When I take my foot off the gas and am coasting or down-shifting prior to a turn, my AFR goes crazy lean.

So, very low MAP but RPM's in the 3-4k range.

Do you worry about these complete no-load AFR's, or do you add fuel to these cells to keep in in the range?

 

If it's in neutral coming down from high rpm or your car is in gear and your not on the throttle there should be little to no fuel added in the engine.

 

Ok, cool. So seeing the AFR peg at that point isn't a concern.
Thanks.

 

In extreme vaccum situations like that, the ECU all but shuts off fuel completely. As an expiriment, start driving down a hill at about 40mph in 3rd and lift off the gas... notice the car starts to slow due to compression... now turn the ignition off... notice the car doesn't slow down any more or less. You lose your brakes and PS, so turn it back on real quick, but the expiriment should show that there is virtually zero ignition taking place.

 

What he said. I came from the Powercards, which indicate that the injectors shut off when your remove your foot off the throttle and turn back on to maintain idle once the engine slows down. Hence the reason many rookie manual transmission drivers are retarded for putting the car in neutral while 'coasting.' One the small scale, more fuel is being used to maintain idle in neutral than for the engine to coast and simple pump air.

 

Helpful explanation. Thanks!