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Old 06-12-2019, 09:00 AM   #1
CorvallisBMW
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Megasquirt help - bucking on light load on/off throttle transition

Hey all, I've been fighting this really awful bucking on light load on/off throttle transitions. Think parking lots, heavy traffic, etc. It's the opposite of smooth and makes low speed driving really tough. I've been reading up on some of the MS forums that reducing timing in this area should help but so far I haven't seen any improvement. I'm wondering if anyone here might have some E30-specific experience.
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Old 06-12-2019, 09:05 AM   #2
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Probably need some accel enrichment. That area of the map may have some lean spots in it that need tuned out. Also make sure your overrun fuel cut is not interfering in that part of the map

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Old 06-12-2019, 10:52 AM   #3
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I may need to zoom in on my map more as I can't really see any lean spots. Is there a difference between overrrun fuel cut and decel fuel cut? Or just two words for the same thing? I seem to recall readings that MT cars really don't want more than a value of '90' for decel fuel cut.
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Old 06-12-2019, 11:39 AM   #4
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I believe thats the same thing.
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Old 06-12-2019, 10:54 PM   #5
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without an MSQ and log we're just gonna be guessing.

Decel enleanment and Overrun fuel cut aren't the same, so l would just watch out for that. In general if your overrun fuel cut is interfering with anything, it's too aggressive and probably set too high on your fuel table's load axis.

Post your MSQ and a log of it happening. In my experience any bucking or hesitation at transitional areas is likely fueling related, and often because of lack of resolution from the off-throttle vac area of the map and the first cell of the fuel map.
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Old 06-12-2019, 11:55 PM   #6
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My personal experience is that a well tuned VE table and spark table will drive very well without any accel enrichment. Its only when you stab the throttle good and proper it needs the enrichment. I drove around with accel enrich turned off while tuning the VE. Drove fine under 95% of conditions. (m20 2.3L)

You haven't mentioned what engine it is?

Also don't worry about any of this being "E30 specific", combustion engine is combustion engine.
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Old 06-14-2019, 07:50 AM   #7
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So changing my overrun fuel cut and that did help with the bucking below 1500rpm.

I probably wasn't clear enough in my first post though. It's not really that the car is bucking on it's own, it's just that the on/off throttle transition is extremely harsh. At low speeds if I need to carefully move between on throttle/off throttle, say around a parking lot or in traffic, the transition throws my head forward/backward. Is there a way to smooth out that transition?
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Old 06-14-2019, 07:52 AM   #8
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Are you sure it's not driveline lash?
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Old 06-14-2019, 08:00 AM   #9
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Tune accel enrichment. I went through the same thing. You'll need to increase the amount of fuel that's supplied once the throttle sees a big change

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Old 06-14-2019, 09:52 AM   #10
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Quote:
Originally Posted by SLEEPYDUB View Post
Tune accel enrichment. I went through the same thing. You'll need to increase the amount of fuel that's supplied once the throttle sees a big change
Unless the OP has an oversized throttle body, I doubt this is the issue.

Again, I would suggest posting your MSQ and a log of the car driving around a parking lot.
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Old 06-14-2019, 02:55 PM   #11
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Quote:
Originally Posted by citizen_insane View Post
Are you sure it's not driveline lash?
It's definitely not driveline lash

Quote:
Originally Posted by SLEEPYDUB View Post
Tune accel enrichment. I went through the same thing. You'll need to increase the amount of fuel that's supplied once the throttle sees a big change

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That might make sense if we were talking about a big change in throttle position, but I'm talking about minute changes, as in going on/off throttle at low speeds and low load. Accel enrichment shouldn't even be needed at 5% throttle opening with a very slow TPSdot.

I should also point out that this happens during the on/off throttle transition, meaning it also happens when moving from on throttle to off throttle, where accel enrich cannot apply.

Quote:
Originally Posted by Jaxx_ View Post
Unless the OP has an oversized throttle body, I doubt this is the issue.

Again, I would suggest posting your MSQ and a log of the car driving around a parking lot.
Working on it.

Last edited by CorvallisBMW; 06-15-2019 at 06:33 AM.
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Old 06-16-2019, 05:12 AM   #12
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No MSQ, no log, not easy to help without it. At least screenshots of your maps and some of a log. Try richening it up in the low load transition areas. No information about your setup but depending on injector size it's fairly common to get down into pulsewidths that are too small for the injectors in low load transition and encounter bucking because of inconsistent fueling. However, lots of things can cause it, sometimes DFCO, closed loop idle activating, timing table issues. If TPS/MAPdot is too slow for enrichment in this situation, then it shouldn't be pulling fuel on the throttle close either, leave decel fuel amount at 100 while tuning maps anyway. Driveline lash is an issue that worsens this in E30s, my car still gets into some light bucking and oscillation despite my extensive time optimizing the tune, I've minimized it but once in a while I still run into it. The lash was there in the much nicer track car I tuned as well, though not as bad as mine, it's probably there in yours too.
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