BMW E38 730d/740d turbo and boost pressure
Why does the E38 730d/740d boost wrongly or go into limp mode?
The cause can be cracked or wrongly connected vacuum hoses, a fault in the boost pressure regulator or the actuator rod, or the turbo itself. The vanes of a variable-geometry turbo can seize on the cam or tear. Replacing the boost pressure sensor does not usually fix this kind of fault.
How do you check the turbo actuator rod (variable geometry)?
The rod's full travel is 10 mm. Fully in (highest boost pressure) it is at 650 mbar vacuum, and fully out (lowest boost pressure) at 0 mbar vacuum. Fault code 1E30 is stored when the boost pressure control has an impermissible deviation, a short circuit or an open circuit.
What is the normal boost pressure of the 740d, and do the turbos last?
The standard boost pressure is about 1.1-1.15 bar. The 740d's turbo is a Garrett GT1749 and has no particular weakness. Durability depends a lot on driving style. A well-reconditioned turbo is no worse than a new one, and a factory-reconditioned one can be had on exchange.
Can the boost pressure regulator be replaced with a working one?
Yes, if the regulator's body and control unit numbers match. Fitting a working regulator has fixed the fault. The regulator's number tells the lever position, and a wrong position means wrong control.