I read up on nj1266's 'how to tune and evo' thread, specifically the part about tuning fuel trims, because my fuel trims are too far off the +-5% that he suggested. I don't even have aftermarket fuel injectors.

They started out as 92,86,100 (low, mid, high), so thinking that the switching of my closed/open exhaust were throwing it off, I resetted my battery/ecu and started a fresh tune to an open downpipe. Upon starting the car, it read 100,100,100, which was perfect, but as I was driving and logging runs for about an hour, it started moving away from 100's and became 108,92,100 (low,mid,high). I quit tuning at that point, cuz my AFR's kept changing after every log as well.

So even though I have stock injectors, do I have to change the injector scaling until the fuel trims get back to 100's, as it says in nj1266's how-to? Are there any other suggestions you guys can give me?

 

Those trims will chang considerably between different gas stations of same octane, even between batches at same place. Put some pump gas with from a station that sells it as a 10% ethanol blend and see how it changes. Lot of work if you ask me.

 

I did a search on the net and found the closest gas station that has an ethanol blend is at the Kennedy Space Center, which is like 50 miles away from me, and it's only for their use. What would this ethanol gas do?

nj1266 said a 5% change is reasonable. Mine's much worse than that, so what could be causing it? Are 711 gas stations any good?

 

I am heavily recommending the Shell V-Power stuff. I seem in all my logs to get better (Less) knock counts with using Shell's V-Power 93. For some reason it seems to be more knock resistant that the usual run-of-the-mill 93.

 

Agreed...

 

Are there any other reasons you guys can think of that my fuel trims may be swerving off 100's?

 

it's a non issue, you're nowhere near maxing out the fuel trims. If you get **** about fuel trims you'll drive yourself crazy trying to make it perfect. close is good enough. They'll always wander around, the key it to get it in the ballpark so it has lots of wiggle room on either side.

 

How are you basing your calculations that you are 'way off'? Is it from that retarded 100 scaling in EvoScan? If so, that isn't percentage.

The real formulas for fuel trims should be (.1961x)-25

This will give you 0 as being pefect and any negative number telling you the percent of fuel the ECU is taking out and any positive number as the percent of fuel the ECU is adding in.

I think you may just be getting confused by the original, incorrect EvoScan scaling. BTW, EvoScan also didn't have the correct request IDs for the fuel trims either, so depending on what yours are, they may be wrong too. Just search for one of the numerous posts where I give the correct requests and formula.

To add to what others are saying, make sure that you don't have any vacuum/boost leaks either. The most common one is putting a breather filter on your valve cover. This will make your fuel trims, especially idle, go very positive.


Eric

 

I agree Shell has terrific gas, unfortunately for me none is around where I live. ;(
i only have BP 92.

 

It could also be the filter throwing off the MAF readings ... I changed my stock filter to another paper filter from NAPA and went from -92 or so to 115 on the low trim (old EvoScan scaling).

Fuel trims are always *****-nilly ... you will never get them perfect.

 

K, that makes me feel better. I wonder if Evoscan 2.0 have wrong formulas? I'll check some things tonight.

 

the old one definitely had things backward, I haven't tried 2.0 yet. Evoscan does them right but uses a different scale.

 

Yeah, I wouldn't worry too much about fuel trims. My car completely stock was 100,108,125 on 91 octane crap in Cali.

 

Is this formula right?

I tried it in the following formats, and EvoScan kept giving me errors..

Eval="(.1961x)-25"
Eval="(.1961*x)-25"
Eval="((.1961*x)-25)"

Shouldn't the second one work?

Or should it be:

Eval=".1961*x-25" (This seems like it would give a false answer...

 

do you have any leaks (boost) in your setup ? id check that first. If you are on stock injectors and rail, your trims should be within tolerance... so if they are way off, aside from different gas being used, a leak will cause them to go haywire