I think the o2 sensor is functioning correctly... hopefully...
This weekend I am going to clean all the grounds (engine/chassis/body) and check the 4 grounds from the ECM....
I have checked the base TBI idle speed (set to 650 RPM)
I have reset the hot idle IAC.
The coolant temp is 190-196F at hot idle....
The MAP value seems correct this morning.....but, I went for a drive at lunch and in gear @ hot idle the map is reading a higher value of 42 kPa ( or Hg 12.4) when I turned on the AC (it's about 85-90 outside today) the value went to 52 (or 15.3 Hg)
Is that too high for hot idle or normal??

 

Also, there are no codes.......

 

With the engine running, MAP is just a measurement of the level of vacuum in the intake. Normal vacuum on a fully functional engine with no base mechanical problems is 18-22 in-Hg - generally speaking 15 in-Hg is considered the lowest acceptable reading.

But the readings you’re getting need to be verified with an actual vacuum gauge - the MAP sensor is calibrated for absolute pressure (hence the term Manifold Absolute Pressure) which uses “absolute zero” or aka “a perfect vacuum” as its starting point whereas a gauge uses atmospheric pressure as its zero point so there’s always a discrepancy between the two.

And yes it’s normal for the reading to increase when the engine is under load (AC on) at idle.

You need to focus on what the o2 sensor is doing at idle if you’re set on figuring this out.

I think you’re misinterpreting the data - the ideal voltage is 0.45 volts - lower than that indicates lean and higher than that indicates rich. It’s not that the engine kicks back into open loop and then the o2 sensor starts reading rich - it’s that the engine is running rich at idle and cooling down the sensor which then results in the return to open loop.

The reason you don’t see it for about 10 seconds is because that’s how long it takes your scanner to communicate, process and read out the data from the ECM. You need to use a meter to probe the sensor circuit and see what the actual voltage is at the moment it returns to idle speed.

 

Hello-

Thanks for the reply!

I will back probe at the O2 sensor connector to check the voltage when it kick back to open loop.

I am going to bench check the O2 heater is working and check the temperature as well.

Today I did notice the O2 was switching rich to lean constantly and then it went into open loop.
At open loop the O2 V jumps to a steady .8V.........

Maybe it is all just the delay in the scanner reading and the O2 sensor is cooling down and kicking to open loop!
I will find out tomorrow and report back!

 

I will also measure the vacuum at the TBI on all ports......
And check the MAP is holding vacuum.....