Is it impossible to run a type IV in closed loop?, Megasquirt Options

[DNHunt]

After about a month of tuning I have a fueling map that seems close. Most of the map yields air fuel ratios of about 13.8. From what I have read this is about where the motor was designed to run and it runs very well.

However, I continue to have problems with overrun. Between 2600 and 3600 rpms under very light load the air fuel ratio is much richer. This is quite disappointing as I would hope to gain some economy here. The solution would seem to be closed loop with the oxygen sensor leaning out the air fuel mix. The problem is that the pulse widths are so short that O2 correction leads to large changes percentage wise in pulsewidth and fuel mix and the car bucks. I have one more idea to try. I am waiting on some new injectors since it feels as if one cylinder goes dry.

If that doesn't help, it appears to be run open loop and send fuel ou the exhaust or run closed loop and suffer some loss of smoothness.

I'd appreciate any advice,

Dave

[airsix]

Dave,
I'm not familiar with the megasquirt, so what I say might not apply, but these are my thougths based on experiences tuning my MIC3.

1) your ve table is 3-d. So you can map different volumetric efficiency at different rpm/load points, right? It sounds like your ECU is programmed for to high of a VE in the 2600-3600rpm range at light load. Lowering the VE at that point in your map would lean out the mixture I would think, while maintaining the proper mixture at high load.

2) Most ECU's have a "minimum injection pulsewidth" paramater. Assuming the megasquirt does too, If your minimum pulsewidth is set too high the ECU won't be able to follow portions of the map because the "minimum pulsewidth" setting will prevent the pulsewidth from being shortened to the proper value. Maybe on overrun or a extremely light load your map is "bottoming out" on the minimum pulsewidth parameter.

Just brainstorming. I don't know much, but I'll help if I can.

-Ben M.