted January 9, 2007 Author Share Posted January 9, 2007 Automatics have a NSS that indictaes neutral and park. SO thats just something to look into.The reason for not coasting to light or stop in neutral is incase you have to take evasive action (the little pedal works as well as the big pedal). There may be a minute amount of increased wear on the tranny, but otherwise its not a big deal. In general you use less gas in neutral at a light then in dirve, but it really is a small differnce, unless you are in a city and get a lot of red lights. Thats what I thought, but I was unsure if the air still being brought into the engine would override the throttle cutoff and make the ECU pump gas into the injector anyhow. I have an EFI Bible for the bosch L-jetronic MPFI in my Zcar, and I knew that IT cut to zero fuel flow at zero gas pedal.. but was unsure how the soob system worked. Since mines the TBI, I could see how it might function more like a carb.. but anyhow, my primary concern was with burning fuel in an elevated amount, at like 2-3K.. im not SO worried about my fuel mileage that i really care to gain by shifting into neutral at a light. to be honest, I just started asking myself the question as I was approaching a light.. and realized I was uncertain of the answer. I didn't anticipate it sparking such a great thread... GD, by all means, feel free to add a link to this thread to your SPFI conversion. You are right; it IS an excellent illustration of why everything needs to be properly connected or fooled. Link to comment Share on other sites More sharing options...
GeneralDisorder Posted January 9, 2007 Share Posted January 9, 2007 Automatics have a NSS that indictaes neutral and park. SO thats just something to look into. SPFI auto's have a park switch only (which happens to connect to the same pin on the ECU as the neutral switch for the manual's). The so called "neutral safety switch" is simply a lockout that prevents the starter from running except in park and neutral. It's not connected to the ECU in any way. The circuit has to be defeated when doing a manual conversion, but it's simply a matter of jumpering two wires in the shifter plug. Beleive me - I've been through the SPFI harness and ECU pinout more times than I care to remember at this point. I've retro-fitted it into two cars, and I have most of the wire colors memorized at this point. I'm sick and twisted I know.... GD Link to comment Share on other sites More sharing options...
Phizinza Posted January 9, 2007 Share Posted January 9, 2007 Phiz, what have you to say against coasting in neutral with an auto?? I have browbeaten some of my friends for upshifting and downshifting their autos, then wondering why they blew their tranny.. I hate the things, give me a five speed or give me death.. or give me a free car that I can't complain about.. Look mate, I can't say for sure as I have never owned a auto (and never plan too, I learned in one and I hated it.) I have just been told by at least 3 different people that it is a bad idea and can cause damage. And damage to an auto is like $$$$... and no home jobbies here either Link to comment Share on other sites More sharing options...
daeron Posted January 9, 2007 Author Share Posted January 9, 2007 Look mate, I can't say for sure as I have never owned a auto (and never plan too, I learned in one and I hated it.) I have just been told by at least 3 different people that it is a bad idea and can cause damage. And damage to an auto is like $$$$... and no home jobbies here either Cool enough, Just had to ask.. You sound like me when it comes to automatic transmissions.. My entire life has been in a shambles starting just around the time my Zcar died and I got this for free.. Driving an automatic is just one of the many things that got shat out upon me at that time... Thanks for the honest answer.. and I will now think twice about it every time, whether I change my behavior or not.. but believe me, it isnt a constant thing. Only an exception makes me do that; my normal behavior is just leaving it in drive, for the various control and "what-if" safety reasons as well as an "anti-murphy ward" with my slushbox, so to speak.. and my hand is still stuck on the shifter most of the time, anyways, just like my left foot still stomps the floorboard like I'm Dr. Strangelove... Link to comment Share on other sites More sharing options...
GeneralDisorder Posted January 9, 2007 Share Posted January 9, 2007 just like my left foot still stomps the floorboard like I'm Dr. Strangelove... Wow - someone that's actually seen it and isn't senile yet. Special edition is out - been meaning to pick it up myself from amazon. Good times. GD Link to comment Share on other sites More sharing options...
nipper Posted January 9, 2007 Share Posted January 9, 2007 Manually shifting an automatic doesnt hurt it. When you shift it manually you are bypassing the valve body. The valve body is a hydraulic computer that balances throttle pressure vs govenor pressure (from the output shaft). The internal spool valves decide when to shift the car. The bands and clutches are applied by these circuits. When you shift manually, you have taken over the job of the hydraulic computer, and the same clutches and bands are still enforce. In some transmissions you bypas the acumulators which are meant to slow down the shifts and make them softer (slips the bands and clutches more). In all honesty, i've known people that have blown trannies that shifted manually. After taking them apart for inspection, to my trained eye, it looked like any transmission that had gone bad (fried clutches), so the tranny would have gone bad anyway. The one way you can damage the tranny slowly is by putting the car in drive without having your foot on the break pedal, this tends to hammer the internal parts. Another is buy going from R to D or D to R without letting the car fully stop. The hard parts were not damaged, and that is the only place where any differnce would have shown up. 80's early 90's automatics were huge power theives, so nothing really helps, as the weak point is the torque converter. If you want to shift a manual, drive a manual, or get a car with a tighter autobox. I ocassionally shift my 97 manually, its a much tighter box, and due to me car accident i cannot drive a stick anymore. nipper Link to comment Share on other sites More sharing options...
nipper Posted January 9, 2007 Share Posted January 9, 2007 (sorry repeated post, ignore this. Site timed out while I posted, so I didnt know it whent through) Link to comment Share on other sites More sharing options...
robm Posted January 9, 2007 Share Posted January 9, 2007 Just to play the devils's advocate here.... Coasting in gear uses less gas, as the SPFI turns it off completely. BUT the car goes further coasting in neutral. SO, is it cheaper to coast a long way in neutral, idling at 1200+ rpm, OR coast a short way in neutral, and keep your foot on the gas for the extra distance it would have gone in neutral? I suspect this is not an easy question to answer. There are lots of variables (like how far can you coast, and how far do you coast) and no easy way to measure the outcome. Driving around town is so wasteful anyway, any difference could easily get lost in the noise. Acceleration is what burns gas, cruising at any speed below 100 km/h uses very little, idling uses very little, and decelerating in gear uses none. The difference between cruise, idle and decelerating is hard to measure against the cost of acceleration. GD, you say you know the pinouts on the SPFI in your sleep. Do you happen to know which of the test sensors has the VSS output available on it? And do you happen to know what the signal looks like? It should be possible to rig up an instantaneous fuel consumption measurement, using this signal and the PWM signal to the fuel injector. With this measurement and a stopwatch or other more accurate instrumentation, it may be possible to answer this question. Link to comment Share on other sites More sharing options...
daeron Posted January 10, 2007 Author Share Posted January 10, 2007 Just to play the devils's advocate here.... Coasting in gear uses less gas, as the SPFI turns it off completely. BUT the car goes further coasting in neutral. SO, is it cheaper to coast a long way in neutral, idling at 1200+ rpm, OR coast a short way in neutral, and keep your foot on the gas for the extra distance it would have gone in neutral? I suspect this is not an easy question to answer. There are lots of variables (like how far can you coast, and how far do you coast) and no easy way to measure the outcome. Driving around town is so wasteful anyway, any difference could easily get lost in the noise. Acceleration is what burns gas, cruising at any speed below 100 km/h uses very little, idling uses very little, and decelerating in gear uses none. The difference between cruise, idle and decelerating is hard to measure against the cost of acceleration. GD, you say you know the pinouts on the SPFI in your sleep. Do you happen to know which of the test sensors has the VSS output available on it? And do you happen to know what the signal looks like? It should be possible to rig up an instantaneous fuel consumption measurement, using this signal and the PWM signal to the fuel injector. With this measurement and a stopwatch or other more accurate instrumentation, it may be possible to answer this question. FREAKY!!! this entire post was played out in my head, as a train of thought, at about four o clock tuesday afternoon.. 12 hours before i read this post... for the record, I will reiterate that the occasional pop into neutral at a red light (so i can take my foot off the brake) is the ONLY "shifting" I do with my auto.. I just find it pretentious for people to flick the things around, and i dont want anyone who would agree with me, getting the wrong impression of me. If I had any choice in the matter this thing would have been a fivespeed last july. unfortunately, life doesn't let me make plans like that. No, something always comes out of left field and screws up MY plans.. allright, plaintive whining mode, off. Link to comment Share on other sites More sharing options...
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