Originally posted by 99SaleenS281
Was looking at something with the autotap tonight and I noticed that my long term fuel trims are both maxed at +25%. My short term trims are fine, hovering right around 0% like they should. I'm wondering why my long terms are so outta whack (maybe this is why my gas mileage sucks right now, like 10mpg).
I've been dyno tuned and all, so my a/f is good, but this problem concerns me. I wonder if it's related to my other problem (fuel pressure surges erradically unless my fan switch is on, see thread here).
Can't say for sure if it is related to your fuel pressure fluctuation problem BUT as far as the LTFTs being 25% Rich (by the way 5% more STFT and you'll be throwing the "Bank X Too Lean" codes), this is an easy one. The answer is in your signature...........C&L 80mm MAF.
Here's what I posted in another thread here on the Corral:
"Exactly BadBud! Just to recap, here's what happens:
1. OEM Meter is calibrated to take into account the ratio of the sampling tube cross-sectional area to main bore cross-sectional area (among other things).
2. C&L effectively alters the ratio between the sampling tube and bore area. The desired effect is that this new ratio (which isn't very accurate) makes the PCM think that it's getting less air so the PCM leans out the mixture at WOT. However, during adaptive operation, the PCM notices that the reported airflow always causes the mixture to be lean so it has to up the fuel trims (enrichen) in order to maintain stoichiometric (14.7:1 air-fuel ratio).
3. The real problem occurs after the PCM has fully adapted to the C&L meter after some miles of driving. When you finally go to test the car on a dyno you run RICH! How is this possible? Open-loop works like this more or less: the PCM looks at the mass-air meter to read air-flow, it then goes to a table and picks the correct injector pulse-width to use to yield some predetermined air-fuel ratio (also in a table). Now, it also has this thing called the Open Loop Fuel Modifier which is essentially a history of how far off the mass-air meter has been when running adaptive. Different model years do different things here BUT they are all based off of the Long-term fuel trims (LTFTs). In general, if the meter is always causing you to be lean by 8% when in closed-loop, your LTFTs will be close to +8% (RICH). Then when you go WOT and your PCM goes open-loop, 8% fuel will be added to the computed injector pulsewidth in order to get the right air-fuel ration. The BIG PROBLEM is that this +8% actually causes you to be RICH by 8% because the LTFTs have adapted to a modified and inaccurate signal!"
So, you should check your LTFTs and STFTs across the rpm/load range by logging them with an OBD-II scanner during different driving conditions. The LTFTs are not just two data values but an array of data with RPM/load as indexes.
You say you were dynotuned. What is the time relation between installation of the C&L meter and the dyno tune? IOW, was the C&L already on your car before you went to get tuned OR did you install the meter after the tune? Did you or the shop tune kill the battery power anytime right before tuning your car (clear the KAM)? If you answer yes to either of these questions, there is a chance that you are running really rich at WOT now. If not, then you are probably OK at WOT. However, when the LTFTs get pegged (25% is maximum) the system tends to behave funny when running in closed loop. Check for posts here on the Corral by "onerichrunner" where he talks about how he used the MAFterburner to correct his out of whack LTFT problems caused by his new cams. Once he started getting the fuel trims down, the PCM started doing the right thing. Until then, the PCM was doing all funky stuff with fuel and timing to get the car to idle (unsuccessfully).
Anyway, a short-term fix is to ditch the C&L and go with another meter like an 80mm or 90mm LMAF. However, this will require a retune. You could also purchase a MAFterburner (my product) and use it to correct your LTFT issue caused by the C&L meter and further optimize your WOT air-fuel curve if necessary. You can also do a search here on the Corral for "MAFterburner". There are a lot of people who have successfully used it here.
For more tech information, see: www.seanhylandmotorsport.com/mafterburner
Or, you can join our tech forum at: www.seanhylandmotorsport.com/forum
Finally, if you have any other questions, you can email me at: [email protected]
In summary, the LTFT problem you have is caused by the C&L meter. I'd bet lunch on it! It happened to me and I've seen it many other times!