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2002 subaru forester surges and hesitates

For the past month, intermittently, my mechanic (a good mechanic) has been trying to figure out what is causing my 2002 subaru forester to surge when when stopped, hesitate intermittently when stepping on gas pedal and generally run crappy. He's



1. replaced the fuel filter, run scans on the check engine light (says oxygen sensor is wearing out - he disconnected it as a test and still runs crappy)

2. checked out timing belt (is ok - put in new one 2 years ago)

3. replaced pcv

4. replaced temperature coolant sensor

5. (check engine code also came back for fuel trim problem - but Subaru doesn't have mass air flow sensor)



This car does have higher mileage than average - 150k



Help!! my mechanic is about to throw up his hands and I need my car back!
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Comments

  • edited February 2009
    I would suggest that he start to focus on the IAC and/or the MAF sensor. At least that would be my next step(s).
  • edited February 2009
    Also check the intake air (vacuum)

    He replaced the fuel filter, but did he run a fuel pressure test on the fuel system while the engine was surging?
  • edited February 2009
    It could be an engine computer problem if nonsense codes come out. Due to a shortage of good head gaskets in Subaru engines, I would wish for a compression test.
  • edited February 2009
    I should add that it was also sometimes idling so rough that it will stall out at stops or red lights. Sometimes it will run fine for a few miles, then start hesitating/running rough again. I've also found that it runs better when temperature (outside) is 40 degrees or above.
  • edited February 2009
    In point number 5 of your original post you state "but Subaru doesn't have mass airflow sensor."

    I think perhaps the engine does have a MAF. On my Subaru the MAF is adjacent to the air filter box.

    I'm in agreement with VDCdriver's advice about the MAF or the Idle Air Control.

    In addition, if the O2 sensor is not functioning correctly it should be replaced. The O2 sensor(s) are very important for correct engine operation. They DO wear out with time.
  • edited May 2009
    Take a close look at this:
    http://www.import-car.com/Article/43516/subaru_tech_tip_diagnosing_idle_surges_on_legacy_models.aspx

    My 2000 Subaru Outback Sedan just starting acting up with the same problem so I'm goint to try that. This has to be related to a sensor providing bad data to the computer about airflow or air/fuel ratios. Somewhere there is a sensor that is only working correctly when the RPM climbs over idle... thus it's working well enough for the computer to believe and react to the data from it. While I have it open, I'm also going to throw in a new air filter.
  • edited May 2009
    I was original poster - When the 02 sensor was finally replaced, car ran fine and runs fine.
  • edited May 2009
    I ran those instructions in the link and it had no effect on my issue. BUT, last night my subaru finally threw an error code. YES! "P0170" = Fuel Trim Malfunction (Bank 1). Called the local dealer and they said the cause is almost always the front oxygen (O2) sensor, but there was a chance it could be something else and the only way to know for sure was to bring it in for a full diagnostic. So, you are probably right jlapro. ;) I'm planning on replacing the front O2 sensor tonight and if that fails, then I'll bring it in. The car has 150,000 miles on it, so the O2 sensors are probably both due for replacement anyway.

    I'll post back on the results.
  • edited May 2009
    By the way. Just wanted to recap my Subaru?s symptoms:
    1) Engine Surge while in Drive and stopped at stop signs and lights. Shifting into neutral stops the surging and idle stabilizes.
    2) Engine occasionally stalls or almost stalls when driving very slowly like through a parking lot.
    3) Engine power is jerky at all speeds (used to be just at slow speeds but getting worse) unless you are deep into the throttle at higher RPM (like 3,000-4,000 + ).
    4) Accelerating from a stop it often times wants to cut out and you have to feather the throttle to get it to go like it should.
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