Can you elaborate? Fuel trims are always trying to maintain zero, through +/- fuel.

 

Thank you for asking! Since gasket matching and installing the 181 my long term fuel trims have been above 7%, some times seeing up to 17% in really hot (high load w/AC) weather. My fuel pressure is correct, so I must assume that after modding the motor is thirsty and adding fuel.

Hopefully with these 15% larger injectors, my fuel trims will settle closer to zero, letting me concentrate on the open loop AFRs later.

The ECU is always trying to maintain 14.7 AFR by manipulating the fuel trims (until open loop).

 

Long term fuel trims are dictated by short term fuel trims. While in closed loop the 181 pulley should not play that much of a factor in trims, since it will not exceed the ECU's ablitiy to maintain 14.7 AFR or a differance of 0% trim.

I'm thinking the larger injectors will only come into play during open loop.

 

The ECU must add or subtract fuel (short term fuel trim/injector pulse width) to maintain an AFR of 14.7 at all times until you pass some threshold and go into open loop.

If your motor is modded and flowing more air, then the ECU will add fuel to keep a 14.7 AFR. After a while the ECU will apply these corrections to an average, long term fuel trim.

I'm sure you are right about the WOT AFRs being effected by the injectors but I think nailing down the LTFTs will make the WOT tuning much easier, with less change due to temp/environment.

 

Agreed.

Please see my STFT in the attached file. The 20% spikes coincide with max boost during open loop (problem) and/or when removing my foot from the accelerator, sending AFR into 20 in closed loop (normal).

Since the LTFT is an average of STFT, I'm not sure how tuning would be easier, considering LTFT is a product of STFT. One could always disconnect the neg battery cable to reset LTFT and then begin tuning.

 

You are correct, but the STFTs are an instant reaction to the voltages of the O2 sensor feedback. The LTFT are the averages of the STFTs.

Think of it this way, your car is tuned and happily driving at 14.7 when all of a sudden you suck a small baby into your intake. The ECU sees the O2 sensor voltage go way high and starts pulling fuel to keep 14.7 AFR. Not much air getting past the baby so your STFTs might be -15 or so. Now being that the STFTs can't adjust fast enough being choked down to -15 it applies a negative OFFSET (LTFT) and starts back at 0.

Ok, I'm tired and being silly, good night.

 

Exactly, BUT LTFTs are applied to your WOT "tune" target AFR.

When you reset your LTFTs by disconnecting your battery, your tune is untouched. Then your LTFTs start adding up again and effecting your WOT AFRs.

The closer your LTFTs are to zero the less they will mess with your WOT AFRs.

 

Yes, I understand the relationship between the O2 sensor, STFT and LTFT. The O2 sensor (narrowband AFR) fluctuates just as much as the STFTs trying to maintain 14.7 AFR. Not good for tunning.

lol..a small baby..lol. Okay, alright, I must compose myself...lol

There is a difference in the way our O2 sensor voltage and STFT calculate AFR. If you see a -15 (negative anything) STFT that is an indication of a rich condition becasue the ECU is taking fuel away to get 14.7 AFR. If you see a spike in the O2 sensor voltage, that is an indication of a rich condition and a valley indicates a lean condition. The LTFT offest will not be a negative, it will be positive. That is an indication of a lean condition or the STFT pulling fuel, not adding it.


Goodnight, I'm going back to celabrate the Saints masscare of the Colts! I'll check in tomorrow to see what else I can learn!

 

You got it 99% right, but the O2 sensor voltage is fluctuating as a result of the mixture crossing from rich to lean trying to keep 14.7. The switch point of the sensor. Notice on your graph how the trims are going up and down across the center (.45volts).

Anyhoo, my fuel trims are now much closer to zero, so these injectors are not a huge step up. I'll get the laptop on there tonight and log some afrs. Cheers! Tuna.

 

just to be clear, is that feet first or head first?

 

Without the ability to modify your tune (inj pulsewidths or duty cycle) in open loop, I'm afraid the corrections to the learned adaptives (LTFT) by the addition of larger injectors will cause any WOT lean condition to be worsened. As you are seeing, the PCM is pulling fuel during closed-loop puttering around to compensate for a larger injector. Once this correction becomes "commited to memory" it will continue to use that compensation in open-loop (WOT), reducing your fuel PW's. If you were rich before at WOT then it's a good thing.....

 

That's exactly why I threw them in.

Again, my LTFTs w/ the stock 072 injectors where a fat +7 to +17%.
This would fatten my WOT AFRs.
Clearing the trims would give me my requested AFRs.

Now my LTFTs are closer to zero and not changing my WOT AFRs.

Worrying about your WOT AFRs before you get your fuel trims sorted seems like chasing your tail to me!

 

Long Term Fuel trims have gone to +3% low load and +1.5% added to WOT. This is after about 75 miles at 56 degrees F outside. WOOT!!!

 

[quote=tunaglove]I installed a set of the 304 44lbs injectors in today. My fuel trims started migrating back towards zero right away after a short drive. Seat of the pants is a big yes, we'llqu see. Tuna[/ote]
Hey tuna what did the 304 set cost you ? The new replacement mercedes # for slk 32 amg injectors is A-1130780123, GOT A PRICE FROM A LOCAL DEALER $181+TAX EACH.From the research i've done , this same injector is used in the 2005-2008 CL and SL AMG 55's.ILLCYA LARRY

 

I scored a used set of 8 off MBWorld for DIRT CHEAP.
Keep watching the For Sale section. These came off a 2006 E55. MB went with bigger injectors in 2006 the earlier E55s use the same injectors as the SRT6.

Those guys are upgrading their injectors over there, keep an eye out.

SRT6 replacement, late 5.5L SC AMG, light-blue: Bosch 0280156304. MB 1130780123, 44-45 lb/hr @ 3 bar, 4-hole.