Subaru Repair: 2000 Subaru Outback losing pressure, Subaru Expert

Question
I recently saw one of your answers and it is the same problem that we are having.  The car while driving decellerates and then picks up again, and then continues doing this.  The problem is the worst about 1 mile into driving and up to 10 miles.  Then it clears.  It also seems to be worst when the gas tank is fill or near empty.  We had the Throttle Position Sensor changed out and it did not make any difference.  The fuel pump was changed out and that corrected the problem for 10,000 miles but now it has come back again.  The garage has checked the fuel pump and has said that it is not showing any problems.  It said that one of the exhaust sensors was faulty and so changed that out today.  How can the Fuel pressure, exhaust back pressure and a throttle position sensor sweep be conducted?

Answer
Lisa,

I don't really understand what you are asking.  Are you asking for the specific test procedure?  

Diagnostics are about a process of elimination and is not a exact science.  

A good Technician familiar with Subaru should be able to diagnose your problem as long as he or she can reproduce the symptom.

The process will vary by the situation and there really is now way I can type out any specific test of a sensor or a device, that is what service manuals are for.

As far as how each Technician will approach the situation will also vary by the symptom, the techs experience and the information available to he or she.    

Keep in mind that there are many things to fail and they can all give like symptoms when they do.  so if a fuel pump fails (which is not common at all) and 10k later a another sensor fails and creates a lean condition the symptoms you feel may be similar.

Justin