There is a lot of time and effort being spent by a number of people troubleshooting manifold boost issues and "Limp Home", with the ALH engine.
I am now retired from a long career in the diesel engine field and began to get interested in turbocharger issues on my Jetta when it went out on loan to a family member but returned on a trailer, months later, not driveable.
In my research on how the system works, the VNT turbocharger, the vanes and actuator, vacuum source and lines, the N75 valve. For some reason, my Bentley manual does not show the MAP sensor. in the text. I found it mentioned in the component locator section of the manual.
On this forum and on You tube i see a number of people trying different things, changing parts, even replacing the turbocharger but I have never seen the MAP sensor being discussed as a possible problem.
The MAP sensor is a Euro version of a manifold boost sensor, and while I have yet to remove, test, clean this one, I am sure it is the cause of the problem. In this case, all the other components check out OK. During operation of the engine, if the ECM is receiving a faulty signal from the MAP then it supplies a different voltage to the N75 to dump the vacuum to the actuator. The VNT vanes then move from the closed position, exhaust gas is routed around the turbine wheel, the turbocharger turbine and shaft slow down, the compressor wheel loses centrifugal force and boost pressure goes down. With reduced boost pressure, the ECM also signals the fuel injection pump to reduce delivery and "Limp Home".
I am now retired from a long career in the diesel engine field and began to get interested in turbocharger issues on my Jetta when it went out on loan to a family member but returned on a trailer, months later, not driveable.
In my research on how the system works, the VNT turbocharger, the vanes and actuator, vacuum source and lines, the N75 valve. For some reason, my Bentley manual does not show the MAP sensor. in the text. I found it mentioned in the component locator section of the manual.
On this forum and on You tube i see a number of people trying different things, changing parts, even replacing the turbocharger but I have never seen the MAP sensor being discussed as a possible problem.
The MAP sensor is a Euro version of a manifold boost sensor, and while I have yet to remove, test, clean this one, I am sure it is the cause of the problem. In this case, all the other components check out OK. During operation of the engine, if the ECM is receiving a faulty signal from the MAP then it supplies a different voltage to the N75 to dump the vacuum to the actuator. The VNT vanes then move from the closed position, exhaust gas is routed around the turbine wheel, the turbocharger turbine and shaft slow down, the compressor wheel loses centrifugal force and boost pressure goes down. With reduced boost pressure, the ECM also signals the fuel injection pump to reduce delivery and "Limp Home".