Jerky engine braking on '13 FB
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Joined: Aug 2012
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From: Detroit
If you're talking about a single jerk that happens on heavy decel (as opposed to vibration), it may be the fuel injectors turning back on once the rpms and intake manifold vacuum have come down far enough. I notice it on my bikes too, and the exhaust sounds a little different once they come back on.
Most emissions ECMs shut them off completely at higher rpms combined with heavy decel. Injecting fuel under those conditions doesn't do any good (except that there is a small cooling benefit), and there isn't enough compression for it to burn reliably anyway, so leaving them on just increases emissions.
An added benefit is that it's impossible to get "decel pop" when the injectors aren't supplying any fuel.
Most emissions ECMs shut them off completely at higher rpms combined with heavy decel. Injecting fuel under those conditions doesn't do any good (except that there is a small cooling benefit), and there isn't enough compression for it to burn reliably anyway, so leaving them on just increases emissions.
An added benefit is that it's impossible to get "decel pop" when the injectors aren't supplying any fuel.
Last edited by Warp Factor; Oct 4, 2013 at 07:15 AM.
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Road Master
Joined: Dec 2012
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From: CO
If you're talking about a single jerk that happens on heavy decel (as opposed to vibration), it may be the fuel injectors turning back on once the rpms and intake manifold vacuum have come down far enough. I notice it on my bikes too, and the exhaust sounds a little different once they come back on.
Most emissions ECMs shut them off completely at higher rpms combined with heavy decel. Injecting fuel under those conditions doesn't do any good (except that there is a small cooling benefit), and there isn't enough compression for it to burn reliably anyway, so leaving them on just increases emissions.
An added benefit is that it's impossible to get "decel pop" when the injectors aren't supplying any fuel.
Most emissions ECMs shut them off completely at higher rpms combined with heavy decel. Injecting fuel under those conditions doesn't do any good (except that there is a small cooling benefit), and there isn't enough compression for it to burn reliably anyway, so leaving them on just increases emissions.
An added benefit is that it's impossible to get "decel pop" when the injectors aren't supplying any fuel.
But you could be partially correct that somewhere around 1700 the injector pulse in changing. I'll have to recheck my AFR table.
However, it does feel more like some sort of slack issue in the primary or with the belt. It's like it lurches, just one time, right before I would normally pull in the clutch. I've always been one to engine brake as long as possible.
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From: Detroit
Well I know for a fact that my injectors are always on during decel because I have adjusted my tune with the PV, and I've actually added a lot of fuel to the entire decel range to get rid of popping. But I also know that the stock tune on these engines also keeps the injectors on during decel, because I saw it in PV and stock engines will pop on decel as well. To my knowledge, there is no point in the AFR tables where the injectors are ever "off." It's always trying to maintain the set AFR for the given rpm and map, even on decel.
That can be supplemented with a wideband oxygen sensor stuck up the exhaust pipe, to get an even better sense of what's going on.
Take a mixture from the intake manifold, at 1/20th normal air pressure on heavy decel, and compress it 10 times on the compression stroke, and you aren't even up to ambient air pressure. In other words, if you were to put a compression tester in the cylinder under those conditions, it wouldn't even be close to budging off zero. The closer to you get to idle speed, the less the intake manifold vacuum drops, and the more compression you have in the cylinder.
The injectors need to turn back on at some point, and that point is a judgement call, taking into account many parameters, including emissions. Except that they need to turn back on above idle speed, or the engine won't idle.
If you turn them back on too close to idle speed, you get a bigger jerk when they come back on.
That's way over-simplified, but I think it puts the general ideas acrossl.
Last edited by Warp Factor; Oct 4, 2013 at 11:27 PM.
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