AFR stupid rich after very short periods of overrun

[David_G]

Hi @ all!

I recently changed to a bigger Throttle Body and bigger Injectors on my '99 Miata with a MS3x Release 1.3.4.

Since then my AFRs go very rich (upto 9.8 ) and I get exhaust-popps after and during very short activations of overrun-fuel cut.
So for example: I step on the throttle and bring revs to 2000rpm and then let go the throttle, fuel overrun activates (because rpms are higher than 1400) until rpms are low enough again. Right after I let go the throttle, AFRs move to about 11, are then about to recover to the normal 14.7 but then just after they seem to recover again, the AFRs go pig rich again and I dont know why.

If the rpms fall from a higher level so that overrun fuel cut is active for a longer period of time, I don't get this problem. My AFRs do go rich after closing the throttle as well but then they just move up to 14.7 again without going back to rich.
Setting the overrun delay to a longer time doesn't help because it just shifts the problem to a higher rpm. What counts, seems to be the time overrun fuel cut is active. If its (too) short, it stays stupid rich, if its long enough, there are no problems. Also if I completely deactivate overrun fuel-cut, AFRs don't stay that rich either, so I guess it has to be some fuel cut setting.

overrun fuel-cut activated:


overrun fuel-cut deactivated:


MSQ and Logs attached

Thanks, David

[elutionsdesign]

Just turn over run off, I find it confuses most people and is rarely actually needed.

[kjones6039]

Just turn over run off, I find it confuses most people and is rarely actually needed.

Agreed! You can accomplish the same effect using nothing but the VE table anyway.........

[grom_e30]

I get a little rich on lifting the throttle for a short decel on my car then a little lean after and all i can put it down to is puddle of fuel that sticks to the intake walls getting pulled off by the vaccume then it taking a while to re astablish its self after.

[David_G]

Thanks for the answers so far.

I live in a very hilly area, so extended deccel situations are very common and I don't want to waste fuel and engine brake if I don't have to.
Sure, I can more or less accomplish the same thing with the VE table, but then I have the problem to be in "simulated fuel cut" everytime I lift the throttle even if its just for the blink of an eye and the throttle response will also be worse in those areas.

I suspected the wetted walls as well which is why I tried to set the target AFRs in the lowest kPa row to something like 17-18. I also set the target AFRs to 8-10 in case the mixture in the cylinders is too lean to be ignitable and therefore showing rich.
I then tried to increase/decrease the size of the fuel adder after over-run ends.
None of what I did had any impacts on the AFR readings at all. But I guess it should, right?

Maybe EAE is the key here but I didn't have a closer look into that and I try to avoid it until I am sure it really helps me here. Unfortunately I'm not familiar with it at all, otherwise I would just try it.