High Negative LTFT

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ne0guri

ne0guri

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The defouler has nothing to do with it, the downstream O2 sensor doesn't control fuelling. It's just a measure of catalyst efficiency.
How are your trims since you installed the Downpipe?
 

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How are your trims since you installed the Downpipe?
No change from before the DP install. Granted I'm non-Si but that shouldn't matter. I find my fuel trims way more affected by temperature, humidity, and gasoline (Esso vs. Shell vs. Petro Canada).

Generally LTFT is around 0 to -5%. STFT fluctuates between -5 - +10%.
 

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No change from before the DP install. Granted I'm non-Si but that shouldn't matter. I find my fuel trims way more affected by temperature, humidity, and gasoline (Esso vs. Shell vs. Petro Canada).

Generally LTFT is around 0 to -5%. STFT fluctuates between -5 - +10%.
Which gas do you find runs best ? I havent had great results with Esso but it may make a difference based on location as well. I actually have the best results with CT gas by my place which is just name brand anyway..
 

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Which gas do you find runs best ? I havent had great results with Esso but it may make a difference based on location as well. I actually have the best results with CT gas by my place which is just name brand anyway..
I run Esso most regularly just because they also have a touchless car wash near my place. Esso gas gets me the trim numbers I quoted above. Shell V-Power 91 definitely runs the best, my fuel trims go significantly more negative running that. I assume because it's 0 ethanol, so the ECU has to inject less fuel to meet targets. Petro is slightly more positive than Esso in fuel trims.

So I suppose for power, I'd rank Shell > Esso > Petro
 


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ne0guri

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Update:

Okay so this morning on my way to work my LTFT started at -15% and eventually settled down to -7% when I got to work (which is normal).

However, I just got back home from work and my LTFT at one point actually went down (or up for you math people) and got to -5%! Since I’ve had this problem, my LTFT has NEVER gone down. But it ended up going back up and is at -7% - -8% now.

The only difference since writing my original post is LITERALLY the weather!! There is some storm coming into Vegas so it’s been only 90 degrees (20 degrees cooler than normal). I will keep testing it out but so far weather and coolness seems to be the biggest factor in my LTFT fluctuations. I still find it unbelievable that my LTFT would skyrocket this much simply because it’s hot. I’ll be heading to California this weekend, which should be cooler, so it will be a good test to see if my fuel trims stabilize while in cooler temperatures.
 

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Update:

Okay so this morning on my way to work my LTFT started at -15% and eventually settled down to -7% when I got to work (which is normal).

However, I just got back home from work and my LTFT at one point actually went down (or up for you math people) and got to -5%! Since I’ve had this problem, my LTFT has NEVER gone down. But it ended up going back up and is at -7% - -8% now.

The only difference since writing my original post is LITERALLY the weather!! There is some storm coming into Vegas so it’s been only 90 degrees (20 degrees cooler than normal). I will keep testing it out but so far weather and coolness seems to be the biggest factor in my LTFT fluctuations. I still find it unbelievable that my LTFT would skyrocket this much simply because it’s hot. I’ll be heading to California this weekend, which should be cooler, so it will be a good test to see if my fuel trims stabilize while in cooler temperatures.
I think it makes sense. Hot air is less dense, which means a given volume of air pulled into the motor would have less oxygen available the hotter it is.

Less O2 = running rich = pull back injection cycle = negative fuel trim.

That's how I rationalize it anyway...
 

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I think weather plays a huge factor with these 1.5 turbos. I generally don't datalog but in the spring I was seeing LTFT's in the 0 to +3 range. With this warm weather they've been more like -5 or -6. The only difference is the warmer weather. Same Sunoco 93 I always use.
 

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My LTFTs had been hovering no more than -3 before it started getting really hot and humid and now I think -7 has been the most extreme, with -5/-6 being about the norm. I'm certainly no tuning expert, but I've done enough reading on the matter back in my WRX days since I had to do some of my own to account for larger injectors and a fueling issue I had. One would think that there should be a fueling table that adjusts fueling based in IAT (in the Subaru world it was a MAF IAT compensation table), in which case the fuel trims shouldn't be affected by temperature too much. Obviously this assumes that the compensation table is accurately compensating for air temp. Since I don't think there's an easy way to access all of Honda's ECU tables, I guess we won't know for sure what's actually going on. I'm so used to the open-source Subaru tuning platform where you can literally see EVERYTHING, getting used to what's available in Ktuner is an adjustment.
 
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ne0guri

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Thanks for the replies everyone!

So another question... at what temperature is considered “heat soak”? Before today my IAT was above 130 degrees, so could that impact fuel trims as well? Is so, then I really need to consider an IC or at least the aftermarket vented front grille.
 


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Hey guys,

I’ve been having some problems with my fuel trims lately and I’m trying to figure out what the problem could be...

Background:

2017 Si with about 18,000 miles

- PRL Street MAF Cobra Intake
- 27Won Catted Downpipe
- 27Won Frontpipe
- 27Won Exhaust
- KTuner + TSP Stage 1

Problem:

So my fuel trims have been all over the place with STFT spiking at the highest I’ve seen -25%! This seems to happen when I’m in boost range and right as I hit the throttle. My LTFT has been swinging from -7% - -14% and lately holding steady between -7% and -10%.

The weird thing is that on my drive to work in the mornings, my LTFT will start dropping. Usually will get between -3% - -7% (depends on how high the LTFT was before). This is when it’s usually the coolest in terms of temperature.

On my drive back home after work, my LTFT will shoot back up and usually gets as high as -16%. Only difference is that it is extremely hot here right now (110-115 degrees) and it’s the hottest when I’m on my way home. Plus there is more of an incline going home so I’m not cruising as I do in the mornings.

These problems all seemed to have started after I installed the Downpipe. On top of that, since installing the Downpipe the weather has gotten extremely hot and even a bit humid.

I’ve cleaned my intake filter and re oiled it without any problems. This seemed to help LTFT for a couple of days before it went back to the high negative numbers. I also took out the MAF sensor and checked to see if it was dirty or anything (though I’m starting to understand that MAF doesn’t directly correlate with fuel trims).

The car itself doesn’t run poorly and I don’t have any MAL lights. I do get backfire every now and then when I’m on 4th gear and let off the throttle. After reading through 27Wons article on fuel trims, I would believe I have injector problems, EVAP problems, or my sensors are not working properly.

I do have data logs that I could provide if anyone was interested in helping me out. It would be greatly appreciated! It’s driving me crazy to see my LTFTs constantly in negative teen percentages. As long as I don’t have any lights, I should be okay to drive right? Thanks in advance to anyone who helps!
Before you do anything, check the condition of your filter.
 

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I have a cobra race Cai tsp stg 1 and my LFTS usually stays at -7 most It hits is -10 im pretty sure your okay man I would be worried if there some super high numbers it's all in the temp and fuel when I use she'll it stays at that I tried arco and it increased as well as my KC it hit .83 on map 3 switch back to shell
 
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ne0guri

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And try cleaning your MAF sensor.
Okay so I sort of did this but don’t think I did it correctly. I’ll watch YouTube videos but am I supposed to clean the actual metal wires? From what I can tell, it’s pretty clean.
 

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It's been awhile since I've cleaned a MAF, but only use MAF cleaner and I don't think you should spray directly onto the metal wires. Might damage them depending on the spray pressure, but hopefully someone will correct me if I'm wrong.
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