BMW M52TU and M54 – Lean Mixture Fault Codes – DISA Valve? (P0174, P0171, P1084, P1085, Etc.)
This answer is applicable for many BMWs in addition to the one listed below.
car year: 2002 car model: 530i I’ve got a 2002 BMW 530i with 2 fault codes p1085 fuel control mixture lean (bank 2 sensor 1) and p0174 system too lean bank 2. Ive looked over the air intake system for any leaks but i have yet to find any. Ive check all vac lines i can find and I also check the intake manifold for leaks and again I found nothing. I’m looking for any possible solution to this issue. Any ideas on what i can check next to solve my issue? Thanks for you help in this matter.
The fault codes that you are showing, would most often be related to a lean fuel mixture. On the BMW M54 6-cylinder engine, this would typically be due to a vacuum leak in the intake system or a faulty intake manifold adjusting valve, commonly called the DISA valve (Differential Intake System Adjuster Valve).
* If there are no obvious potential vacuum leak areas (such as the rubber upper and/or lower intake boot or the plastic “F” vacuum hose connector on the boot), have the intake system smoke tested. This involves injecting a special smoke into the intake system and looking for areas where the smoke is leaking out. This would indicate a vacuum leak (unlike Lucas electrical systems, which leak smoke from the wires when they fail ….. sorry, old British sports car joke). Common vacuum leak areas would include the hoses and oil-separator/check-valve system for the crankcase ventilation system, the throttle-body gasket, the intake manifold gaskets and the various smaller vacuum hoses that run under the intake manifold.
* The DISA valve is a common failure item. The flap door breaks and the valve no longer properly controls the intake air flow through the manifold. There are no obvious external clues to this, but this failure can cause the leak mixture fault codes (various different codes can be generated, including the ones in the title to this posting). The valve must be removed and the control flap inspected for broken pivot pins. The valve is relatively easy to remove. It is secured to the driver’s side of the intake manifold by four bolts.