I put my Snap-On MT-2500 scanner on my '94 Blazer (W) after I got it running well (I had another thread about the tune-up and disconnected vacuum line). All the values looked good except the Air-Fuel Ratio. It should read about 14.7. It read 0. Why would it read 0 in closed loop mode?

 

What you are looking at is likely the fuel trim which is a % off of baseline used to modify the baseline values depending on what the O2 sensors are telling the ECU. 0% means it is operating at baseline values with no modifications.

 

I don't think it's the fuel trim that I'm viewing. It's labeled A/F Ratio. Short-term and long-term fuel trim are both labeled separately. When I put the scanner on my wife's '93 Jimmy it says 14.7. Did that measurement change between the '93 and '94 CPI motors?

 

If I were you I wouldn't pay attention to the value. Watch the trims to see how well the engine is operating in relation to the fuel map values and watch the O2 sensor output in mV. Narrow band sensors such as these trucks have a non-linear response so accurate AFR outside of their typical 14.7:1 operating range isn't that reliable.

 

That's exactly what I wanted to know. Everything else is good: LT fuel trim is 128, O2 sensor is oscillating normally. I'll ignore the 0.0 air fuel ratio.