[norcal_not_nevada]

I have been experiencing power issues for a couple of years now with my 03 Forester. It has about 186k miles on it now.
I finally got everything together and tried to do something about it.
For a while my acceleration was poor at best, and going up a hill was a struggle.
I've replaced Spark Plugs, Wires, Fuel Filter, Air Filter, all the fluids, Seafoam through the gas tank a couple of times, and have a can of Seafoam aerosol I have yet to put into the throttle body.
Has anyone had similar issues? I am worried that after all this it means that my exhaust has a leak or blockage somewhere. I should also add that every now and then I get a P0420. But I always pass smog.
Or is it simply that the car is getting on in years?

 

[Kevin]

Your cat or O2 sensors are the issue here and the symptoms you describe fit exactly.

Take a look at these threads:

• P0420 - Catalytic Converter Efficiency Below Threshold
• $5 fix

 

[2.5x_sleeper]

@norcal_not_nevada I have some questions:

[1] Do you have any other CEL codes?
[2] Is your gas mileage normal?
[3] Has your front O2 sensor been replaced?
[4] Did you install NGK spark plugs?
[5] Is your timing belt maintenance up to date?

From the Subaru technical service manual:

4. Poor acceleration

1) Manifold absolute pressure sensor
2) Intake air temperature sensor
3) Throttle position sensor
4) Fuel injection parts (*4)
5) Fuel pump and fuel pump relay
6) Engine coolant temperature sensor (*2)
7) Crankshaft position sensor (*3)
8) Camshaft position sensor (*3)
9) A/C switch and A/C cut relay
10) Engine torque control signal circuit
11) Ignition parts (*1)

*1: Check ignition coil & ignitor ASSY and spark plug.
*2: Indicate the symptom occurring only in cold temperatures.
*3: Ensure the secure installation.
*4: Check fuel injector, fuel pressure regulator and fuel filter.
*5: Inspect air leak in air intake system.
*6: Adjust accelerator cable.

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]

 

[norcal_not_nevada]

@Kevin I’ll have to check out the $5 fix. That’ll be a good stepping off point before I make and huge rash decisions regarding my exhaust.
@2.5x_sleeper
I have no other CEL codes.
Gas mileage is a little worse than it was a couple of months ago but that is due to my spare tire mounted on the roof
I have never replaced the O2 sensors since I got the car 9 years ago.
I did install NGK sparks&wires.
The timing belt was done prior to me purchasing the car at 110k miles.

As far as what the manual says, I’ll have to start going through those. I did replace the MAP sensor about a year ago.

 

[2.5x_sleeper]

The front O2 sensor can be failing long before it sets a CEL code. The Denso O2 sensor are on the expensive side, so you might want to check the other items first. :wink:

Do you have a way to verify the engine is transitioning from "open loop" to "closed loop"?

Open Loop:
When the engine is first started the system goes into open loop operation. In open loop, the ECM calculates the Air/Fuel ratio based on inputs from the coolant and MAF sensors. The system remains in open-loop until the following three conditions are met:
• Coolant temperature above 105°F.
• Oxygen sensor has varying output voltage (indicating it is hot enough to operate properly).
• A specific amount of time (preprogrammed into the MEM-CAL) has elapsed.

Closed Loop:
When all of the above conditions are met, the system goes into closed loop operation. In this mode, the ECM calculates the Air/Fuel ratio based, mainly, on input from the oxygen sensor and maintains the Air/Fuel ratio at the desired 14.7 to 1.

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]

 

[norcal_not_nevada]

I can confirm that the coolant gets above 105F and I assume the predetermined timeframe is met every time. I guess it all loops back to O2 sensors again. Looks like I know what I’ll be doing next weekend now! I’ll check back in with my results. Hopefully all goes well.

 

[2.5x_sleeper]

The Subaru engine ECM works best with Denso O2 sensors. Your Denso aftermarket OEM style O2 sensor is 234-9015. I've seen good prices on Amazon with free shipping. :wink:

As to the rear O2 sensor, it's only purpose on the '03 is to monitor the exhaust for excess O2 & it's not used for engine management. We've had the $5 fix on my son's '03 X for years.

► Disclaimer - always verify any information you read on the internet, especially if you're going to spend money on parts! :icon_eek:

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]