Idle RPM+AFR Oscillations
 

I cannot seem to work out the cause of my RPM and AFR oscillations at idle. The car has a very distinctive 820-920 RPM oscillation with a period of ~4 seconds and AFR fluctuates along with RPM. Using Idle RPM Timing Correction greatly reduces the amplitude of the RPM oscillations but does nothing for AFRs. I have tried controlling every variable I can think of to figure out what is causing this but nothing seems to change it.
The basics:

• 94-95 MSLabs MS3 Basic
• Low comp. 1.9L, turbo, etc etc
• ID1050X w/ supplied deadtimes, voltage corrections, and small PWs
• Fuel pressure sensor shows constant on-target fuel pressures
• Battery voltage calibrated to DMM reading at injectors
• Spartan3 wideband via CAN, no voltage offset concerns

What I have tried thus far:

• Lock idle timing
• Set all idle VE cells to the same value
• Lock IACV duty
• Disable EGO
• Blocking off any other variable air sources (PCV, BOV)

How can I be getting these 100RPM oscillations with constant timing, fueling (or close to it), and IACV duty? A log showing the issue is attached, including with Idle RPM Timing Correction enabled at the end to combat the oscillations.

The only thing that I see closely correlated to the oscillations is my battery voltage which switches between 13.6V and 13.7V. This could just be a direct result of the RPM changes, but my working theory is that the voltage oscillations are causing my deadtimes to change and creating a feedback loop of some kind. I am also growing suspicious of the injector settings from ID -- my VE table is bizarre (very limited range of VE values, odd hotspot at low-load/high-RPM, very high VE at idle, etc). Thoughts?

Couple things I see. First off, you have idle VE enabled, so although your main fuel table is locked at 74, the idle ve table is not. Your timing correction is a bit aggressive, and I'm not sure why you're trying to idle at 15.4. I've also found that most BPs are much happier at 950rpm vs. 850, especially with anything but stock engine mounts, which most of us have. EGO will also help regulate AFR, monitor it at idle and make sure it's around 96-98% once warmed up. I've made all these changes and probably some more and attached the tune, try it and see if it helps. I've always used ID's settings without issue,

Curly, have a cat...

thanks for the tips, I’ll give it a go this week. I should have noted that during testing I set the entire Idle VE table to the same value. Is it even worth using idle VE though? I had hoped it would help with the AFR swings but doesn’t appear to do much of anything other than being another table to tune.

I don't think it's necessary, no. I've always felt like if the idle table is different than the main fuel table, there's a bump when transitioning in and out of it. And if they're the same, why bother? I feel like I can get EGO working as good as any benefit idle VE can get me.

As you can see from my sig, I've moved to a G4x on my car, but the when I had a MS and on the current MS's I tune at work, I follow the above advise. And the G4x doesn't have any sort of idle VE, fyi.

How do you like the Link? Comparison with MS3?

I don't use idle VE, but do use idle Advance. Allows more advance as soon as throttle is pushed off idle.

Sorry to thread jack OP, but I love the Link. I've had a DIY ms1, ms1 pnp, ms2 pnp, and a MS3 basic. I've tuned around a hundred megasquirts in Miatas, mustangs, and a Roush '32 Ford Hotrod thingy, among others. The Link allows me to look up things and change things MS doesn't let you to, so I really appreciate it for that, and I recommend it to anyone who asks. It's a little bit less PNP friendly, but that being said, the basemaps are, in my opinion, much better.

As an example of the adjustability, I've never liked WUE. It's a blanket percentage that doesn't make sense for all RPMs/load. I use a table in the Link to change my AFR targets, so it's a bit richer around idle (13.5 instead of 14.7) when cold, but I don't want 10.3 instead of 11.5, so in boost, the target doesn't change. See my table below.

Again these are only two things I like about the Link, I could probably talk to you about it for a hour, but I'll let OP keep his thread.


https://cimg8.ibsrv.net/gimg/www.mia...518dec77bc.png

And like I said the pinout is crazy helpful. I think everyone on here with a MS has at one time or another wondered if all the unused plugs in the engine bay go to a usable pin on the ECU, and we usually end up doing the PITA solution of routing wires through the firewall to the expansion plug.


https://cimg6.ibsrv.net/gimg/www.mia...1c0c601dad.png