Lean after WOT

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bstevenson

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Jun 1, 2005
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I just got my new cooleeze megatmouth dp on and my new aem wideband and took my car for a little test ride. When I first start the car, the wideband shows a little lean for a minute (mid to high 15's). after running for about 15 seconds it goes down to the high 13's. So I took the car for a test ride and it was running good afr's while cruising so I decided to get on it a bit and check it out, I slow down and hammer it through second and third and my afr's stay down around 11-13 but as soon as I left off the gas and let it coast to slow down, afr's go of the gauge. . .lean. I realize that I am not giving the car any gas so it will lean out a bit, but I just wanted to check and see if it is normal for this to happen or if its something I need to fix.
Thanks
 
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bstevenson

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Jun 1, 2005
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jetjock said:
When you close the throttle the ecu shuts the injectors off to limit HC emissions so yes, it's normal. When the engine is first started it's supposed to run rich. Everything you're seeing is normal.
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Awesome, I knew it would go a bit lean after I let off the throttle, but I didn't know if it should shoot all the way up like that.
Thanks a lot!
 
jetjock

jetjock

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Jul 11, 2005
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Redacted per Title 18 USC Section 798
What The Boss said. Hot idle should be stoichiometric (14.7) to a bit leaner. I checked yesterday on my gas analyzer in prep for a smog test and mine is running at a lambda of .99 (14.6 AFR) during hot idle and steady state cruise. Cold idle will be richer. How richer depends on many factors.

The fine print: On deceleration the ecu looks at the AFM and TPS. If I recall correctly it also looks at vehicle speed but without yanking out the books don't hold me to it. Anyway, it knows the engine is decelerating so it cuts injection. When it starts injection again is a bit complicated to explain but it's done in such a way you won't feel it.

Even with the injection cut there's still some fuel left that would cause a large HC spike so the dashpot holds the throttle plate cracked for a second or two, allowing a bit of air to come in and cleanly burn most (but not all) of the fuel that's left over.
 
Nick M

Nick M

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Sep 9, 2005
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jetjock said:
What The Boss said. Hot idle should be stoichiometric (14.7) to a bit leaner. I checked yesterday on my gas analyzer in prep for a smog test and mine is running at a lambda of .99 (14.6 AFR) during hot idle and steady state cruise. Cold idle will be richer. How richer depends on many factors.

The fine print: On deceleration the ecu looks at the AFM and TPS. If I recall correctly it also looks at vehicle speed but without yanking out the books don't hold me to it. Anyway, it knows the engine is decelerating so it cuts injection. When it starts injection again is a bit complicated to explain but it's done in such a way you won't feel it.

Even with the injection cut there's still some fuel left that would cause a large HC spike so the dashpot holds the throttle plate cracked for a second or two, allowing a bit of air to come in and cleanly burn most (but not all) of the fuel that's left over.
Click to expand...
Yep.
 
jdub

jdub

Official SM Expert: Motor Oil, Lubricants & Fil
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Feb 10, 2006
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jetjock said:
Lol, I keep seeing that in posts but with different names. What the heck does it mean? Since this time it's mine I hope it's a good thing ;)
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Means he tried to add to your rep, but couldn't cause Lagged already did on an earlier post. And yes, it's a good thing :biglaugh:
 
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