Subaru Outback BP9 2007 EJ253 2.5L – lean faults and sluggish on acceleration – Air flow meters, insist on genuine parts for fuel delivery products

Previous Repairs, and the short story

The vehicle was delivered to us after talks about the original issue with the vehicle, which was poor performance and fuel trims high +45.

During the Conversation about the vehicle it was decided that a new air flow meter be installed and the vehicle test driven to confirm the fuel trims are coming back into line (+ or – 6%)
The vehicle was fitted with a new aftermarket air flow meter and fuel trims checked on the test drive.

The vehicle had been towed back to the workshop with the vehicle running rough and missing at idle with a very low idle.
This is when we where called to get our hands dirty and work out the issue that may have been masked by a faulty air flow meter.

Testing of the vehicle as it was given back to client.

The vehicle was dropped off – barely running, the vehicle cranked over very slowly, once running it was evident that the battery had given up the ghost and the throttle body was dirty as the low battery cleared all the fuel trims and engine management learning.
The vehicles battery changed and charge rate checked.
Next it was to test the vehicles engine management for any fault codes and conduct a fuel trim test and air mass meter test, following the test to check oxygen sensor and catalyst efficiency, this is the interesting part.

The first thing to note in the series of images and data sets is the idle Air flow the vehicle is a 2.5l (EJ25)
Idle Air mass – New After Market Air Flow Meter – 2.2g/s

Test 1 – Conducting Tests with the present faults and aftermarket AFM

Test 1 being Performed during warm up and the alternator output voltage checked. the engine has higher than expected idle fuel trims. +10%

The first thing to note is the long term fuel trims keep adding sitting there at idle and the oxygen sensors appear to be happy but the LTFT just keeps adding (strange).

Test 2 – Test drive with Aftermarket AFM

Fuel Trim chart after test drive high but nothing to see too crazy

Test 3 – AFM/VE test with aftermarket AFM

VE calculation – of Engines beathing capability VS the AFM output.

The above 2 images show the Aftermarket air flow meter working in the parameters that i would expect to see, as in the numbers are out but it doesn’t immediately point to any particularly problem.
(Thoughts on the issue with high 14+ air flow VE factor camshafts slightly retarded, bad fuel and the timing is out during acceleration. )

The engine was checked for air leaks and the air box air filter for correct fitting etc, no leaks no Evap valve stuck open, nothing.
Upper Cylinder clean was conducted to check if there could be any thing we might have missed. This resulted in a wasted can of upper cylinder cleaner (only for the test results – engine internals might have liked the cleaner operating environment haha!)

At Request from client the old air flow meter was installed to verify the original issue with the vehicle and it wasn’t mis-diagnosed in the first place.

Test 4 – Original MAF

Original Air flow meter and previous symptom, witnessed.

The Poor drivability of the vehicle and the air flow meter output being way less than expected for the engine to run properly +45% to 50% fuel being needed to be added to the basic feed forward system.
Fuel Trims Very BAD (chart not saved and uploaded) but the Idle air Flow was 3.2 g/s

This air flow meter was cleaned and checked again after reinstallation.

Test 5 – Start up with cleaned AFM

Test 5- Cleaned afm instantly removing the corrupted fuel trim table.

Test 6 – Test Drive With cleaned MAF

Test 6 – afm test data cleaned afm fuel trims good subaru

The air flow meter test after cleaning the test result shows the idle is 2.8 to 3.2 g/sec and bouncy, the air flow meter output did not peak or reported less air when engine is accelerated at higher Rpm, resulting in fuel trims ok +4% at idle and higher at high rpm +10

Test 7

A known good genuine air mass sensor was installed to the vehicle,
what a difference the vehicle sings now!

Test 7 – Known Good (TEST) AFM fuel trim test.

The Fuel trim table is now correct and the idle grams is 2.7 g/s which experience has told me to be right for this vehicle and engine.

the Fuel trims are now correct the VE table was filled out and documented in the image below.

Test 8 – Air Mass Test Known Good sensor

Test 8 – Known good afm cleaned – fuel trims good – air mass not expected but car good

The known good sensors output was lower in the mid range and correct at higher rpm which proved the output voltage was reached at maximum air flow, what was weird was the fuel trim table was still correct. so the vehicle is now fixed.
We concluded that this chart did not make sense but the vehicle performed correctly and the fuel trim table ideal so the vehicle was sent back to the owner FIXED.


After 2 and a half hours in the conclusion is aftermarket air flow meters may claim to work, but OE/ genuine sensor is suited to engine and engine ecu’s fuel mapping to combat all driveability issues,
This is not the first time aftermarket sensors have been installed and failed, 2 weeks prior a unit had been fitted to a Toyota Aurion with the same results of making the long term fuel trims add.

like we are learning all the time about aftermarket parts…….. Genuine is the only way.