Can't diagnose the Subaru!

subaru
outback

#1

My 2008 Subaru Outback with 40K miles has been at the Subaru dealership for 15 days and still no closer to fixing or diagnosis. CEL light and cruise light flashing, and running poorly…Initially, I would step on the gas and little to no acceleration, then finally stalled out. The fault code found was P0171 with a lean mixture. The dealership changed all the fuel in the car, changed the intake manifold gasket, fuel pump and finally the egr valve. None of this has worked. They are trying, but they are supposed to be the experts. Any ideas on what could be wrong with my car?


#2

The fuel injectors should be the next step. Maybe a bad batch got through Quality Control. Could also be ignition related Coils, spark plugs, etc., although they seem focused on the fuel delivery system.


#3

Thanks so much. I just phoned the dealership and told them to check those items. I’ll let you know what I find out. Thanks again!


#4

MAF sensor or Oxygen sensors are probably the culprit.

MAF sensor tells the ECU how much air is entering the engine to be burned.
The ECU then tells the Fuel injectors how long to stay open in order to add fuel.
Spark plugs ignite the fuel.
Oxygen Sensors then tells the computer how much Oxygen is present in the exhaust, and enriches or leans the mixture to keep it inside its preferred range.

If the ECU doesn’t get the correct information at the beginning or the end of the process, then you get the issues you are having.

BC.


#5

Thanks very much. The latest is that the engine computer is faulty…Will keep you posted. Thanks again!


#6

I would consider a vacuum leak or a leak in the intake tract; the latter of which could affect the MAF sensor.

A vacuum gauge can be connected in seconds to the intake manifold and this will reveal instantly if a vacuum leak exists. The intake tract will require a careful visual inspection. An '08 with 40k miles is really not aged enough to suffer rotted or cracked vacuum lines but something could be amiss due to a prior servicing if someone inadvertently dislodged a vac. line, breather hose, etc.

The problem though? Few mechanics use a vacuum gauge at all even though it’s one of the handiest tools that one can own.

Offhand, it does sound to me like they’ve been guessing; and making a number of bad guesses at that. By that I mean they’re replacing things that really should not be behind this problem.
It’s very easy sometimes to assume a problem is worse than it really is and you would be surprised at how often the cause is something very very simple.