Understanding relationship between MAF Sensor and Computer.....

[Epitome]

I hope someone can explain this for me. Here are some of my observations and thoughts and want to know if anything needs to be corrected in my mind. I started having a problem with my stock T starting to knock at about 65 mph and continue to get worse if I were to stay in it (I had always been able to run up to 18lbs or so with my 18 degree JC street chip with 0 knock). I noticed that my formerly perfect MAF sensor was now only maxxing out at about 225-227 when it had always gotten to 254-255 with no problems before. I got an MAF translator and a 3" sensor and now it is running perfect again. So......my thoughts are this: the computer was reading the maf sensor lower than the actual amount of air flowing in and therefore not enriching the mixture as much as it actually needed, thus creating the lean condition and detonation. However, I am not sure the computer actually does this type of adjusting and I would have thought that the O2 sensor would have told the computer to raise enrichment. Perhaps the differential between the MAF reading and reality was too great to overcome? If anyone that can explain this all to me understands my question, I'd appreciate a response

 

[MistaScott]

You are correct. The o2 only reports the exhaust o2 content to the ECM which then adjusts fueling. If your MAF calibration is off, it fails to report the correct amount of air entering the engine. Say it reports 220 at WOT instead of 255 (which is quite a ways off). The o2 sees that the exhaust is lean and reports back to the ECM. The ECM adds fuel. If there is so much air getting in that the ECM cannot compensate, you end up lean and usually with a 150 BLM.

 

[BoostKillsStres]

And keep in mind that at WOT the o2 sensor isn't used at all for fuel calculations so between the old MAF and the new MAF you now have your ~20 g/s of fuel calculation back in the fuel delivery. Fuel delivery would only be somewhat equal in these two situations if your BLM cell 15 was much higher with the old MAF and is now lower w/ the new MAF.

Hope thats clear as mud

 

[Epitome]

I wasn't sure....but now I see that I basically had it right Feels good to have it running perfect again.

 

[Epitome]

I GUARANTEE that there are a whole pile of us Buick guys running around with cars that "just don't seem to run exactly right" because of MAF sensor problems. All it takes is what seems to be a "slightly" lower reading and all kinds of things happen. This should be one of the first things I would check from now on. If you don't see 250-255 at WOT, don't do another thing until that is fixed. Thinking back a few years, when I first got my GN, my T (which had a properly working MAF sensor at the time, 255 everytime) would run much better (GN's MAF sensor read 235 max at WOT). I swapped the MAF sensors at the time and whammo, GN ran great. We should have a sticky post somewhere to remind everyone how critical checking the af reading is.

 

[V6 Beast]

Originally posted by Epitome
I GUARANTEE that there are a whole pile of us Buick guys running around with cars that "just don't seem to run exactly right" because of MAF sensor problems. All it takes is what seems to be a "slightly" lower reading and all kinds of things happen. This should be one of the first things I would check from now on. If you don't see 250-255 at WOT, don't do another thing until that is fixed. Thinking back a few years, when I first got my GN, my T (which had a properly working MAF sensor at the time, 255 everytime) would run much better (GN's MAF sensor read 235 max at WOT). I swapped the MAF sensors at the time and whammo, GN ran great. We should have a sticky post somewhere to remind everyone how critical checking the af reading is.

That is exactly why most chip guys lock the MAF at 255 when WOT, at least on race chips.

 

[BJM]

Here is another issue most people are not aware of.

The BLM fuel trimming is done over a range of RPM and MAF readings causing the computer to modify usually about 10 of the possible 16 cells. Cell 15 is the "WOT" cell and in a normal chip without a BLM or MAF lock it is used to trim any power enrichment mode fueling.

The thing is the learning mode for that cell is done for flow roughly between 30 g/s - maybe as high as 80-100 g/s. Your slowly dieing MAF got trimmed at those levels and if the calibration goes screwy up around 200 and up the computer won't know.

The WOT fueling is determined using data from way down on the MAF curve and the ECM must assume everything is okay.