Hello. I have a 2008 with 165k+ miles. I have RRracing tune, intake with CF elbow that incorporates a breather filter, and sikky headers. Also been trying the E30 blend.

My LTFT is about 23% @ idle. But drops to the teens when moving. I've been reading that this is supposed to be single digits and indicator of a vacuum leak possibly.

Is this normal with the tune, since i thought i should be getting a CEL with that high of an LTFT and I'm guessing the tune is masking it from throwing the CEL?

TIA

 

You have to remember as the engine ages and the cylinders don’t seal evenly anymore, the short-term fuel trim may trend high or low most of the time. The PCM has the ability to learn this trend and store it in memory, and it will use that number in the fuel trim calculation to compensate for the changes causing the trend. That’s long-term fuel trim (LTFT). Since fuel trim is an ongoing calculation, keeping one factor in the calculation constant makes it possible to return STFT to its normal range, enabling a faster and more accurate reaction to bigger changes in operating conditions, such as acceleration. In general, it takes anywhere from five to 20 seconds for LTFT to change, and since it’s stored in memory when the engine is switched off, that same LTFT value is used next time the system goes into closed-loop operation.

 

LTFT is also used in open loop as a correction factor. With ~10 quarts of oil, this can be 25 - 30 minutes from start up before you hit closed loop proper. There's a pretty decent amount of information about this in TIS, and it started in the 80's with TCCS.

 

The tune does not prevent a cel from being thrown. You will get a cel if your LTFT is over 39%.
if you’re worried about it, you should datalog your a/f ratio to make sure it isn’t running lean at WOT.

my idle fuel trims are quite off too (30%!) and come back to the teens while driving. I’m searching for a potential vacuum leak.

 

The whole purpose of blending e85 is it's higher resistance to knock and since stoich for e85 is somewhere around 9.8:1 AFR ( 91/93 stoich will be 14.7:1)this will result in slightly leaning you out. The slightly lean condition creates more power.
So yes, your fuel trims will be abnormal compared to straight 91/93

Blending e30-e40 I've observed 13.5 : 1 AFR under WOT

Stoich blending would then be around ~12:1 AFR
See below for more calculations!
http://www.ecmtuning.com/images/foru...5%20Blends.xls