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Discussion Starter · #1 ·
I'm curious, if you coast to a stop in gear (say, third), obviously depressing the clutch before the car stalls, does the fuel shut off at that point? I've noticed that my Cyberdyne reads LEAN (as in no lights) when I coast down in gear. This means no fuel to the cylinders right? Does this also mean no spark? Any help, or comments?
 

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Well there certainly is no additional fuel being delivered but there is always a spark and flowing fuel to keep the engine going.
 

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it's my understanding that the fuel injectors are completly shut off when deaccelerating. all the fuel is just routed back through the return line. if there was fuel being burned or cumbustion taking place the engine wouldn't deaccelerate. ever watch an egt gauge while deaccelerating? it will go from 900c to 0 fairly quick depending on how long you are deaccelerating.

bill

------------------
95 tsi-awd
91 tsi-fwd
95 civic turbo
82 gti
 

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I always thought there had to be air/spark/fuel even when deccelerating to keep the engine from slowing to fast but I guess the Throttle boby plate is open wide enough to the specific rpm so it won't just stall.
 

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Discussion Starter · #5 ·
Hmmm...if the two conditions are the same (idling and decelerating in gear), then why does my a/f gauge read full lean (no lights) on deceleration and bounce around at idle. Also, the EGT drops to almost nothing in deceleration, and stays at a reasonable level at idle. This tells me that they can't be the same condition.
 

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When you take your foot off the gas when you decelerate the thortle plate shuts. The position of the plate would be the same as if you were idling, right? When you're idling the ECU is in a closed loop measuring the incoming air and giving the right amount of fuel (also monitoring O2 sensor). So you would be getting fuel and spark. If you didn't get any fuel the car would stall completely. If you were decelerating and ran out of gas I'm pretty sure the car would buck and stall out. The RPM's slowely drop off because the transmission is still in gear and the wheels are turning the drive shaft, which is turning the flywheel (engaged clutch), which is turning the crank, and moving the pistons.

Sound reasonable?

Zach
'92 Laser RS Turbo AWD
"RSX"
 

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>
(idling and decelerating in gear), then why does my a/f gauge read full lean (no lights) on deceleration and bounce around at idle. Also, the EGT drops to almost nothing in deceleration, and stays at a reasonable level at idle. <HR></BLOCKQUOTE>

Here's a theoroy:
Let's think about NA engines. As the piston pumps, the vacuum created sucks in air. Now look at what is happening when you decelerate in gear. The throttle plate shuts (like at idle), TPS tells the ECU its at idle, you get only so much fuel. But the pistons are sucking in more air at 3K+ RPM's than at 750, so you get A LOT more air than at idle. Your A/F gauge is wired to the O2 sensor and it reads too much air in the combustion mix, hense the lean reading. The issue with the EGT is confusing. Perhaps the excess air has something to do with it. Maybe the air that was not consumed in the burn acts as a heat sink and absorbs the heat???

Problem with theory:
Wouldn't the excess air be compensated for by the ECU via karmon vortex measurements and related fuel maps?

Answer:
Perhaps the ECU adjusts the FPS (fuel pressure solenoid) because of the TPS reading. That would explain the variable vacuum readings at idle and deceleration on the boost gauge. (That's the way mine and tons of people have it wired)

Just theorizing, could be wrong,
Zach
'92 Laser RS Turbo AWD
"RSX"
 
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