I've got a problem with bucking on deceleration whenever my FD gets below 2500 rpm regardless of gear. It only happens on trailing, or closed throttle; light throttle immediately smooths it out. The bucking is accompanied by backfiring. My car has a healthy stock motor with catback, efini y-pipe, and downpipe. I also have a Power FC with a stock map. I've down some searching, and my first thought was TPS out of adjustment but all the readings are in range. Also, my idle is great (750 rpm, smooth) which is usually not the case when the TPS is out of adjustment. Could it be the ISC? I know the FC stock map is very rich, and I get a lot of carbon buildup on the exhaust. Is excessive fuel in the that part of the map the cause?
Thanks in advance.

 

how old are your plugs and wires?

 

both new, 9s leading and trailing

 

Im interested in this also, my car does exactly the same thing. Always has.

I have a theory regarding this..and that is that it happens when the car transitions from complete fuel cut to single rotor fuel cut.

(the stock ECU initiates fuel cut when coasting in gear, and cuts fuel to only one rotor at partial throttle or lower rpm when not on the gas)

My observation, is that it happens when the RPM's drop below a level where it will build boost. The tone also changes completely when it drops below that level.
I think its going from complete fuel cut - to one rotor only fuel cut.

 

Interesting theory, what rpm does the bucking/backfire begin for you? It's exactly 2500rpm on mine. Does that correspond with single rotor fuel cut? Is that point adjustable on a Power FC?

 

So called back firing (which it is not) is caused by too much fuel that is not burning in the engine but in the exhaust.
Since you have a PFC, is the F/C (fuel cut back on point) set lower like it should be?

 

Thanks, can I check this setting from the commander? I don't have a datalogit

 

yep, on commander go to settings
rev/idle
look for F/C AE, F/C E/L, F/C A/C. I have mine all set to 1500.

 

It happens at just over 2k rpm for me. It dosen't really backfire though, it just jerks and tells me "oi, downshift!!"

In all honestly im not sure the exact time that the single rotor fuel cut happens, or under exactly what conditions.
I have just read that it does happen, so it's just a little theory of mine that sort of makes sense

 

thanks! I will check my settings, and (if different) I will try yours and report results.

 

like stated above this is caused by the PFC Idle fuel cutback options where it starts to add fuel on decel

if you release the clutch it will smoothen out and the RPMS WILL DROP , this function is there in order to keep the car from stalling when you press the clutch and rpms drop it slows that drop in rpms down a bit.

higher it is less chance of stalling , But you get that buck , adjust it until the car almost shuts off .

people with light weight flywheels may need to put this option higher then those with OEM flywheels as there is less momentum and revs drop much quicker .

change your driving style a bit to make sure you don't go into the buckzone . down shift down shift

 

Thanks to everyone who responded. cewrx7r1 was exactly right...F/C AE was set to 2500 which corresponds exactly to when the car would begin to buck. Changed the setting to 1500 per ksu-chewie and problem solved. Car is perfectly smooth under trailing throttle down to 1500, and below that the syptoms are much milder. Makes the car so much more enjoyable in traffic/city driving.

 

Not really related but wasn't there something about having to turn off Decel fuel cut off when you premix? So that it never cuts off the fuel?

 

you would buck every time you let off the throttle not very pleasant

 

I thought the bucking was related to the fuel kicking on and off?

 

I know on my s2000, it would buck badly when I stayed off throttle near 2000 rpm because that's where I had my DFCO set at on my AEM. So it would cut fuel then put fuel back in repeatedly.

 

Dang my car has been doing this for years... Didn't bother me much but I guess I'll give it a shot.

 

What's the solution for those using the stock ECU? My FD just started doing this a few weeks ago.

 

You may want to adjust he TPS even if it does appear to be in range.
Adjusting the TPS fixed that problem in my FD even though the adjustment was in spec.
Mine was near the edge of spec, and setting to midrange fixed the problem.

 

Yeah, I spent the best part of a summer evening readjusting the TPS to end up with 0 change. I've also tested fuel pressure and it's spot on.

My air fuel gauge indicates that as the car starts to descend into it's bucking fit the car runs leaner and leaner till it goes full lean at the bucking.