2006 X Trail QR25DE , no MIL/codes, MAF fault, intermittent, solved.

History: 74K miles from new until onset intermittent possible MAF failure, no codes, engine reverted to lack of power/limp mode. Started from cold, perfect. vehicle runs without problems on local short trips but fails at unpredictable times from there on.

Trouble shooting sequence: Changed Crankcase sensor, camshaft sensor, tested MAF.= 1.2v cold, 1.12v at warm idle & cleaned it with contact cleaner.). Tested low speed cooling fan, coolant temperature sensor, changed the air filter, changed the fuel filter, tested fuel pump pressure (head height > 4 feet, wired an LED across EVAP solenoid pump to check operation while driving & tested the fuel cap, all appeared normal but they would be without the fault showing. Inspected throttle for smooth butterfly valve operation & spring return action. Problem still exists.

Plan B was have the local garage scan the system, this was done & no problems found.

Whats left ? TPS & accelerator sender and O2 sensors. By this time I had invested in an ELM327 v 1.4 code reader & downloaded the TORQUE LITE program. Passenger logged the readings by camera, then inspected the data this all looked OK. The only anomaly was after an event the return to a lumpy tickover showed the MAF reading sitting at 5 to 6 gps where I expected around 2.5 gps. On that basis I ordered a new MAF & some magic happened. Hind sight, MAF should have been first changed irrespective of all else. But this was a red herring since further recurrances occurred but not as significant. With air con on, both radiater fans are on full, this improved the car operation and in this mode no further glitches happened. How were the fans helping the system? Directly behind the fans is the exhaust manifold and upper O2 sensor which is limiting the temperature peaks of the pre catalyst exhaust pipe. So next, the upper O2 sensor was changed & tests at approx 70mph without the air con fans operating carried out which seems to have solved the problem.

Analysis: Closed loop amplifier & hot wire drive transistor within the MAF partially failed when hot. The assumption that ambient air passing the maf will keep it cool was notneccesarily valid. The heat increases in the engine bay from exhaust pipes etc depending on engine load/ambient temperature when driving. This heats the air intake tubing externally by convection & conduction on the outer surface, including the maf connector & maf circuits. Scanning with an infra red temperature monitor puts the air intake surfaces well above well above 60 degs C. The only predictable thing about this failure mode is that it is unpredictable in time after starting a trip. Similarly, the O2 sensor has deteriorated to the point where random partial failures could also occur. Since the Maf works with the O2 sensor to maintain the correct air/fuel mixture, either or both in combination can cause the exhaust emissions to reach a situation which falls out of engine computer control, thus causing the engine control to fail temporarily without error codes being generated.

Hope this helps X Trail community.

Printed resistors, ceramic capacitors, foil conductors, single amplifier, aluminium bonds plus a conformal coating on the hybrid circuit. Failure after 14 years service due to external heating from engine bay.

QR25DE engine control system.

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