Tuning with Fuel trims...Cobalt 2.0

[Sunburst_SS]

Well, I was going to tune the VE tables on my car. Until I actually get a wideband I was going to do it with the fuel trims. Do you guys use the STFT or the LTFT?? I've heard if I was going to use the short term trims, it would be best to disable the Long Term fuel trims?? How would I go about doing that?

[Doc.SS]

Things are gonna go BOOM. Wait for a wideband or take it to a shop with a dyno and a sniffer.

[Garrett]

was is the difference between long trim and short trim tables?

[Sunburst_SS]

short term fuel trims are pretty much "real time" fuel trims....what the engine is running at at that particular time.

Long term fuel trims are an average of past short term trims that the computer compiles over a period of time.

[Garrett]

so we have to ajust the short trim, and the PCM will ajust the long trim automatically when the engine will run right?

[Sunburst_SS]

well, if you are tuning with fuel trims......you reset the trims to zero before you go and log the car. So technically if you tune the short term trims to be within specs, then the long terms trims will also be in specs.

[Garrett]

how could I reset the long trims?

[Mobile Chassis Dyno]

You do it with the VCM controls under fuel and spark. I have done a several cobalts so free to ask for help.

[SJSchafer]

To elaborate on what DOC said, tuning with trims can result in really bad results. Here's why. You trims lock in at PE, so once you floor it, they are not changing, which means you cannot tune PE by using your trims. If you are tuning your MAF by trims, using the paste percentage method, you are going to screw your high end MAF all up. MAF is a 3rd order polynomial, which means that the table needs to increase exponentially. If you have have 3rd order polynomial equations down, then you may be okay, otherwise you are going to leak spike like crazy when you enter PE, and knock like an SOB.

In VE you will screw yourself as well, because even though it appears that you hit each cell independantly, they all rely upon each other. You're hitting one cell on your histogram, but the ECM is actually looking at the 4 or 5 that surround it.

Long story short (too late), get a WB. You can get an LC1 for like $250 and be safe(er).

[REDFOCZ]

how could I reset the long trims?

Here you go

[REDFOCZ]

I know it is not connected, I just wanted to show him were it was at.

[Sunburst_SS]

heh, I got my LM-1 in the mail friday....no more tuning by fuel trims here Now just to set up the wideband, woo hoo

[SJSchafer]

heh, I got my LM-1 in the mail friday....no more tuning by fuel trims here Now just to set up the wideband, woo hoo