Fuel trims wideband

Still having some issues with my fuel trims I have managed to get my mid and high trims at just about 100% my low trim goes right back to pig rich 81.2%. problem is I can lean it out a bit by decreasing the fuel pressure(below 37psi my chip setup by keydiver chip)I can get all the trims above 100% but then I get excessive knock either in 3rd gear or WOT up to 14 counts sometimes more. Mods are MHI big16g,fic 850cc,walboro 255 rewired,evoIII manifold and O2 housing,buschur racing 3" turbo back exhaust,aem wideband,dsm logger,new O2 sensor,evo ix mas,aeromotive afpr,keydiver 91-94 ecu for current mods.Only exhaust leak is a small one after my test pipe clamp its also after my wideband O2 sensor. Did a basic boost leak test nothing conclusive I am still reading 18-20hg at idle so I dont think its a vacuum leak the car now pops or backfires under hard acceleration which would signify a rich condition well it goes away when I actually add fuel not when I take away fuel????? My wideband also doesnt change value at idle when I add or subtract fuel just stays at 14.9-15.0. I cant seem get all my trims at 100% or so not have knock or have the wideband read correctly.I am very frustrated the idle part drives me nuts my logger shows 81.2% at idle but my wideband reads 14.9-15.0(good) isnt this contradicting I also notice the wideband reads extremely rich when I lift off the throttle like in the 10's shoudlnt be the opposite. Well sorry for the long post but I am up to my neck in frustration just cant seem to get this thing correct please help thanks......

 

Fuel trims being one way or another doesn't mean you are rich or lean. It means the ECU is making a certain percentage correction to maintain a stoichiometric AFR during closed loop operation. This is why your wideband still reads around stioch even though you are adding/pulling fuel. So, if your low fuel trim is below 100% I do believe that means the ECU is having to make a negative correction to maintain a stoich mixture around idle speed and really low load cruising. If you're pulling fuel by reducing fuel pressure it should bring the low trim back inline.

BUT, at WOT your fuel trims don't mean a thing. You're at a far higher MAS signal than is used for the fuel trims. Also, at WOT you're in open loop, so there is no O2 sensor feedback to maintain the target AFR. Therefore, by decreasing your fuel pressure you're running leaner than the target AFRs encoded in the chip for open loop operation because fuel flow in the chip is modeled after stock base fuel pressure. Since there is no O2 feedback in open loop the ECU doesn't know it's running leaner than the "asked for" AFR.

In short, you need a way to adjust fuel other than global adjustments via fuel pressure. To bring your fuel trims inline and maintain proper open loop AFRs will require at least an SAFC.

 

So I guess I need a at least an safc then? think I may just go ahead and get a dsm link V3
then that way I can at least tune the damn thing. It makes sense the way you explained it
guess at least I should adjust the fp to the point where I am hitting my target afr at WOT.
It makes sense because even at WOT the best I am seeing is around 12:1.Should I at least give
it some more fuel to bring the target afr 11:1 closer for and just ignore the damn logger?

 

Start at 37 psi base fuel pressure and go from there. If it goes leaner than you like at WOT, then bump it up a bit until you're at an AFR that you aren't getting knock. DSMLink is probably the best way to go. You can sell your chip and get v3 lite and still be a bit above the cost of an SAFC.

So long as you're within the ECU's range of adjustment you will always be around stoich in closed loop mode even though your fuel trims won't be perfect. I ran a Walbro 255 with a stock FPR and my AFR in closed loop was always around 14.7 The ECU simply made the proper correction to maintain stoich. If you're seeing around 14.7 in closed loop and you don't have a way to adjust the fuel in the MAS hertz range that the fuel trims update, then just forget about them. Focus on WOT/high load/open loop where a bad tune can damage your engine, but at the same time keep an eye on your wideband while you're cruising just to make sure you haven't made adjustments that the ECU can't compensate for in closed loop.

Fuel trims being inline mean you have a well calibrated setup. Well calibrated means the air and fuel models in the ECU are very close to real life air and fuel flow going through the MAS and injectors. If real life flow and ECU models match up well then the ECU only has to make small corrections in closed loop to maintain stoich. Small corrections mean your fuel trims will be near ideal. In fact, if you have a well calibrated setup you can lock the car in open loop and always come within 0.1-0.2 of the target AFRs at all RPM/load points with no ECU correction.

 

Ok point well taken I will go make the necessary adjustments with what I have. I can get a good deal on the V3 I bought mine from 1glink and they have a buy back and trade in program that I will take advantage of. So moral of the story is I guess the wideband at WOT/open loop is a better judge for tuning with what I have until I can get my hands on a dsm linkV3 thanks for all the info man really appreciate it....

 

You'll like v3. The direct access tables are wonderful, though a little intimidating at first.