How does the pcm determine barometric pressure?
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Joined: Nov 2001
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From: St Louis, MO
How does the pcm determine barometric pressure? How does the pcm determine barometric pressure?
I always thought this was only measured by the MAP sensor when the ignition key is turned on. Today I was in Phoenix and decided to log the drive home with EFILive V6. I started at an elevation of 1,100 feet and drove to the base of the ski lift here which is 9,300 feet. This is the highest point on a paved road that I have found.
The baro reading started at 96.7. At the end of each WOT climb it changed: 95.9, 93.4, 88.2, 80.8 for an instant, then 81.5. It held 81.5 for almost 3000ft elevation change, but during that time I did not enter PE (Cell 22). At the ski lift I turned the key off and MAP read 72kpa.
I also logged the drive back to my house (7,000ft). My next start baro read 72kpa. Even the lightest application of throttle that put it into Cell 22 would update the baro reading: 74.5, 76.8, 77.9. When I turned the key off, the MAP read 78kpa.
I never thought it could calculate this while driving, but it appears all it takes to update the baro is to enter PE mode. It looks pretty accurate to me. I hope someone else finds this interesting.
Eric
I always thought this was only measured by the MAP sensor when the ignition key is turned on. Today I was in Phoenix and decided to log the drive home with EFILive V6. I started at an elevation of 1,100 feet and drove to the base of the ski lift here which is 9,300 feet. This is the highest point on a paved road that I have found.
The baro reading started at 96.7. At the end of each WOT climb it changed: 95.9, 93.4, 88.2, 80.8 for an instant, then 81.5. It held 81.5 for almost 3000ft elevation change, but during that time I did not enter PE (Cell 22). At the ski lift I turned the key off and MAP read 72kpa.
I also logged the drive back to my house (7,000ft). My next start baro read 72kpa. Even the lightest application of throttle that put it into Cell 22 would update the baro reading: 74.5, 76.8, 77.9. When I turned the key off, the MAP read 78kpa.
I never thought it could calculate this while driving, but it appears all it takes to update the baro is to enter PE mode. It looks pretty accurate to me. I hope someone else finds this interesting.
Eric
Joined: Nov 2001
Posts: 1,707
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From: Anderson, Indiana
Re: How does the pcm determine barometric pressure? It was my understanding that the newer systems could update baro at key on and WOT; both being conditions where the PCM "knows" what the MAP should be and then calculates the BARO. I'm not sure how it can make an accurate determination if you're just getting into PE; but I guess it can. 
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91 Z28 LS2 408CI, LS9 Supercharger, LPE GT7 cam, Yank3000, 3450 raceweight.
Latest numbers: 9.71 ET, 141.42 MPH, 1.40 60' , 610 RWHP Mustang Dyno
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91 Z28 LS2 408CI, LS9 Supercharger, LPE GT7 cam, Yank3000, 3450 raceweight.
Latest numbers: 9.71 ET, 141.42 MPH, 1.40 60' , 610 RWHP Mustang Dyno
www.speartech.com
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Re: How does the pcm determine barometric pressure?
Somehow it does, a couple times I had less than 50% throttle when it hit cell 22 and updated baro.
Thanks for the post and info.It now makes sense that PCM would need to Verify pressures before entering PE.
joel
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Joined: Dec 2001
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From: Santa Clarita, CA
Re: How does the pcm determine barometric pressure? Interesting experiment, thanks for the info. Speaking of info I was curious after reading your post, and found there is some more info posted as the Detailed dianostic info sticky in this forum. If you click on the link and then go to the Manifold Absolute Pressure Sensor section, they mention the MAP output is used to let the control module make altitdue adjustments etc. Pretty slick cars we have.
Regards,
Regards,
