Air/Fuel Ratio OBDII

[Nader]

I recently purchased some hardware and software to read diagnostics through the OBDII port in my Viper It actually works great and can not only check and report trouble codes but also deliver many of the engine diagnostics such as coolant temp, air intake temp, boost/vac, throttle position, RPM, Engine load, Timing advance, Fuel trims,and a variety of other information.

I was really looking to log air/fuel ratio but come to find that only the Fuel trims can be pulled through the OBDII port. The fuel trims that can be seen real time and logged. The fuel trims you can log are SB1, SB2, LB1, LB2.

Can someone please give me some back ground on what fuel trim is and what the differences bwtween the four are? Also can you give me some insight as to how it relates to the air/fuel ratio? Is it possible to see air/fuel through the OBDII?

The software is called REV and is run with the iPhone. It is actually really cool.

DevToaster - Rev - iPhone app development, Great iPhone apps, and more

Thanks.

Tom

 

[Viper Specialty]

Short Term Bank 1 [Driver]
Short Term Bank 2 [Passenger]
Long Term Bank 1 [Driver]
Long Term Bank 2 [Passenger]


More Later...

 

[KNG SNKE]

Was waiting for that app to come out. Now I want an iPhone

 

[Nader]

Dan I knew if anyone was going to know it would be you. I look forward to hearing from you.

Short Term Bank 1 [Driver]
Short Term Bank 2 [Passenger]
Long Term Bank 1 [Driver]
Long Term Bank 2 [Passenger]


More Later...

 

[Viper Specialty]

OK, better late than never!

First off, you have to understand how a PCM like the Viper works. It is what is called a Narrow Band Controller, and works within a small AFR, of approx 14.4:1-15.4:1, rather than the full sprectrum of Pig Rich at 10:1, and INSANELY lean, at 20:1 as some Wideband systems are capable of reading. In Closed Loop, meaning the PCM is reading the O2 sensors, the target AFR is always 14.7:1, or Stoich, meaning a perfect burn, all fuel is consumed, and no extra oxygen is left over.

The Base Fuel Tables represent what the engine SHOULD require in uS [time pulse] to attain 14.7:1 at a given RPM and Manifold Pressure [Load]. As always, these base numbers are multiplied by or added to correcting factors such as intake temp, coolant temp, altitude, etc. However, to adjust for changing conditions such as Fuel Filters clogging over time, wear, spark plug condition, etc... the computer must have the ability to adapt its basic fuel tables. Rather than alter the base fuel tables directly, they add in a multiplier called a "Fuel Trim", which always starts at Zero percent, and either adds in fuel, or removed fuel, in order to satisfy the engines requirement to get back to 14.7:1, which the O2 sensors are reporting back to the PCM.

The Short Term Trim, which is constantly being corrected as the engine is in closed loop to attain 14.7:1, has the ability to correct as much as 33.3% error in the fuel system. Short Term Trims are NOT permanent, and are cleared whenever the engine stops, or closed loop turns into Open Loop where the O2's are not being read. Now, this is where you add in Long Term Trims. If the computer sees that the engine is ALWAYS requiring 10% more fuel under certain conditions, it will move the Long Term Trim to 10%, in an effort to keep the Short Term Trims as close to Zero as possible. Long term Trims are "Permanent", they are stored in the memory and not cleared when the engine is turned off, and also WILL be multiplied even when the engine is in Open Loop or Wide Open Throttle. Also of note is that Long Term Trims will only update once the engine is fully warmed up, and if no check engine lights or faults are present, in an attempt to keep them from being skewed by faulty data. The Long Term Trim has the ability to correct up to 33.3% as well, so you have the ability to correct a total of 66.6% of the fuel demand with both long and short term trims, which is quite substantial in any normal case.

Lastly, there are TWO sets of trims, one for each side of the engine, bank 1 and bank 2, and they are completely independant of each other.

 

[LynnS]

Excellent Information - Thanks Dan!