2005 Forester XT sporadic lurching, misfiring,

[jameswooder]

175kms, good condition. Vehicle sat for 10 days over Xmas. When started I noticed a slight to moderate, sporadic lurching, misfire. No particular rhyme or reason. No code triggered. No dealership where I live. Tried injector additive to gas, no luck. Local mechanics no help. Got a tip that someone with the same vehicle has a a similar problem with no warning code and replaced the knock sensor. DId that yesterday and $300 later no luck. I am frustrated. Now looking at maybe sourcing a second hand oxygen sensor. Any thoughts or ideas appreciated. One weird anecdote; when you hammer the thing it responds without an issue, then when you back off to steady state driving it comes back. Also seems to be worst when slowly accelerating in 1st and 2nd gears. Thanks

[GeneralDisorder]

This is a turbo charged model. You need to proper scan tool with live data. You need to check fuel trims and O2 sensor data. Your symptoms match a variety of potential issues. O2 sensor is the most likely candidate - get a new one. Used is just silly and at that mileage it should be replaced anyway. Could also be an intake leak, etc.

 

There is very little difference between that engine and a 305 HP STI engine. It's a highly complex, production rally car engine. You need to treat it with proper respect and care or it will be a steaming pile of aluminum with quickness. Do not troubleshoot with Visa. Get a scan tool and learn to use it or find someone that can.

 

GD

[wtdash]

^+1

 

Unfortunately, that front 'upstream' O2 sensor is an expen$ive Wideband- type AF sensor - you can find the Subaru OEM parts on ebay or amazon for much cheaper than the dealer. ......REALLY should be new and OEM quality.

 

The O2 sensor should be in the exhaust header/manifold: http://www.scoobymods.com/showthread.php/front-oxygen-sensor-o2-sensor-9398.html

 

 

 - I'd also clean your MAF/IAT sensor - carefully - as they can cause issues. 

 

FYI:

1.  This model also has a Catalytic converter in the up pipe (before the turbo) w/w/an EGT sensor. Highly suggested to replace the up pipe w/an STi catless version (find 'em used on Subie sites).The CC in the up pipe can melt / break / come apart and send all that mess into the turbo. There is a resistor-mod (same as on WRX) to eliminate the EGT sensor - it's only there to 'save' the CC from melting so not required w/a catless up pipe.

2. Read up on removing/replacing the screens / filters in the oil lines that go to the heads and turbo....they require maintenance.

3. Change your oil EVERY 3K miles / 4500 kms...one of the best/easiest ways to keep the turbo engine happy.

Edited April 13, 2017 by wtdash