OBDII Data: Barometric Pressure

 The barometric pressure in my area was 30.18 so lets compare some cars. The 2009 Hyundai Elantra Barometric Pressure pid was in PSI rather than inHg so math, 14.5 PSI x 2.036 = 29.52 inHg. When you see about 14 or 15 on that pid you are seeing PSI. The Hyundai has an actual Baro sensor soldered into the printed circuit board of the PCM. We will call that 29.52 close enough. The Barometric pressure from the weather report is usually corrected to sea level so subtracting .3 would be the raw (what the car feels) pid. 29.88 inHg. A faulty Baro reading on the Elantra would cause a subtle shift in calculated load, and some related shifting concerns. 

   Lets take a look at the pid on the 2008 Chrysler 300C. The gold standard is met, 29.8 from the Chrysler Baro sensor, I believe it is also a part of the PCM on this model. 

Chryslers are big on logic, and rationality. When you turn the key on prior to start it will compare things, taking it all into account. For instance, since the engine is not yet running the MAP reading and Baro reading should match. It will monitor throttle position, map and baro once running to see that the readings make sense. It uses that Baro pid to calculate pressure ratio. If it sees something irrational it can go into limp mode, or disable the cylinder deactivation since it can't determine the load values. 

2000 Ford Taurus 3.0 DOHC: RED FLAG. 

The Ford is showing a Baro pid of 13.2 psi, or 26.87 inHg. Now the Ford doesn't have an actual Baro sensor. It calculates the Baro based on MAF sensor readings. Seeing the Baro off makes me suspicious of the MAF sensor. 

Interestingly enough the g/s of the MAF was 3.96 so slightly high but not alarmingly so. What seemed out of place is the long term fuel trim of 7%. Off into theoretical land, since the Baro pid was 26.87 perhaps the computer sees the higher altitude than actual and is using a fuel control strategy for "thin air". 

A quick wide-open-throttle gave me 104.4 g/s airflow before the rev limiter kicked in so lets clean that MAF and see what happens. It is not easy to safely reach that lower spool which would be the culprit but we'll give it a shot.

After cleaning i am idling at 3.43 g/s. Baro is now changed to 26.8. WOT is 110.79. Cleared the KAM and Baro went to 28. Time for the test drive. It may take awhile for the PCM to recalculate that Baro pid but it is headed in the right direction. 

TEST DRIVE: WOT load hit 95%. g/s airflow reached 127.35, LTFT is now 0, Baro is still 28 but it is just a slow learner. Major improvement.

Thanks for reading.

Kenny@GGAuto.Repair