Low fuel trim help!!

[matt98eclipse]

Hey guys i installed a aem wide band and at idle it read right around 14.5 to 14.7 but on my datalogger my low fuel trim shows 81.2 percent for some reason what would cause this?? Please help.

 

[jrohner]

I see from your profile you are using a MAF Translator. You need to adjust that to get your fuel trims right. What Hz of airflow does your logger show at idle? If it's off, check for leaks, then adjust the MAFT so it shows less airflow at idle. For some reason it is probably showing higher than actual airflow. Is your MAF installed before or after the turbo? How are your fuel trims otherwise? Free revving different RPMs, Cruising?

The 1g's I've looked at have always had fuel trims that move around a lot, but the 2g's seem really accurate.

 

[matt98eclipse]

I just bought a safc2 so i will be installing that really soon and at idle im seeing about 37hz. The maft is installed after the turbo.

So i will be haveing the maft and safc2 installed but make sure i zero out the maft and just tunin with the safc2.

Im thinking base fuel pressure is set lower then its suppose to be would this have somthing to do with it?

 

[mattdev]

I too have the MAF Translator, and in my experience, I have to richen things wayy up to get a level fuel trim. I was getting a CEL for fuel trims while on stock injectors. The MAF was set to 450cc, so I didn't think much of it.

After getting 650cc injectors, I went to compensate for them with the Translator set at the correct value (650cc). My car would not idle correctly, and definately was not driving. After logging, my fuel trims read as lean as the computer could adjust. (In 2g +17%)

I currently have my Translator set to 480cc injectors, just to get a level fuel trim on 650cc injectors. Now I may have something wrong, but it goes to show that setting the MAF at the correct injector size does not do the trick.

As for your wideband reading stoich, check for exhaust leaks and be sure it is calibrated correctly, if you have not already done so... It could be that the computer is adding/subracting just enough fuel to correct the mixture, and the wideband reads the corrected mixture. You're computer sees it before it is compensated however...

Make sense??

 

[jrohner]

Im thinking base fuel pressure is set lower then its suppose to be would this have somthing to do with it?

That would cause your trims to be high, not low. The trim being low means the ECU is leaning the mixture out (too rich), high means the ECU is richening the mixture (too lean).

If the trim is in the 80's, your afr can still show 14.7 because the ecu can adjust that much. If it's rich (14:1, 13:1, etc), then you are off too far for the ecu to adjust it.

If all of your trims show low, just adjust the MAFT as though you have slightly smaller injectors that you really do. You could possibly have a 1g non-turbo FPR on your car, the higher pressure would cause the low fuel trims.

Did you get the car with the MAFT already installed or did you drive it with a stock MAS for a while (with it showing correct trims) and then switched it yourself?

 

[steve]

That would cause your trims to be high, not low. The trim being low means the ECU is leaning the mixture out (too rich), high means the ECU is richening the mixture (too lean).

If the trim is in the 80's, your afr can still show 14.7 because the ecu can adjust that much. If it's rich (14:1, 13:1, etc), then you are off too far for the ecu to adjust it.

If all of your trims show low, just adjust the MAFT as though you have slightly smaller injectors that you really do. You could possibly have a 1g non-turbo FPR on your car, the higher pressure would cause the low fuel trims.

Doing great up to the point where you suggest the OP tell the MAFT he has smaller injectors. I think he really wants to tell it he has bigger injectors so that it will pull additional air out and not run so rich.

 

[matt98eclipse]

So basically i should try and just lean it out on the maft by setting it to the bigger injector size and see of that helps but wont the wideband read even leaner?

 

[steve]

While in closed loop the ECU works to maintain the 14.7:1 ratio your seeing by including and updating the fuel trim. Right now the feedback from the stock narrowband sensor is causing the ECU to remove as much fuel as it can to reach 14.7:1 and that's why the trim is 81.2. By telling the MAFT you have bigger injectors it will lower the airflow reported to the ECU so that the ECU thinks it need less fuel to reach the stoch. The O2 sensor will tell the ECU that it's now leaner than it was, the ECU will subtract less fuel than it was and the fuel trim will start to move back towards 100% all while still targeting the same closed loop 14.7:1 AFR.

If you go too large the trims will go past 100% and the ECU will start adding fuel to what it calculated is needed from the airflow counts.

That bad part right now is that your getting more fuel than expected so once you go open loop the car will always be richer than the target maps since the trims aren't used to correct the calculations.

What are your mid and high trims?

 

[jrohner]

Doing great up to the point where you suggest the OP tell the MAFT he has smaller injectors. I think he really wants to tell it he has bigger injectors so that it will pull additional air out and not run so rich.

Yeah, that would be correct, sorry.

It's set in the ECU how much fuel to give, but the ECU removes (<100 trims) or adds (>100 trims) fuel to get to 14.7:1 AFR using the o2 sensor. Your low throttle AFR will stay 14.7 unless the ecu can't compensate enough (you are way off), or if it is in open loop for some reason -- bad o2 sensor, temp sensor, engine coolant too cold, etc. You want your trims to be close to 100 (0 on a 2g) because otherwise you can't get an accurate tune, and because you could at sometime need more correcting which the ecu couldn't do if you were already so far off.

You can do rough tuning right away, but the fine tuning must be done once the engine has reached normal operating temperature. It can also help to make tuning easier if you clear the ECU first (take the negative battery cable off).

You still have yet to tell us what your other fuel trims are. What are the trims at everywhere else?

 

[Black_Bullet]

I have a maft setup in blow thru.
I logged last night and despite the AFR looking normal on the WBO2, my low trims were 83%
my mid trims were 89% and my hi trim was 100%.
Most of my tune is taken care of with a eprom chip because I have 850cc injectors and I run the nt fpr with 48psi base fuel pressure, but I tried setting the translator to just 450cc and zero'ing everything out and it ran so rich that the ecu couldnt pull out enough fuel in the fuel trims.
I then set the base injector size to about 500cc (base7) since I calculate that the 48psi base fuel pressure will give 450cc injectors a flow rate of about 505cc's and I then only had to pull back my mid 1 click lean on the translator, and my hi 2 clicks back. This brought me to were im at now which is much closer than when I set the maft base on 450s. Im going to try going to base 8 settings next (510cc) and that may lean the curve just enough to get the trims were they need to be.... Basically the key is getting the fuel trims as close to 100% as possible, a tad less or more than 100 is still ok though, and you want to get your WOT afr to go richer and richer in a linear fashion on as you go up in rpm and come onto boost and then settle at were ever you believe your WOT afr should stop at.
( i.e. 10-8 - 11.5 )

What is your base fuel pressure, and make sure you dont have boost leaks as Ive found they affect it some also despite it being a blow thru setup.
These translators are far from exact and they seem to be randomly fickle from setup to setup.

Also if you have the AEM UEGO wideband you have to have it in the right mode or its not getting the right reading I think. Though Im not positive what mode it needs to be in. I have the UEGO and Ive been running it in p4 ( nerst cell mode ) which is suppose to be the closest, but Ive heard p3 was the one to use. I tried both but I dont remember what the differences were. Maybe selecting the mode is only important when you are actually hooking the white wire up to the ecu for the ecu to use???