still chasing cause of my boost problem. had a DRBIII plugged into the truck and noticed that the MAP sensor reading was 14psi at idle and was consistently around 14 psi higher than my boost gauge at high psi. so does the MAP sensor read absolute pressure, as in at sea level its measuring atmospheric pressure at 14psi when there is no boost? or could the sensor be bad and reading too high, therefore causing my wastegate to open too early creating my low boost problem?

running out of ideas here i've replaced the electric solenoid that controls the wastegate and still can't make over 20psi boost. with the wastegate vacuum hose pinched off it spools 45psi easy so for some reason the computer is opening the wastegate way too soon. any other ideas?

 

Could be a bad map sensor. Do you have any codes with that?

I don't think it measures the air pressure, but sea level (you're at 300ish feet) is around 15 psi.

 

That's what they read at idle. Why I don't know.

 

Lets see...... MAP...... Manifold Absolute Pressure.
Have read others say they read 14 psi at idle

 

the solinoid does not open the wastegate. here is how it works. the he351 was designed with emission controll in mind. with no voltage on the solinoid the boost is 20 psi with 12 volts on it it increases to 32 psi the ecm checks to see if a load is attached it also self checks at each start. get a test light, unplug the solinoid and connect the light across the two pins from the ecm. turn the key on and the light will flash on full brightness for about 1 or two seconds if it does not the ecm is bad. if it is you can place a 120 ohm resistor across the ecm pins and wire the solinoid to a ignition hot lead. or i can send info and pix of how to make it fixed for about 75 cents and 30 minutes labor.

 

It reads air pressure. You have to consider that the air around you is at pressure already. Thats why they read something at idle. Some readers don't compensate for this so you gotta look at the weather channel, see what the current pressure is, then subtract that to get boost at idle. A scangauge is one that you have to do that with.

 

INTAKE AIR TEMPERATURE
SENSOR/MAP SENSOR
DESCRIPTION
The combination, dual function Intake Manifold
Air Temperature Sensor/MAP Sensor is installed into
the top of the intake manifold.
OPERATION
The combination, dual function Intake Manifold
Air Temperature Sensor/MAP Sensor is installed into
the top of the intake manifold with the sensor element
extending into the air stream.
The IAT portion of the sensor provides an input
voltage to the Engine Control Module (ECM) indicating
intake manifold air temperature. The MAP portion
of the sensor provides an input voltage to the
ECM indicating turbocharger boost pressure.

 

Disconnect the Intake Air Temperature sensor harness connector.
Check connectors - Clean/repair as necessary.
Measure the resistance of the Intake Air Temp sensor.
Is the resistance between 300 and 90k ohms?
All
Yes ! Go To 2
No ! Replace the Intake Air temperature sensor/MAP sensor.
Perform POWERTRAIN VERIFICATION TEST VER - 1 (DIESEL).
2 Reconnect the circuit.
Turn the ignition on.
With the DRBIIIt, erase DTCs.
With the DRBIIIt, read DTCs.
Did the DTC reappear?

Yes Refer to the INTERMITTENT CONDITION Symptom

Start the engine.
With the DRBIIIt, read the MAP Sensor voltage.
Is the pressure reading of the MAP sensor less than 35.85 in Hg?
All
Yes ! Go To 3
No ! Replace the MAP Sensor.
Perform POWERTRAIN VERIFICATION TEST VER - 1

Operate the engine under normal driving conditions (no load) for at least 30 seconds.
Monitor the MAP sensor reading with the DRBIIIt.
Did the pressure reading from the MAP sensor go above 22.5 in Hg for more than 2.5
seconds?
All
Yes ! Replace the Map sensor.
Perform POWERTRAIN VERIFICATION TEST VER - 1 (DIESEL).
No ! Refer to the INTERMITTENT CONDITION Symptom

 

14.7 psi is atmospheric pressure at sea level. The scan tool is reading the MAP correctly.

MikeyB

 

What MikeyB said. Pressure at idle will vary based on weather and altitude. On a standard day at 5K feet it would only read 12.2.

 

This is interesting, and you learn something new every day.

From the times I used a programmer that read boost from the map, it would read 0 at idle...

I would still check the resistance on it IAW the values I posted above.

Could your wastegate be bad?

Have you tried to find someone with a smarty to disable the wastegate?

Have you made sure your connection to the wastegate is good?

 

Many of the electronic gauge subtract the ambient pressure from the MAP to display boost. The sensor in the intake just before the turbo provides the ECM with the ambient pressure.

 

wow you guys are awesome thanks for all the info! been on the road for a week so i haven't been able to check anything yet but i'm going to tear into it this week.

 

in regards to the wastegate, what happens when you pinch off the vacuum line to the wastegate doesn that basically disable it and keep it from opening? because i see plenty off boost with the vacuum line pinched off. all connections look good

 

That's correct. But it should throw an overboost code.