I agree with Dick's explanations, though he left out the O2 sensor feeding back a Lambda “excess oxygen ratio” value, which accounts for much less fuel injected than the air passing through the MAF sensor would indicate. We can’t allow all the O2 to be consumed during ignition or there isn’t any left for the catalytic converter to work with, but if we let ALL the air in at low load we get too many NOx compounds, so EGR is increased to displace some of the fresh air that would otherwise enter the cylinders.
The ECM supports two “generic” SAE Mode 1 pids for air pressure:
PID 0B - Intake Manifold Absolute Pressure (kPa)
PID 33 - Absolute Barometric Pressure (kPa)
Your generic scan tool or app may do the math and display "Boost" = MAP - Barometric.
A T1N specific tool like the Autel MD802 or AP200 can decode the Mode 21 data block that contains all three absolute pressure sensor values.
If Intake Pressure (sensed in the filter box on the clean side of the filter) is much below ambient Barometric pressure (sensed under the dash by the ECM) then there is a restriction in the air intake, likely a clogged filter, but could be a plugged fender grate or hose.
If your MAP (sensed on the hose rising from the cold side of the intercooler) is near the Barometric Pressure then the turbo is idle.
With around 14 psi ambient, I will occasionally (though rarely) see 35 psi MAP, or 21 psi "Boost". A more typical MAP value for my driving style is 18-24 psi, or 4-10 psi of boost.
An off-thread (but related) adventure tale:
I got to watch my Arduino display as the ECM coped with a split induction hose today... we felt a subtle saw-toothed surge on our way out of town yesterday, but things looked okay. On our way home this afternoon, it was much more pronounced as the engine hunted for a combination of turbo actuator (85%-60% = 15% to 40% vane engagement), MAP (18 to 24 psi), and likely an EGR and fuel quantity that would maintain the cruise control's set speed of 65 mph. I dropped to 60mph, but the symptoms continued and it finally gave up and went into a No-Turbo limp mode. It could achieve 60 mph down hill, but with just 125 ft-lbs coming into the torque converter the van slowed to 40 mph on the slightest uphill grade. A key-off / key-on reset helped, but LHM would return at the next hill, even driving without cruise and holding MAP below 20 psi. I found it interesting that the LOD value re-calibrated while in limp, staying pegged at 100% with a MAF of around 150 g/sec at around 2500 rpm and 14 psi MAP. After the key reset, MAP could (briefly) reach 24 psi (10 psi "boost") and the MAF would climb to about 190 g/sec, with a LOD of 65%. I didn't note the EGR commanded... and am wishing I had asked my copilot daughter to take a video of the Arduino screen (or perhaps had calbiker's logger sketch loaded?)
At the next town I found an auto parts store, and used my MD-802 (don't leave home without it!) to scan the ECM before shutting off the engine: "Pending/Stored" codes 2359-1 (Charge Pressure Control - charge pressure too low) and 2359-2 (Charge Pressure Control - charge pressure too high). Not unexpected. I shut it off and popped the hood, expecting maybe a loose clamp or an obscure hose tear somewhere, but was instead presented with a 2" tear front and center.
Good karma? I'll certainly take an easy diagnosis over a head-scratcher any day, or a crawl under the radiator checking hoses.
So a roll of silicone self-stick tape patched the hose, cleared the DTCs from the ECM, and we were on our way... remaining two hours home without incident; no codes pending.
The patch will likely hold for a while, but I'll order a set of induction hoses first thing in the morning.
-dave