MSPNP MSPNP specific Megasquirt related discussion.
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Pop/Backfire on throttle tip in

 
Old 05-07-2017, 12:26 AM
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Default Pop/Backfire on throttle tip in

Hey all. I've been struggling with this issue for a while, can't find anything related to it and my fixes thus far have been unsuccessful, hopefully someone wiser than me can help.

Ever since I've installed my MSPNP-Pro for my MSM (note, this is the Australian version), I have been getting a pop and sometimes a loud backfire when pressing the throttle between 2k and 3k RPM, this is when changing gears or when re-applying throttle after coasting. It doesn't seem to happen above 3k and doesn't happen when increasing already applied throttle.

Please see attached tune and log. I've marked the log just after instances of the pop (18.8s, 103.7s, 246.6s)

The car is running FF640 injectors, but the issue first appeared on stock injectors when the MS was initially installed. I'm not sure if related, but even with fuel cut disabled (currently enabled), AFRs in overrun appeared as though there was no fuel (ie: AFRs of ~22). I've tried to increase my overrun VEs to almost silly levels, but still the issue persisted. I've also experimented with increased AE, but no improvement.

I also sometimes see the Idle PMW increase after a short overrun/decel (but CLI not activate), which would increase the oxygen (right?) leading to a leaner condition, is this normal/correct?

Thanks. Hopefully this is something obvious and I'm just n00bed, because this has been driving me nuts!
Attached Files
File Type: msq
2017-05-07_14.19.00.msq (263.5 KB, 123 views)
File Type: msl
2017-05-07_09.44.56.msl (4.41 MB, 52 views)
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Old 05-07-2017, 01:57 AM
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Greetings Lachy!

My MS was doing this too, then after fiddling with just about everything, I think I got it sorted.

Your Accell Enrichment comes on much stronger than mine, and your %WOT is also more agressive than mine, especially in the 2-3k RPM area, so little throttle movements will be dumping in more fuel than you need, the marks in the log happen just after massive "Fuel: Accel Enrinchment" spikes of 150 - 200% which support my theory.
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Old 05-07-2017, 03:25 AM
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Hey Cus, thanks.

I agree that my AE is aggressive, but my logs never spike rich, they still hit lean. Maybe it's way too much and failing to ignite? Could the pop be unburned fuel in the hot exhaust?
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Old 05-07-2017, 06:15 AM
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I'm going to say that's exactly what's happening!

On closer inspection, in your AE settings there's a "Full Accel below (RPM)" - yours is set to 3000, bring that down to something closer to 1500 or so for a start (I'm at 1300). This will create a "maximum" slope up to the "Zero Accel Above RPM".

If your log at the first mark you're at 2200 RPM and just apply 5% throttle, but you get as much extra fuel as if you were at 900RPM and add in as much air. I don't know the practical reason, but engines need less AE as the RPM increases. So currently up to 3000RPM You have 100% AE, 4000RPM is 50% AE, and 5000RPM is 0% AE. By moving the 100% knee lower, you can make 2200RPM have more like 80% so you're not putting as much fuel in for a smaller amount of air.

Second mark: small movement again (32.2%TPSdot) but you get an extra 4% duty cycle, when the VE Table is only commanding 2.9% it spiked up to 6.9%, but the kPa has only gone up about 5.

Third mark: same deal, 15 - 30kPa change, (so lots of vac, to slightly less vac) and it puts in >200% fuel, TPSdot is 27.4%/s - maybe consider a low-threshhold in your TPSdot curve:
instead of having:
Added: 0.0 / TPSdot: 0.0
Added: 65.0 / TPSdot: 50.0
Added: 83.2 / TPSdot: 141.4

try:
Added: 0.0 / TPSdot: 0.0
Added: 0.0 / TPSdot: 35.0
Added: 55.0 / TPSdot: 176.0

That will mean AE ignores very small throttle inputs, and then starts ramping in much more slowly, which seem to be where the pops are all coming from AE starting off too "hot"

Fourth Mark: You snap the throttle open a whopping 25%, up to 40%, once again at 2200RPM and gain 40kpa more air (60ish->100ish) and hit 393.9tpsDOT, which takes you to 18% Duty Cycle when the engine was only after 9ish. You can also see on this one where after pulling throttle out, your MAP is still increasing, this is what the %WOT curve is for, Your TPS gets all the way back down to 28% before the MAP starts dropping off, so about 28% throttle @ 2200RPM would be 100% throttle for you, any "extra" throttle won't add any more air, so setting the WOT% curve means you can also limit the amount of "max throttle" the AE calculations will see. (ie: a throttle snap at 2200RPM of 100% will only need the same amount of fuel as one to 28%)

Hrm. Might have got overly involved in that analysis.... Still, was a good distraction from work...
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Old 05-07-2017, 06:16 AM
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I'm going to say that's exactly what's happening!

On closer inspection, in your AE settings there's a "Full Accel below (RPM)" - yours is set to 3000, bring that down to something closer to 1500 or so for a start (I'm at 1300). This will create a "maximum" slope up to the "Zero Accel Above RPM".

If your log at the first mark you're at 2200 RPM and just apply 5% throttle, but you get as much extra fuel as if you were at 900RPM and add in as much air. I don't know the practical reason, but engines need less AE as the RPM increases. So currently up to 3000RPM You have 100% AE, 4000RPM is 50% AE, and 5000RPM is 0% AE. By moving the 100% knee lower, you can make 2200RPM have more like 80% so you're not putting as much fuel in for a smaller amount of air.

Second mark: small movement again (32.2%TPSdot) but you get an extra 4% duty cycle, when the VE Table is only commanding 2.9% it spiked up to 6.9%, but the kPa has only gone up about 5.

Third mark: same deal, 15 - 30kPa change, (so lots of vac, to slightly less vac) and it puts in >200% fuel, TPSdot is 27.4%/s - maybe consider a low-threshhold in your TPSdot curve:
instead of having:
Added: 0.0 / TPSdot: 0.0
Added: 65.0 / TPSdot: 50.0
Added: 83.2 / TPSdot: 141.4

try:
Added: 0.0 / TPSdot: 0.0
Added: 0.0 / TPSdot: 35.0
Added: 55.0 / TPSdot: 176.0

That will mean AE ignores very small throttle inputs, and then starts ramping in much more slowly, which seem to be where the pops are all coming from AE starting off too "hot"

Fourth Mark: You snap the throttle open a whopping 25%, up to 40%, once again at 2200RPM and gain 40kpa more air (60ish->100ish) and hit 393.9tpsDOT, which takes you to 18% Duty Cycle when the engine was only after 9ish. You can also see on this one where after pulling throttle out, your MAP is still increasing, this is what the %WOT curve is for, Your TPS gets all the way back down to 28% before the MAP starts dropping off, so about 28% throttle @ 2200RPM would be 100% throttle for you, any "extra" throttle won't add any more air, so setting the WOT% curve means you can also limit the amount of "max throttle" the AE calculations will see. (ie: a throttle snap at 2200RPM of 100% will only need the same amount of fuel as one to 28%)

Hrm. Might have got overly involved in that analysis.... Still, was a good distraction from work...
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Old 05-07-2017, 09:35 AM
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im not seeing on your logs why you'd be getting a pop/backfire, your logs indicate the opposite problem -- too little fuel -- not excessive. But I do see the RPMs get jagged where you mark.

currently, your accel-pump is basically tuned not to work -- you're not injecting enough fuel with tip-in. I agree they are aggressive. Every time you add throttle where you mark, you've double your PW, I dont think your AFR gauge is fast enough to catch it, but i think you're injecting way too much fuel and causing it to explode.

But I disagree with Cus's suggestion. I believe it will actually make it worse (it may solve the bangs, but it's going to make the car feel jerky and unsmooth)... you're filtering out all AE below 35 %/sec and adding little above it -- AE should be tuned to work just above your idling RPMdot.



I'd start with something more like this:




what i do find interesting in your log is that your idle valve keeps opening on decel, yet it doesn't seem to meeting requirements to open.

Last edited by Braineack; 05-07-2017 at 09:55 AM.
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Old 05-07-2017, 03:34 PM
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Thanks Brain, I'll give something like that a try. I was wondering if it was something like too much fuel but not being accurately detected.

As for the idle valve, that's got me confused. I wasn't sure if it's normal behaviour or not. It seems to open in the 25-33% range (usually form one to the other), then gradually back down to 0% as regular throttle resumes. I wondered if it was getting ready for CLI, but those values aren't near the high RPM values for my initial values table. Faulty idle valve?

Also, should normal target AFRs be achievable in overrun (when overrun cut is not enabled)? Or is it normal for the AFRs to peg off the end at 22?
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Old 05-08-2017, 03:56 AM
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I made changes to my AE along the lines of Braineack's suggestions. The drive home this afternoon definitely seemed like an improvement Bangs and pops are mostly gone now, just the occasional fart. Still some fine tuning to do, but it's on the right track. Attached new log and current tune.

Now I'm just trying to get to the bottom of the overrun. Can someone tell me if I should be achieving normal target AFRs in overrun without fuel cut? I will re-enable fuel cut, but I would like those cells to be correctly tuned regardless.
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Old 05-08-2017, 09:43 AM
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wait, you're not using fuel cut for overrun?
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Old 05-08-2017, 04:34 PM
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It wasn't enabled initially as part of the original DIY basemap, and for about 6 months I'd left it off. Recently I turned it on because I was fed up with worrying about overrun and my MPG was going to crap. So, I turned it on, got it working nicely. In the first log/tune it's fuel cut is enabled.

It's now disabled again in the latest log/tune because I don't think the overrun cells of my VE table are any good at all and I want them to be at least reasonably accurate. They would mess with transitions from overrun (~15kpa) to cruise/accel (>40kpa) if they were too rich/lean right? But I've never been able to find an answer to the question of AFRs in overrun with fuel enabled. I assume I should be able to achieve reasonable AFRs, but I never have been able to get anything other than 22.
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Old 05-08-2017, 05:17 PM
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it was for sure cutting fuel in that last log -- keep overrun turned on.

your WB will peg 22% becuase there's no fuel... that's good.

you could stand to tinker with the settings to make it engage and disengage more smoothly.

turn on progressive cut
increase the delay to 1.5-2sec.
increase the lowest rpm to return fuel to 1500rpm.


you can time the EGO delay by reading logs, seeing the moment PW is returned and when the AFR reacts.
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Old 05-08-2017, 05:52 PM
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Originally Posted by Braineack View Post
it was for sure cutting fuel in that last log .
But it wasn't meant to be cutting fuel. However it certainly looks like it was. Should active fueling in overrun see afrs around 14-16%?

Anyway, I'll take your advice and re-enable with your suggested settings, you've got the experience and the knowledge and I don't want to go in circles second guessing you only to ultimately end up right were you said I would. I'll make the changes today and we'll see how the drive home tonight behaves. It's certainly better already
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