L-Jet going full Rich on Deceleration Options

[Brian Fuerbach]

1974 1.8 with L- jet. Been playing with a wide band air fuel meter and noticed that when I lift the throttle the AFR goes to max value on the gauge. I thought it was supposed to go lean. I checked the throttle position switch and it checks out fine. Going to check the wiring harness next.

What else should I check? Decel valve?

[emerygt350]

Let's just blame autocorrect on all of that....

Really an interesting topic. I wonder if the afm shut off wasn't taking advantage of residual pressure. Sure the pump will stop when the afm closes but residual pressure will give another second of gas. If the engine is still running the afm will open again.

Just went for a little test run. Did the downhill drag (not intense, just normal kind of hill driving) and when I got to the bottom my idle was about 200 low. Engine was quite cool though. 186 head temps. It was hard to tell if I was fighting a computer induced rich condition rather than fouled plugs. The car ran fine beyond the low idle. By the time I got to the next stop sign and the temp came back up to 260ish, idle was back at 900.


I did notice that my vacuum on decel was 23. And that was not rpm dependent. If I was dragging down the hill at 4k rpms, the vacuum was the same as 1800 rpm.

In a 2.0 we have the mps measuring plenum vacuum for normal operating conditions (and at high vacuum fuel duration is cut) but we also have a more active tps, and one of its jobs is to tell the ecu to ignore the mps at closed throttle.

Need to get my new o2 sensor to play more with this... I just have to imagine that the engineers would have cut fuel almost completely during decel.

[wonkipop]

Let's just blame autocorrect on all of that....

Really an interesting topic. I wonder if the afm shut off wasn't taking advantage of residual pressure. Sure the pump will stop when the afm closes but residual pressure will give another second of gas. If the engine is still running the afm will open again.

Just went for a little test run. Did the downhill drag (not intense, just normal kind of hill driving) and when I got to the bottom my idle was about 200 low. Engine was quite cool though. 186 head temps. It was hard to tell if I was fighting a computer induced rich condition rather than fouled plugs. The car ran fine beyond the low idle. By the time I got to the next stop sign and the temp came back up to 260ish, idle was back at 900.


I did notice that my vacuum on decel was 23. And that was not rpm dependent. If I was dragging down the hill at 4k rpms, the vacuum was the same as 1800 rpm.

In a 2.0 we have the mps measuring plenum vacuum for normal operating conditions (and at high vacuum fuel duration is cut) but we also have a more active tps, and one of its jobs is to tell the ecu to ignore the mps at closed throttle.

Need to get my new o2 sensor to play more with this... I just have to imagine that the engineers would have cut fuel almost completely during decel.

yeah emery

what interested me was it does it cold but not when warm.
during warm up i believe the engine must be getting 2-3 times amount of enrichment.
and with vac working on fuel pressure regulator, fp goes down to 28.

and injectors won't squirt below 26. thats the bit i have to check. someone must know.
its not something i found in manual. it was just something someone wrote somewhere.

so maybe it can dip down enough when its cold to truly cut the injectors briefly.

because it don't happen warm.

as always. i dunno.

and i have read and read and read on the fuel cut off thing.
its not in this early version of L jet.
but i'm guessing that in a situation where the decel is disconnected and there is no action on the flapper that the throttle switch controls the fuel amount. so it delivers idle fuel portioning all the way down. ??? but not a cut off.

some slightly later bmws have the fuel cut off on decel. fuel is cut off at higher revs then as it drops to around 3000 revs fuel is resumed. but not it seems in these babies.

the decel does a couple of things.
it sort of simulates the way a carb can come down smoothly and resume idle.
ie it keeps combustion going and lets it subside gently to idle level.
but it also cleans up emissions because its letting in fuel on the way down via the flapper (i think?) and its burning it with air from the decel valve. and on top of that with an original distributor it retards timing back a little from centrifigal position and burns it slightly later - even past the exhaust valve and into upper stubs of exhaust manifold.
i think that is how they did it with these early L jets in relation to emissions.
all of which would lead you to a leaner condition measured with a probe. because its also burned up excess hydrocarbons in upper exhaust. these don't pop or detonate later in exhaust causing that usual burble - rather they get burned as they leave.
i think i have all that correct for a stock set up.

brians is a little harder to understand.

like how long is this rich thing persisting.
is it all the way down the hill/mountain coasting. or is it just initially on backing off to coast.