I think you might be missing the point with the temperatures. The reason for waiting 10 -12 hours, is to make sure ECT and IAT have stabilized with ambient temperature. All three temperatures need to be checked simultaneously to make an accurate comparison. The more places you check temperature, the greater the odds are of finding the inaccurate sensor. You can also check TFT, (Transmission Fluid Temperature) if your scan tool will access it. Your readings from the two sensors showed a 5 degree difference, which is excessive. We need the third temperature to prove the "odd man out" theory. Measuring resistance at the sensors is fine, both sensors are thermistors and should have identical resistance values. However, that doesn't tell us what the PCM is receiving, and that's what matters. When we figure out who the "odd one" is, then we start checking resistance on the circuits at the sensors and the PCM connector if necessary.


Your key on engine off, (KOEO) MAF reading should be zero. If you unplug the MAF, it should also read zero, which is most likely what you'll find. If that's the case, I would suspect a faulty MAF, but don't expect the SES light to come on if you don't replace it, and don't expect a difference in fuel mileage if you do.


When the MAF reading is high, the PCM increases injector pulse width to richen the fuel mixture. If the MAF is falsely high, the oxygen sensors will detect a rich mixture, the PCM will shorten injector pulse width, and the fuel trims will go negative on both banks.


The rule of thumb that you referred to regarding MAF is "ballpark" correct, not gospel.

 

My scanner will read transmission temp so tomorrow I will check it and compare all three. I just didn't think 5 degrees difference would cause much of a problem.

The MAF reading does drop to 0 when the connector is disconnected then when reconnected it reads 2.4gm/sec.

How would the computer react as far as fuel trims on a cold engine where the o2 sensors are not functioning yet and the MAF has a falsely high reading with the MAP and TPS reading a normal idle condition.

 

TFT doesn't affect driveability, but in addition to ECT & IAT, it will help in pinpointing false data. Having an accurate ambient reading will give a benchmark for the comparison. Who knows, there might be more than one sensor giving false data.


The oxygen sensors monitor fuel mixture, but only after they warm up. Heaters within the sensors are used to bring them up to temperature quickly. When they reach a certain temperature, they create voltage that the PCM uses to detect & control the fuel mixture. During open loop operation, oxygen sensor data is ignored, and the PCM uses a fuel management strategy that relies on MAF, TPS, BARO, MAP, IAT, & ECT data. If any of that data is incorrect or missing, it will affect how the engine runs. When the PCM switches to closed loop operation, it uses the oxygen sensor data for fuel control, which is a far more accurate strategy. If there is a problem with any of the sensors, including the oxygen sensors, driveability symptoms will change instantly when it switches to closed loop.


Oxygen sensor data is used to calculate fuel trims, so they are inaccurate during open loop operation.

 

Checked temps this morning after car sat all night. Both ECT and IAT where 37f. The weird thing is the transmission temp is showing 30f. The outside temp was 36f according to the car temp ind. My digital thermometer was showing 38. I guess the ECT and IAT sensors are closer than we thought.

See pics below.

 

Before your next cold start up, connect the scanner. Watch when it switches to closed loop, and see if the problem starts immediately upon switching.

 

The problem is there as soon as I start it and then after it stumbles around for anywhere from 15 to 45 seconds it seems to smooth out and run ok even before it goes into closed loop. By the time it goes into closed loop it has been running good for a couple min.

It has 235,000 miles on it but has always ran great with no problems. I am sure the MAF is jacked up because it isn't zero when car is off and when I first start it shows 12 to 14 gm/sec and slowly comes down to about 5 to 6gm/sec at which time is when it seems to run good. I think I am going to order a MAF sensor and see how that effects it.

I will keep you updated.

 

The problem is there as soon as I start it and then after it stumbles around for anywhere from 15 to 45 seconds it seems to smooth out and run ok even before it goes into closed loop. By the time it goes into closed loop it has been running good for a couple min.

It has 235,000 miles on it but has always ran great with no problems. I am sure the MAF is jacked up because it isn't zero when car is off and when I first start it shows 12 to 14 gm/sec and slowly comes down to about 5 to 6gm/sec at which time is when it seems to run good. I think I am going to order a MAF sensor and see how that effects it.

I will keep you updated.

 

I've seen that same thing, (MAF not reading zero with KOEO) several times on vehicles, and never found a solution. I spent some time experimenting with additional grounds, installing MAF sensors, PCM's, etc and couldn't get it to zero. My 98 Blazer has that exact same scenario, but it doesn't pose any driveability issues. MAF is the only reading that isn't exactly what normal values should be. The 04 MAF starts at zero, and yes, even tried swapping that MAF too, with no change. It's perfectly normal for MAF to read high at start up, and drop down as RPM drops. A more accurate way to read MAF is measuring frequency. Most higher end scanners and DVOM's have that capability. But even that doesn't start out at zero.


For the extended idle: log the readings of the IAC at start up. Then check them again at operating temp, in park or neutral, closed loop, at idle, all accessories off. Post the readings.