[Yoitsasubaru!]

Alright so I’ve been having issues with FKL for a while now. Car is a 19 sti with basic stage 2 mods and a protune. I never see any form of knock under WOT but it’s had this issue since I bought it where under low loads (12ish% throttle) between 2500-3000 rpm’s I’ll get fine knock learn numbers. Most days it’s only around -2 but last night on my way home cruising in 6th gear light throttle it hit -10.85 fine knock. Very concerning but I didn’t hear any audible pinging and nothing crazy seemed to be going on with the car…. Any thoughts?? I’ve been trying to diagnose this basically since I got the car any help is appreciated!

 

[yamahaSHO]

Do you have high fuel trims in that region and do you feel the "stumble"?

 

[praha-sti]

Rich or bad sensor? Does it always do that in the same area? Old gas? In boost or decel?

 

[Yoitsasubaru!]

Rich or bad sensor? Does it always do that in the same area? Old gas? In boost or decel?

Fuel trims seem okay in that area and it usually happens before boost or coming up to boost. Car has 45k on it I was thinking possible maf or 02 sensor but I hate to just buy and throw parts at it till it’s fixed…. I do know that my bpv is leaking and I’ve been waiting on the one I ordered for almost a month but haven’t really gotten into boost since finding that leak.

 

[AustinSedz]

Your tuner needs to address the issue.
I have the Cobb 'stumble fix' which basically moves the 2500rpm stumble area up to 3500rpm and makes it less.
I wasn't getting as bad as you are getting, but I was getting enough Cyl3 roughness at the stumble zone at 18-25% throttle that every tank or so of gas I would get a CEL because I was exceeding the roughness threshold.

Full throttle pulls were no problem, smooth and consistent, but light throttle around the 'stumble zone' saw fuelling issues that would result in problems like roughness and/or FKL.

Apparently the STi fuel system is sorta meh in stock form and the 'stumble' is actually a fuel-pressure choke zone.
The car has no means to monitor it (we don't have fuel pressure sensors at the rail, stock) so the car runs a little rough here.
Not so noticeable in stock form, but magnified when you crank everything up.
I've heard of people installing the Cobb Flex kit (which adds a fuel pressure sensor) and that being used to monitor the pressure dip at the stumble, factoring that value into the tune, and tuning out the roughness and FKL using the fuel pressure readings, but now with greenspeed it's no longer possible to plumb that system in on the Accessport.
The ultimate solution as proposed by my tuner was dual aftermarket fuel rails and lines to remedy the known issue 100% (and switching to ECUtek - all of this was $$$$).

My guess is that because you don't have the stumble fix kit, your issue is simply moved down the rpm range into the 2500 area.

My tuner was able to tune out the roughness and FKL at my 3500/20%ish area, and it involved modifying the 'cruising maps' (I think I recall him calling it).
I am not a tuner, so I could be wrong, but usually the cruising maps are a product of the full-throttle maps.
They can be modified separately, but im pretty sure the cruise map is only altered in these types of cases where the WOT map is good but you have issues at low or partial throttle.

I had my car tuned in late 2021 on the dyno and my tuner offered to road-tune the cruising maps to clean up the roughness issues as my 'complimentary revision', even though it was almost a year later.

Deff approach your tuner with this issue, as he should be able to solve it.
Anything above -2.8 is indicative of a problem, and I imagine a regular -10 is probably causing the DAM to drop too, which while not damaging will more drastically effect the overall smoothness and power.

 

[AustinSedz]

Also forgot to mention, my 'roughness issue' would usually always happen around -2 to 2psi of boost, so yea, light throttle.

 

[Yoitsasubaru!]

Also forgot to mention, my 'roughness issue' would usually always happen around -2 to 2psi of boost, so yea, light throttle.

Austin thank you for such a detailed response!

This definitely sounds like it is very similar to my issue and I’m currently in the process of prepping the car for a retune and going with a different tuner so I will definitely mention this to my new tuner during the process.

On the bright side this has helped me track down other various small problems (vacuum leaks and such 😅) YamahaSHO mentioned this right off the bat as well so hopefully this is something that can be tuned out. While having heard of the stumble for a a long time I never know it could cause this much disturbance in the ignition timing.
BTW -10 fkl isn’t an average case this was an extreme case normally the worst I’ll see is in the -2 —4 region with occasional -5.6 or so and yeah every once in a while a dam drop.

I’ve seen tons of unfinished years old threads on this so I’ll make sure to update post tune it might be a few weeks or so.
Thank you guys for all the responses!

 

[praha-sti]

If it's in the transition from vacuum to boost, it's usually the stock fuel pressure regular that simply can't keep up ... but it shouldn't happen on stock cars, but as you mod -- the stock parts perform poorly.

 

[Yoitsasubaru!]

I’m wondering if anyone has any other ideas on this? I’m on a new tune now and the same thing is happening. Fkl hit -5.6 today while cruising at 65ish and jumped to -7 after starting to learn away. I’m on a new tune now same results

 

[JohnSub2013sti]

What's your map and baro reading at idle. / Can you pull the AFR learning table (closed loop)?



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[Yoitsasubaru!]

What's your map and baro reading at idle. / Can you pull the AFR learning table (closed loop)?



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AF learning is around -5 at idle since the new map. Do you think this could just be a o2 sensor or something? Is baro something I can monitor on AP? I personally haven’t checked that and don’t know much about it something to do with altitudes?

 

[Yoitsasubaru!]

Also I’ll check map reading when I get out of work tonight and update

 

[JohnSub2013sti]

Barometric pressure should be available in access port. It's atmospheric pressure

Your baro should read 14.7 psi (absolute pressure). The only reason I was wondering what the value is was so it can be crossed off as a potential broken component. If it was broken, it would read 7psi or something, which is the sensor lower limit. It messes with the AFR if broken


Same thing with the map. At idle, I would expect maybe 6-10 psi absolute pressure. && when the vehicle is off it should read 14.7psi absolute. So if you check that, we could cross it off as well.

For the learning values, I was trying to determine if you were running lean. Like in the case of incorrectly scaled maf. But a negative value would indicate that your running rich and fuel is being pulled in closed loop. (Using o2 sensor readings)

A bad o2 sensor could definitely be the culprit. This is 100% a closed loop fueling problem. I'm just not sure how to test the o2 out tbh. If you had a voltmeter, you could measure the resistance of it just by unplugging it. I belive theres a ohm range for a good sensor. Have you checked its AFR readings through access port ?

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[Yoitsasubaru!]

Barometric pressure should be available in access port. It's atmospheric pressure

Your baro should read 14.7 psi (absolute pressure). The only reason I was wondering what the value is was so it can be crossed off as a potential broken component. If it was broken, it would read 7psi or something, which is the sensor lower limit. It messes with the AFR if broken


Same thing with the map. At idle, I would expect maybe 6-10 psi absolute pressure. && when the vehicle is off it should read 14.7psi absolute. So if you check that, we could cross it off as well.

For the learning values, I was trying to determine if you were running lean. Like in the case of incorrectly scaled maf. But a negative value would indicate that your running rich and fuel is being pulled in closed loop. (Using o2 sensor readings)

A bad o2 sensor could definitely be the culprit. This is 100% a closed loop fueling problem. I'm just not sure how to test the o2 out tbh. If you had a voltmeter, you could measure the resistance of it just by unplugging it. I belive theres a ohm range for a good sensor. Have you checked its AFR readings through access port ?

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Awesome info man much appreciated! My baro pressure is right where you say it should be.

manifold absolute is slightly below 5 at idle? Is this still normal range or could that indicate a leak somewhere?

I have been absolutely beating my head off the wall with this issue since I got the car lol I might just go ahead and buy a new maf and front o2 if the car passes smoke test next time I have a day off because I’m ready to just start throwing parts at it at this point…

 

[Yoitsasubaru!]

Awesome info man much appreciated! My baro pressure is right where you say it should be.

manifold absolute is slightly below 5 at idle? Is this still normal range or could that indicate a leak somewhere?

I have been absolutely beating my head off the wall with this issue since I got the car lol I might just go ahead and buy a new maf and front o2 if the car passes smoke test next time I have a day off because I’m ready to just start throwing parts at it at this point…

Edit: yeah I monitor afr on the accessport every drive. Tbh nothing crazy on there sometimes some fluctuations between shifts which I assume is normal but idle is steady around 14.7 for the most part. WOT (which I haven’t done much of at all in the last month+ with these issues coming back everytime
I think I have them figured out) is always 11.14 and no knock events once the car has built some boost…

 

[JohnSub2013sti]

I would say that's fine for a map reading it means you have good vacuum. You definitely don't have a leak (since you have 5psi). If your reading is 14.7 psi, when the key is turned on but the car is off, then the map is good, too.

11.4 AFR is what to always expect at wot. It's the sensor limit. The sensor isn't accurate in open loop fuel (aka wot). It should read around 14.7 the rest of the time

(Below would be my train of thought)

• baro (good)
• map (good)
• vacumm leaks (good)
• maf (idk how to check) (volt meter prob)
• o2 (idk how to check) (volt meter prob)
• knock sensor (idk how to check)
• try octane boost, maybe

These are just parts, though. I'm not confident enough to talk about what could cause this in a tune. I'm actually surprised your tuner didn't diagnose this

And np from the info, hopefully it helps, and you figure it out

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[Yoitsasubaru!]

I would say that's fine for a map reading it means you have good vacuum. You definitely don't have a leak (since you have 5psi). If your reading is 14.7 psi, when the key is turned on but the car is off, then the map is good, too.

11.4 AFR is what to always expect at wot. It's the sensor limit. The sensor isn't accurate in open loop fuel (aka wot). It should read around 14.7 the rest of the time

(Below would be my train of thought)

• baro (good)
• map (good)
• vacumm leaks (good)
• maf (idk how to check) (volt meter prob)
• o2 (idk how to check) (volt meter prob)
• knock sensor (idk how to check)
• try octane boost, maybe

These are just parts, though. I'm not confident enough to talk about what could cause this in a tune. I'm actually surprised your tuner didn't diagnose this

And np from the info, hopefully it helps, and you figure it out

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Unfortunately right now the car is my daily and whenever I’ve called the tuner with an issue they want me to drop the car off for an extended period where I need a loaner….

The tune that I am on now is a basemape e-tune where I asked the tuner to do what he could in a basemap to combat a fuel stumble issue.

I was thinking about the way you worded your response on my way home last night though “a closed loop fueling issue” and i decided to go the route of getting a new front o2 and maf sensor because you’re 100% right about this. The car is only having these issues when deciding it’s fueling based off these sensors!

Hopefully this leads somewhere I will update again when I get these parts in. I’m waiting to finish my e-tune until I can get some results with this issue…

 

[JohnSub2013sti]

Sounds good man keep us updated. If you can return stuff. Check if just changing the o2 sensor fixes the problem first. Before changing the maf. Did you get the new bypass valve on, btw ?

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[Yoitsasubaru!]

Sounds good man keep us updated. If you can return stuff. Check if just changing the o2 sensor fixes the problem first. Before changing the maf. Did you get the new bypass valve on, btw ?

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I did get the new bpv on. Had to use a plastic airline fitting to extend the bit of vacuum line that goes from the manifold to the bpv because the supplied piece of line wasn’t quite long enough…

My parts came in today so Wednesday I’ll try changing the o2 before the maf and see what results I come up with.

My AFL has been creeping lower into the negatives at cruising speeds and is now -17.something when I’m at light throttle so I’m hoping that’s indicating the o2 is failing….

Thanks again for all the helpful replies fingers crossed for some results! I really need something to go right with this car lol

 

[Yoitsasubaru!]

UPDATE: I ended up changing out the maf and front o2 yesterday with no results. Took the car for a drive after install and started getting similar FKL reading almost right away even after resetting the ecu. Also long term fuel trims worked their way back into the negatives by the end of a 15/20 minute drive was down to -5.47 I imagine it’ll just keep creeping lower as I continue to drive.