Over-run fuel cut problem

[murjam]

Hi!

I am running VEMS 3.8 with the newest firmware (as of 2016-11-24 -- sorry, cannot point out the exact version right now) on my daily driver. It is a BMW M52 engine -- not that it matters.

Now, to the problem. I have set over run fuel cut to kick in while decelerating in gear just to improve fuel economy slightly.
The parameters I can set there are:
* Fuel cut below kPa
* Fuel cut above rpm
* Fuel return rpm
* Fuel cut delay

So far, I have set it up to kick in almost without any delay (0.1s) over 1500 rpm, and return fuel at 1100rpm. What I have seen is that sometimes I actually start decelerating even lower rpm and the fuel cut does not kick in. So, I took the approach to lower the 1500rpm to about 1300rpm.

But when the fuel cut starts over 1300rpm, it also kicks in when the idle for what ever reason (usually after a cold start) reaches 1300rpm. I do not want that to happen!

Is there something else I can configure for the fuel cut not to kick in when idling. With Megasquirt I know I can set the kPa limit under which the fuel cut should kick in, but currently with VEMS the "fuel cut below kPa" works as a separate function -- whenever the rpm is below currently set 18kPa, it cuts the fuel. It has nothing to do with the rpm nor TPS position! My car idles at about 25kPa the lowest, it never comes down to 18kPa.

Also, the TPS is set up correctly and shows 0% when the throttle is depressed. CL Idle is activated under TPS less than 0.39% or something like that, and works well.

As always, the story became rather long, but maybe there is someone who is willing to read the issue and give it some thought.. Thanks for any feedback and ideas!

[Sprocket]

So far, I have set it up to kick in almost without any delay (0.1s) over 1500 rpm, and return fuel at 1100rpm. What I have seen is that sometimes I actually start decelerating even lower rpm and the fuel cut does not kick in

That is correct. Anything below 1500rpm and TBS below idle threshold, fuel cut is NOT active. RPM has to rise above 1500rpm first, then when TPS below idle threshold, fuel cut is active. Fuel cut inactive below Fuel resume threshold of 1100rpm. Fuel cut below 18kpa is just that, cut anything below 18kpa.

If you want my honest opinion, I think you have the RPM values far too conservative and too close to normal idle conditions. I usually set fuel cut threshold to 2300rpm and fuel resume to 2000rpm, which is well clear of a cold idle speed of 1250rpm and after start added rpm of 200rpm which gives an idle speed at first cold start of 1450rpm diminishing to 1250rpm over ~20seconds and then diminishing to 850rpm warm idle.

I have fuel cut below KPA set at 10 with a normal idle ~35kpa. Overrun engine KPA drops significantly below idle KPA when the car is 'driving' the engine. Usually overrun KPA starts to rise towards idle KPA as RPM drops anyway so should only ever be active when the throttle is snapped shut on high RPM overrun condition (greatest manifold depression).

You could try some slightly lower RPM values than I have used, but I do think you need to increase your own RPM values a little. Perhaps set Overrun KPA to 10 and see how you get on.

[murjam]

Thanks for your response!

As I understand, it might be close to impossible to currently achieve what I'm trying to.

I really like decelerating in gear, even when driving quite low rpm . I.e when going slightly downhill, I like the car to maintain the speed without applying brakes nor throttle (and fuel). This often happens even in 5th gear at about 1500rpm.

The over run fuel cut works 95+% perfectly for me if i set the "rpm above" to 1600rpm, but there are occasions when I'd like the fuel cut to activate even at about 1400rpm.

It really helped me achieve this if there was an option to only activate the fuel cut over some CLT value or x seconds after start.

Thanks for clarifying the good use of the kPa limit!