Idle control vs vvt

Hi! I've got a m50b25tu running on an ecumaster emu classic. This engine has a switched (+25 degrees when on) inlet cam. I want to turn it off at idle, however the amount of time it takes for the cam to fully retard is longer than it takes for the rpm to drop from when I turn the cam off to my idle rpm. As such the cam is still moving while the idle control is attempting to start controlling the idle, and it drops significantly before steadying out. I've attached a screenshot of this happening in the log. Does anyone have any ideas what I could try to correct this?

The Timer Fuel Correction is a fuel enrichment that activates when vanos is disabled. I am using windowing(and table switching) to turn the cam on/off at different rpms for high and low load, and this helps to smooth out the fuel ratio when I shut it off at higher rpm(4000/4400). My air fuel ratio is correct for the initial drop, gets very rich(0.80) for the first peak and stays rich until it smooths out. The Timer Fuel Correction is over by the time it gets to the first peak, so I dont think its impacting this lean condition. Closed loop lambda control is off but having it on didnt improve anything.

I am using PID control on the idle valve duty cycle. Disabling PID control does not seem to change this behavior. I am not using any sort of tip out enleanment or overrun fuel cut. I tried overrun fuel cut last night but it was backfiring very loudly when I'd get on the throttle after coasting. It says vtec, but it is actually controlling the bmw vanos. I'm running 15 degrees of ignition timing at idle. My air fuel ratios in the relevant cells are correct when under load.

It never stalls and after it oscillates a few times it steadies out and idles smooth. If I coast until the cam is completely retarded or simply disable cam control it does not have this problem at all. I would be ok with having it idle high for a second until the cam finishes moving, this felt like what the oem ecu did. There is an option to increase the idle valve dc when idle control starts, but even at its max of 40% dc increase it doesnt fix the dip.

Thanks for the help!

Graham,

Thank you for sharing your thought process and steps you've taken. It all sounds great.

That ECU isn't one I have much experience with, and I try to provide suggestions that may help in general, so please forgive me if I suggest something that's not possible on that system.

You mentioned the stumble doesn't occur when VANOS remains in the off position the whole time. Do you observe brief rich operation on tip out still? Perhaps the rich condition exists (to a lesser degree) and is compounded by the VANOS fueling, putting things over the edge to being rich enough to stumble?

Either way, tip out enleanment may help the situation. While I expect you to generally run VANOS from around 2000-4000 RPM, if you want to set up a tip out enleanment independent of VANOS fueling, you could tune it to stabilize lambda with VANOS off, then re-enable VANOS and see if the rich dip is less severe and avoids stumbling.

Another option would be mechanically increasing airflow via throttle stop so your idle valve duty uses a range of perhaps 10-40% vs what appears to be about 23-40%. This would give you a larger control range for your experimentation.

Your log image doesn't show what ignition timing is doing, but if timing is part of your idle control system, it may be dropping to a level that reduces tq output despite the extra airflow you're providing. If that's occurring, increasing idle target temporarily is usually the solution.

I don't know what your options for this are off the top of my head, but if you can target +500 RPM for 1 second, taper to +0 over the next 1-2 seconds, then you may resolve fighting idle ignition compensation, idle duty will remain higher, and by then your cam should be where you want it.

If that's not possible on your ECU, a less ideal bandaid may be to hijack a moving idle speed offset. Most systems have an RPM target offset or alternate idle target speed for moving idle vs. stationary idle. Perhaps you could use that to achieve your goal by idling higher when the vehicle is >= 2 MPH, combined with disabling VANOS < 2 MPH.

All that said, airflow stability doesn't seem ideal when the cam is in motion as you approach idle, so the temporary high idle does seem to help things stabilize before the engine returns to normal idle.

Please keep us posted.

Thanks Mike! I increased the throttle stop to where it will idle at 710-750 with 0% dc on the idle valve. I then set the idle valve dc to 7% when warm, which was enough to get to my target of 800rpms. I don't really understand why but this seems to have completely resolved the issue. I can't actually drive it tonight but just revving it and letting it fall in the driveway, it seems perfect. I'm going to leave this unresolved until I get to go for a drive tomorrow and can confirm its working 100%.

It now catches at 1000, then slowly drops it to 800, exactly like I wanted it to. Somehow the cam movement seems to have little to no impact now. I was not using any sort of ignition control, I figured since I have an idle valve that I aught to be using pid control on it instead of the ignition timing. After trying it out, the ignition control seems much better. With idle valve pid control off and ignition pid control on it both responds faster and is more stable than with idle valve pid on and ignition pid off. I'm going to be sticking with this, it seems like the better solution.

I'm not really sure why this works. Shouldn't it be the same? The idle valve was switching to its idle on dc when the tps hit 0%, which I think means it should be a stable amount of airflow from the moment the throttle stops moving. I'm not sure how moving some of that airflow from the idle valve to the throttle plate changes anything. Is it because the idle valve outlet points at the back of the throttle plate instead of towards the ports, and this was interrupting flow somehow?

The ecu is currently not seeing vss at all. I have an open input so I can give it that but I'm not sure I need to at this point. As the car is both NA and slow right now, there isn't any real need for limiting power by gear or speed. As long as the idle control works as well when moving as it does in my driveway I think I can skip that one without any downsides.

Fantastic! Your idle valve has a fixed range of control. You were using its maximum flow at max duty, without getting sufficient total airflow to get the idle stability you needed on idle re-entry, which does require more airflow than steady state idle in order to slow engine speed deceleration as the engine falls to idle speed.

Now you've shifted the airflow range associated with valve control by increasing base airflow past the throttle body, so when your idle valve is at a certain position, like the base position at entry to idle mode, you are getting similar flow via the idle valve, but added to the increased base airflow past the throttle body, achieving a greater total airflow into the engine.

I like how you made the adjustment, shutting the system off, adjusting mechanically to hot 7xx RPM idle, then re-enabling to hit your 800 RPM target. You always want to leave a little room for idle duty to reduce towards zero, without being bottomed out, for situations where less airflow is required than usual. This will avoid exceeding your idle target.

Another way to do this is increase base idle position via throttle stop while watching idle valve duty. This assumes closed loop idle control. While increasing throttle opening with the throttle stop adjustment you keep going until you're achieving target idle speed with the margin between current idle duty and 0 duty that you want to leave in reserve.