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I have a stock 6.5 installed in 96 3500hd and looking to keep it that way. I recently replaced the injector pump and accuired the GM Tech 2 to get it in time. The truck starts very well and sounds good, pulls a trailer with a load ok but seems to lack power. When using the Tech 2 I noticed the boost pressure reads 14 psi at idle of 700 rpm, then when under load advances from 14 to 26psi. I have an axuillary boost gauge installed that reads at idle 0psi and advances to around 15psi under lload. Thinking I had a bad boost sensor on the intake, I replaced it only to have the same results, 14psi at idle. I'm guessing that if the ECM thinks it has more Boost than it actually has, it will limit power. The Tech 2 tool has a manual operate test to operate the wastegate valve, and it is working properly. Is there a way to calibrate the actual boost pressure or could somthing else be causing the erronous boost pressure reading?
 

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Welcome to DieselPlace

The stock programming of the ECM will not allow boost levels to go over 12psi.
If the ECM detects levels above 12psi the ECM will fuel limp the engine to protect itself and set the CEL on the dash.
 

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Thank you. Any sugestions on how to resolve the issue?
Check for any stored codes.
Go through all grounds using the grounds link in the 6.5L FAQ page
 

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That is normal.
You have to subtract the BARO reading from the BOOST reading to get actual boost.
BARO should be about 14psi and is collected when the engine is first started.
Both readings come from the same MAP sensor on OBD2 trucks.
 

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Manifold Absolute Pressure is what the electronics are using, the signal from the sensor is varying voltage. The reason for this is operation at altitude where atmospheric pressure is lower.

The engine control monitors the volume of diesel injected, since liquid density is less temperature dependent, this is am adequate measure of mass flow. Air density is both temperature and pressure dependent, if absolute pressure and temperature are known, density is easily calculated.

Most computer controlled engines use both MAP and mass flow sensors for initial fuel trim. Temperature is also used, mostly for timing but also a check on MAF and MAP. Most modern diesels use all three as soot and nitrogen oxide emission control.
 

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The Tech II reads atmospheric pressure not PSI. Little confusing but that's what you are reading which is normal.
 
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