Thread: Unplugged my MAF and still running lean???

I thought the PCM would default pretty heavy with fuel and run rich? The engine is the L36, so NA. I'm just doing the basics learning my tuner now before I start it on my top swap later this summer. Anyway, is my thinking wrong? I'm scanning in process of a VE tune and LTFT are positive at 2-5 with the MAF unplugged at idle, up to 9-10 driving. Thoughts?

I cleaned my MAF today since I was on hold for the tune, no real changes.

TIA,
Slim

pretty normal.

Also, "trims" reflect air temperature when the maf is unplugged, it doesnt read the O2 sensor at all, it effectively goes into open loop.

You ****tin' me? Positive is rich, negative is lean? I know this is basic but I was told the opposite! He told me positive is adding fuel, negative is taking away fuel. Bastard lied or is an idiot (like me apparently, oops!). The VE tune goes on. Thanks for helping out this noob.

Slim

lean is positive = pcm tryin to add fuel to compensate
rich is negative = pcm tryin to pull fuel to compensate

Is your car actually responding to the VE tune?

Most of the newer cars tend to ignore the VE and use the default maf table instead.

Originally Posted by DoubleBlown

lean is positive = pcm tryin to add fuel to compensate
rich is negative = pcm tryin to pull fuel to compensate

So, this is what I thought and puts me back to square one. I thought the engine would run rich with a MAF failure (or disconnect). Right now, MAF disconnected, LTFT running positive 2-5 at idle and up to 9-10 with moderate driving. Using "Jerry's handbook" VE tune methodology I thought I would be getting negative numbers and trying to lock those down between 0 and -3 when PE mode kicks in.

With the LTFT so far in the positive, how do I flip that around? Please show me if a better tuning (DHP) guide if one exist but I have been using Jerry's guide for my NA motor until the parts are on order and ready to swap. TIA

Matt, I'll tell you if I ever get it done. A lot of learning.

GTPpower, I understand the fuel trims do not define rich and lean, but like a lot of things, they can be indicators.

Originally Posted by slimguns

So, this is what I thought and puts me back to square one. I thought the engine would run rich with a MAF failure (or disconnect). Right now, MAF disconnected, LTFT running positive 2-5 at idle and up to 9-10 with moderate driving. Using "Jerry's handbook" VE tune methodology I thought I would be getting negative numbers and trying to lock those down between 0 and -3 when PE mode kicks in.

With the LTFT so far in the positive, how do I flip that around? Please show me if a better tuning (DHP) guide if one exist but I have been using Jerry's guide for my NA motor until the parts are on order and ready to swap. TIA

I have ssaid many times the "handbook" is complete and utter bullcrap and should be burned, as there is no accurate information in there.

Also, "trims" reflect air temperature when the maf is unplugged, it doesnt read the O2 sensor at all, it effectively goes into open loop.

the LTFT number is just the PCM's thoughts on how to compensate for air temperature based on your IAT reading... Trying to tune any VE table (which is never used when the maf is plugged in btw) is stupid as hell in general, even dumber trying to do it using the LTFT value as it is not reflecting anything other than what the air temperature is reading.

Should I skip the VE tune and move on or...? I guess I was just looking at the handbook to learn the basics. Where should I start then?

ould I skip the VE tune and move on or...? I guess I was just looking at the handbook to learn the basics. Where should I start then?

There is not much to do really. "tuning" fuel on a car is usually no harder than fiddling around with the PE table (time table and the base PE coolant table), and the injector constant. I have yet to find a car that "needed" a maf tune. With any car you are going to get goofy trims all the time because o2 sensors are from from highly tuned peices of machinary, so letting your car trim a bit in one direction or the other is most likely a result of some type of mechanical injector variance OR a mechanically altered maf sensor (dirty, screen removed, bad intake setup, ported TB). Any MAF variance is surprisingly linear acrossed the whole range of the maf, and instead of making some crazy custom maf curve up, I usually just adjust the injectors to the average LTFT you see driving around, it has yet to fail me. In terms of "locking trims" you may want to trick things a bit by manually adjusting a spike into the MAF table where it typically finds a locking trim to prevent things from getting too wild there.

Whoa it was my understanding that long term fuel trims are cummalative averages of short term fuel trims.

When the maf is plugged in, this is vaguely true. Long terms are mostly just values coming off the o2 sensor and on OBD2 cars not particularly related to the short term. The short term can reflect many things in a obd2 car.

and how the hell is ve not important to dial in? The more accurate the ve tune the easier it is to dial in maf correctly imo

The VE table is the "map sensor calibration" and the maf table is the "maf sensor calibration". When the maf is plugged in the map sensor is largely ignored, and when the maf is not around the map sensor is used (in most cars, others still dont use it).

The only ONLY time your car is going to use the LTFT data value is when the maf is plugged in and functioning correctly. There is some ability for the PCM to remember trim values after a failure, but these stored LTFT values will still be modified by the IAT correction fuel table (hidden table, I cant seem to find it but I havent looked hard because I dont care).

Typically when an O2 fails, it fails low which tells the pcm it's lean and the pcm dumps a ton of fuel to compensate.

While true in OBD1, the OBD2 systems catch failing O2 sensors a bit faster usually due to the fact they are internally heated, so its somewhat unlikely for a o2 to fail like this without a major code showing up. Not wrong, just slightly unlikely in reality.

Originally Posted by BillBoost37

Had two cars with bad heaters in the O2's pretty close together. Sure it typically throws a code. Sometimes though, it's dead on startup...so after your 10 seconds when the ICM hands off to the pcm, the car runs horribly. Take a look at the O2 on a tool and it's sitting around 100mV and dropping. May run so badly the PCM doesn't have time before the stall to test and toss the code.

Merely saying, it does happen.

Sounds good to me.

Its not the 'ICM handing off" its basically just going near closed loop idle, as it does not try to go into any of the o2 values for a period of time.