How much should pressing the brakes impact fuel trims?
 

I've posted some other threads on this... I'm in the process of trying to diagnose some odd tune issues with my Harrop supercharged car and Ecutek tune. While doing one of my many datalog sessions I noticed some odd behavior that I'm curious if it's normal or not...

With the car fully warmed up and at idle my short term trims are ~-10% (Yes, I know this means I need to have my MAF scale tweaked). That's not the odd part, what's odd is when I step on the brake pedal I notice the AFR lean out then the fuel trims jump back up to basically zero for a few seconds as the closed-loop brings the AFRs back to 14.7.

Is this normal? I know the brakes are vacuum assisted and pumping the pedal does change the vacuum load on the engine, but should it be THAT drastic?

If it's abnormal, what on earth could be causing it? If it's to be expected then I'll disregard and move on to the next unlikely thing that could be causing my problems :(
-Matt

I've noticed the same behaviour with my car. I always figured it was to do with the brake booster. My car is NA. I am also fairly certain other cars I've owned have done the same thing.

Idle ST and LTFT fluctuates quite a bit depending on lots of stuff (even switching on/off the lights). As far as I can tell the ECU resets idle LTFT to +10 on every restart and then it works it's way back down as I suppose it's switching through the different idle tables.

Also your engine idles in these 4 fields marked grey. There is stock +5% load compensation.So you get 5% extra fuel right there, if you didn't change it.

Simply changing those 4 fields will change your idle LTFT. You can give yourself whatever idle LTFT you want by changing those cells.

But why do you think idle LTFT is so important? You are not driving the car in idle. If you don't have high LTFT in the relevant areas that you are actually in while driving, why bother?

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