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Old 07-08-2019, 05:57 PM   #11
Aqua Puttana
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Default Re: what is max boost

Different things are under discussion vs a mechanical gauge tapped into the manifold.

The Boost displayed by any performance monitor is a calculated value. As has been mentioned, there are a number of sensors which can be involved.

My mechanical boost gauge is not a differential pressure gauge. It is a simple direct read Bourdin Tube gauge. Until I start the engine the atmospheric pressure is equal on the inside of the tube and the outside of the tube. Whether I am at sea level or in the mountains the gauge reads "0" with the engine not running.

Watching a Scangauge for pressure readings at various altitudes with the engine not running is using sensors. That is not the same as a mechanical gauge.

vic
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Old 07-08-2019, 06:18 PM   #12
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Default Re: what is max boost

There’s a difference between “boost” and “boost pressure”. Boost calculates the differential from ambient, while boost pressure is absolute, hence the difference between 20 and 35 psi.
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Old 07-08-2019, 11:21 PM   #13
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Default Re: what is max boost

Quote:
Originally Posted by marklg View Post
The Autel provides three pressure sensor readings:
The T1N Sprinter has a barometric sensor on the ECM board itself,
an ambient sensor buried in the MAF
the MAP (or "boost pressure") absolute sensor plugged into the intake hoses on the battery side (near the Intake Air Temperature sensor).
3 sensors, 3 pressure readings ... all referenced against 0 psi. They're all "absolute" gauges.
Quote:
As I understood it, the vehicle subtracts the readings from two sensors to determine what it thinks the boost is. I think it is the boost and atmospheric sensors, but there are three to choose from it seems.
Your display (such as the ScanGauge II, but not my ScanGauge 1) may be willing to calculate and show relative "boost" (i.e. 0 to 19), but the Sprinter doesn't bother with it.
The Sprinter only cares about MAP and ambient.

When the engine is off, or just idling, all three pressures should read very close to the same number.

--dick
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Get YOUR Sprinter's full configuration datacard by entering your VIN to https://www.datamb.com/
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Old 07-08-2019, 11:51 PM   #14
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Default Re: what is max boost

Quote:
Originally Posted by autostaretx View Post
The T1N Sprinter has a barometric sensor on the ECM board itself,
an ambient sensor buried in the MAF
the MAP (or "boost pressure") absolute sensor plugged into the intake hoses on the battery side (near the Intake Air Temperature sensor).
3 sensors, 3 pressure readings ... all referenced against 0 psi. They're all "absolute" gauges.

Your display (such as the ScanGauge II, but not my ScanGauge 1) may be willing to calculate and show relative "boost" (i.e. 0 to 19), but the Sprinter doesn't bother with it.
The Sprinter only cares about MAP and ambient.

When the engine is off, or just idling, all three pressures should read very close to the same number.

--dick
I have the Autel AP200 and also another ODBII dongle and the Torque app. The Torque app displays net boost. Doesn't the switch to LHM depend on the sprinter seeing the boost it expects?

I can see the raw sensor readings using the Autel and the calculated boost using the Torque app, but not both at the same time.

Regards,

Mark
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Old 07-09-2019, 12:15 AM   #15
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Default Re: what is max boost

Quote:
Originally Posted by marklg View Post
I have the Autel AP200 and also another ODBII dongle and the Torque app. The Torque app displays net boost. Doesn't the switch to LHM depend on the sprinter seeing the boost it expects?
Torque does the "net boost" calculation.

To answer your question *precisely* (it's a chicken-or-egg issue), i'd have to know what the ECM programming is.
But i can guess:
"When the Sprinter is running and you're asking for a certain amount of power (by your foot's position on the pedal), it thinks "to deliver that power, i'll have to burn XXX grams of fuel per cylinder charge, and that amount of fuel demands YYY milligrams of air (YYY = 14 * XXX)".
I'll increase the turbo's flow until the MAF sensor says i'm getting that mass of air ingested.
(i will look at ambient pressure to perhaps limit the boost to less than 20 psi over ambient).

If the MAP gauge says i'm not getting the total pressure i seek, i'll cry foul and : (a) stop trying to use the turbo (b) perhaps illuminate the CEL (c) lodge a P-code as a temporary thing.

The next time the engine is started i will turn off the CEL light and try using the turbo. If it seems to work, i'll forget the P-code."


I added the (parenthetical) "limit" clause since i have seen the Sprinter not reach 30 psi MAP when i'm at 9500 feet, yet it didn't claim "foul".

So for the *burn* it doesn't care about "relative boost" ... but it probably does calculate it to prevent "over boosting" ... since the hoses etc. are living in a "relative" world with respect to burst pressure. (although i'd hope they'd be able to withstand at least 25 psi .. but then there's the resonator which have a long track record of not being able to survive even their "design" pressure.)

--dick
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Get YOUR Sprinter's full configuration datacard by entering your VIN to https://www.datamb.com/
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((as always: this post may go through a couple of post-posting edits... so maybe give it ten minutes before commenting))
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Old 07-09-2019, 04:38 AM   #16
Nautamaran
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Default Re: what is max boost

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
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Last edited by Nautamaran; 07-09-2019 at 06:35 AM.
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Old 07-17-2019, 06:51 AM   #17
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Default Re: what is max boost

Update: my silicone-tape patched hose held up just fine, so I’ll be hanging on to it as a spare now that I’ve replaced the cold side pipes. The underside of the lower hose was wet with oil, so it appears the clamp at the sensor manifold has been loose for a while? For the record, the torn area was cleaned with an alcohol swab (all I had) then the patch tape stretched TIGHTLY around with a 50% overlap on the outside of the bends, and 2 1/2 layers (down, back, and half way down) on the area.
I drove with it for about 20 hours with no sign of leakage, with 20 psi boost (34 map, 14 iap) typical on hills and green lights.

-dave
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