Aeroforce/ KR Question

[outonthetiles]

I just got an Aeroforce Interceptor Scan Gauge for my 2000 Impala LS. The car runs great, the only problem I ever have is an occasional stumble at idle after the car has just been started, but it has never really bothered me and I don't think that is related to my question.

My Aeroforce is showing up to 10* of KR on WOT, and also a few degrees in partial throttle. My 02 LS only shows up to 2* KR on WOT and none at partial throttle so I don't think this is normal.

My mods are a JMB Intake, a 180* t-stat, and a ZZP S1 Voltage Booster. I know I shouldn't be seeing KR with this setup.

I don't know where to begin in diagnosing this issue. It doesn't seem like false KR because it is steady, not intermittant. I'm thinking maybe a vac leak somewhere?

I'm also wondering what my short term/ long term fuel trims should be looking at, as well as MAP kpa and any other parameters I should be looking at for a stock l36.

Thanks for any help!

[Fivefingerdeathpunch]

You have KR because you are stock.

You have stock manifolds, stock downpipe and are running 87 most likely.

All stock cars knock.

If you added a plog and downpipe and ran 93 octance fuel, I bet all of the KR would go away.

[outonthetiles]

I have some SD headers to go on it soon.

Why is this car seeing so much KR when my identical 02 sees very very little?

[Fivefingerdeathpunch]

All cars are different.

None will act the same.

Factory freak?

Who knows, but I'm sure those headers will help alot and same will a high octane fuel and tune.

[outonthetiles]

Cold a small vac leak cause this? Is there anything else I can look at that may be diagnostic such as LTFT's or MAP kpa? I just want to make sure I'm not running lean or anything, and I don't have a wideband yet.

[Fivefingerdeathpunch]

STFT should be right around 0.

LTFT should be around 3-4 of 0.

So if its pegged at 16.4, you have a vac leak.

Map in KPA at idle should be around -19 to -20 idle. I see -20.3 on my supercharged car.

[outonthetiles]

I'll check the LTFT/STFT's after work, but I think I remember them being small numbers. MAP Kpa was around -19 at an idle. I'll try a tank of 93 next time I need gas. It just seems strange that this car with only 78k miles would be seeing so much more knock than my 02 at 265k. Better compression in the 00?

[outonthetiles]

Went for a cruise after work. At an idle, LTFT's seemed to be around 5 while STFT's bounced around zero. Drove about 10 miles and while driving, everything seemed to average zero but I did see LTFT's as high as 8 and as low as -7. STFT's seemed to average around zero as well but they did spike at 14 and -16. In the 02, however the STFT's averaged around zero but LTFT's stayed put around 13.2, so I have an issue there.

Edit: The 02's LTFT's were at -13, not +13 whenever I'd press the gas. At an idle it's -6ish.

[Z34Phoenix]

what are the LTFT's at when WOT? and what MAP kpa do you see at WOT?

[outonthetiles]

At around 9:30 AM, on the drive to my garage I was seeing 0.7 LTFT's and 99 MAP kpa at WOT. On my drive home at 1PM, LTFT's were 3.9 and MAP kpa was still around 98-99 at WOT.

[Z34Phoenix]

sounds like trims are off a bit... dont really remember what the ltft's locked in before i ever tuned the car... but if it is adding almost 4% then it knows things are lean and its trying to add fuel. as for the MAP that sounds fine. one trick i have learned to see if your intake system is a restriction is to have the engine off and key on. then read the MAP. down here in florida we see anywere from 100-104 at times. when at WOT you should see the same as when the key is off if your intake has no restriction. if it is less then something can be swapped for more flow and gain some power in the process.

[outonthetiles]

I already have a JMB fenderwell intake with a K&N cone, and yeah my MAP kpa is about the same at WOT as engine off and key on. I might try replacing the upstream O2 sensor as it is original to the car.

I must be way off with my understanding of how fuel trims and open/closed loop fueling work. My understanding is that fuel trims are only used when in closed loop, using the upstream O2 sensor to calculate how far off the fueling is, and correcting it with fuel trims. I also thought that in WOT, it is in open loop. If so, i thought fuel trims wouldn't be used. Can someone enlighten me on how this works? I'm probably way off.

[Z34Phoenix]

you are correct on your understanding of when the O2 is used... however when at WOT (or any time in PE power enrichment) the last value seen for the LTFT is locked in and used for the entire WOT/PE fueling. so if you are adding 5 or 10% more fuel before you go WOT, then during WOT the pcm will keep adding that same 5 or 10%. This is why it is so critical during tuning to get your LTFT's to lock in at Zero just before and while in WOT/PE as the computer is not adjusting the trims anymore.

[matt5112]

I'm not convinced the PCM uses the last value.

It seems the PCM has a separate fuel trim cell for PE.

[Z34Phoenix]

where would that value be stored... actually more importantly where can we scan or check to find it then? I am starting to see some tuning differences between my 96 DOHC's PCM logic and my 98 L36's logic. the DOHC's reacted to changes in the VE and the L36 one doesnt. I have noticed there is a setting in tiny tuner that i can toggle to use the VE for air fuel calculations... i might just activate this one of these days so i have more control over the values. all this aside, it is still important to have your trims leveled out before attacking the rest of the fueling. it deff helps to see the results of tuning if the vehicle does not have a wide band installed. I deff miss my wide band. i just need to get a new sensor and weld in a bung... but no real need until i get the topswap ready.

[matt5112]

Log Fuel trim cell.

For MAF/VE corrections, disable fuel trims by unplugging front O2 sensor then compare commanded to actual AFR and correct MAF by that factor.

Fuel trims wont play into it.

I also noticed that option for VE, curious if it works.

[outonthetiles]

I cleaned my MAF sensor, now my LTFT's are 0.0 at WOT. Still getting 7ish* of KR though, and LTFT's at partial throttle are around 4.