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Tuning fuel injector shutoff/overrun

Posted: Sun Jun 23, 2019 8:13 am
by 2x0
I've been battling issues with severe jerkiness when my injector shut off and turn back on under light throttle conditions, for example at my work campus I drive pretty far staying at low rpm in 1st and 2nd gear to stay under 15mph, and in doing so the injector will occasionally shut off make my car buck embarrassingly.

I don't understand what the MAP kpa values are for in the fuel overrun cutoff table vs RPM. I've tried lowering them but the lowest it will let them go is to 8? Why can't these be any lower? What do these values mean?

Also I've had the TPS setting anywhere from 0.3% up to 1.5% and it does the same thing. Have toyed with the delay from 10ms to 900ms and doesn't help much, just makes it more unpredictable with higher delay time.

What if I increase the fuel resume to say, 3000 or 4000rpm so my injectors don't shut off when at low RPM cruising at low speed? Will this be bad for my engine? I would still want them to shut off between higher rpm shifts, since there will be a lot more fuel going in then. But at low rpm and high vacuum there shouldn't be much fuel?

Any other recommendations would be appreciated. I have 1000cc injectors and e85.

Re: Tuning fuel injector shutoff/overrun

Posted: Mon Jun 24, 2019 9:21 am
by Spunkster
Post your calibration and a data log showing the issue.

Re: Tuning fuel injector shutoff/overrun

Posted: Mon Jun 24, 2019 9:26 am
by crluver123
1) fuel overrun is not what you should be messing with. change all those values back to default.
2) if the injectors do not shut off when at zero throttle, no matter what rpm you are at, fuel will pool up and cause a severe rich condition when you get back on throttle. Which will make the jerkiness way worse, and can damage the engine from extra fuel washing oil off the cyl walls and contaminating/diluting the oil
3) calibrate the TPS so zero throttle = 0%, don't experiment with other values. set it correctly and forget it.
4) check your distributor adjustment with a timing light while locking the timing to 15* through smanager. If the distributor is too advanced, it will cause these issues.
5) You may need to retard the timing a little in low load lower rpm areas. Too much timing will cause jerkiness and stutter
6) if fueling in these areas is an issue, it's the fuel TABLE you want to adjust, not the overrun parameters.

Re: Tuning fuel injector shutoff/overrun

Posted: Mon Jun 24, 2019 12:58 pm
by 2x0
Thanks for these suggestions.

I may have not worded what I said correctly when I said I adjusted the TPS setting. I meant in the fuel cutoff/overrun table it has a value to set below what % throttle the injectors turn off at. What should this be set at? I would just put it at 0% but I fear the throttle may not always actually read 0% when I am off of it.

I do not have problem with AFR tuning at low RPM or in high vacuum, they are fine. The problem is that if I am trying to maintain a speed in 1st or 2nd gear, at say between 2000-3000rpm. This requires very little, almost no throttle at all. So when I am moving along suddenly the parameters are met where it shuts off the injectors and tries to decel rapidly, causing the car to buck. Since it slows so much when the injectors turn off, almost inevitably my foot will be pushed into the gas pedal slightly so that the injectors come back on because it reads above the minimum TPS again, and so on and so forth. The bucking continues until I either gas it harder or take my foot off completely.

I just want to be able to maintain a constant speed in low gear without this happening.

Re: Tuning fuel injector shutoff/overrun

Posted: Mon Jun 24, 2019 12:59 pm
by 2x0
And I will record datalog tomorrow and post it up. Thanks again.

Re: Tuning fuel injector shutoff/overrun

Posted: Wed Jun 26, 2019 9:17 am
by 2x0
Well my tuning laptop is not able to get online at the moment, so I have no way to upload the tune quickly.

BUT, I think I have gotten the problem under control. I adjusted the fuel overrun cutoff TPS to 0%, and the delay at 250ms. I increased the load values to 9, 9, 11, 12, 18. It seems to have done the trick. I was worried that with the TPS cutoff set at 0%, sometimes off the throttle it might not go all the way to 0 and the injectors would stay on, but that has not been happening. It is much more predictable now.