Question about 1996 Subaru Legacy

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97 legacy no boost

Had to replace the engine but now I have no boost from the turbos. The mechanic has had it for 4 days and still cant find a cause. He's taken it to an auto electrician who cant find any electrical faults.

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The second turbo simply follows the first when the system activates it by bringing it in parallel with the first. The first turbo is only designed for a 1200cc motor so it can not deliver enough compressed air at revs for a bigger motor so they arrange to bring in a second turbo of the same size when the first one runs out of puff.
The first turbo is a semi-mechanical device. The ecm raises the boost level at which it operates but it should be producing some boost regardless.
So concentrate on the first turbo.
Isolate it from the ecm control system
(There is a misleading tip on youtube involving pulling hoses but it is misguided, the comment is ignorant, and I do not recommend it)
Instead, as per the manual, on the first turbo remove the system pressure lines from exhaust and wastegate. Loop a short piece of vacuum hose from the primary turbo output to its own wastegate.
Blank off the system hoses you disconnected. Fit a boost gauge, drive it and observe the boost gauge readings (manifold pressure). If the primary turbo is OK this will make it operate at a lower boost level (5-7psi ?) than the system control (10-17psi) but it will be a purely mechanical operation which will isolate where the fault is.
If there is no boost and the pressure stays zero down then the turbo is not working. Check for the wastegate being stuck or maladjusted. If that is working then you bave a faulty turbo for whatever reason. Maybe there is a loose bolt stuck in it from the engine rebuild?

If the turbo is working OK then it points to an electrical issue with the the boost control solenoid which is inside the wing near the vacuum tank beside the battery compartment. The control voltage from the ecm usually pulses it on/off at a fast rate to give a measured control and it may be stuck. Maybe it is the ecm or maybe the valve. But before you go dig it out, first check the pressure lines such as 10 and 23 are connected or not reversed at their connectors where they go through the wing (just behind the battery)
Good luck

Posted on Feb 10, 2010

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Posted on Jan 02, 2017

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The car goes in "limp home" mode!!!
After scan the EDC-15P engine control unit with a VAS, VAG or KTS -BOSCH diagnosis tool you will find this: Fault code: P1557-Turbo Boost pressure control exceeded
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3. This is the most difficult work. The problem is that the soot particles deposits inside the turbocharger plugging the variable nozzle geometry mechanism = adjustable vanes of the turbine. If the turbocharger actuator is not able to adjust the turbine vanes the charge air pressure increase too much and ECU (engine control unit) go in "limp mode" = engine protection software. As a result the "limp mode" engine still running until you turn the engine off (ignition switch off) and back on when the "limp mode" is deactivated, but the fault still remain in ECU memory!
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