I ran my 2002 Forester by the mechanics who usually check out my car, pretty intuitive guys.   The diagnostics were showing lean fuel, P0171 I think.   Put your foot on the gas and it lugs, not the usual alertness and power.  I was thinking letting the car run low on gas, maybe sucking scum from the tank into the fuel filter could be the cause.   I started filling up with premium and used some STP fuel treatment and today some Lucas fuel treatment, says to use 3 oz per 10 gallons.   I ordered a fuel filter and some catalytic converter cleaner.  The mechanics were thinking something may be broken inside the Cat. converter.  I replaced it with an after market unit in the winters of 2021-22.   Any recommendations on this?

Contact the seller of the CAT and see what they have to say.

This was the Walker unit I got:   https://www.amazon.com/gp/product/B001AGPD96/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

Bought in early 2022.

Lean fuel - check for a vaccuum leak in the engine bay.

Does it do all the time, once in awhile, only at start up, on highway, driving slow....?

I did not see any vacuum leak;  I had it checked out by the mechanic and their computer code device which came up with the same code as mine.   It's pretty continual acts as though the clogging builds up, why we thought it was a the cat. converter.   

 

Diagnostics look pretty complicated as recommended on Amazon's site.   Any good mechanic could nail this down in person, but not the Hispanic immigrant mechanics except their "instinctual" judgments.

This is their usual diagnostics on the Amazon site, apparently the warranty is through Amazon:

 

1. Use an OBD-II scannerNote: Refer to the user manual for the exact location of the OBD-II port.Note: Refer to the user manual for instructions on how to do this for different models.Note: Error codes related to the catalytic converter usually start with P0420 to P0439.
   1. Locate the OBD-II port under the dashboard.
   2. Plug in the OBD-II scanner and turn the Ignition to ON
   3. Scan for error codes
   4. Interpret the codes
2. Perform a visual inspectionNote: Refer to the user manual for the exact lift points on your vehicle.Note: Typically located between the engine and the muffler.Note: Physical damages may require immediate replacement.
   1. Safely elevate the vehicle using a car lift or jack stands.
   2. Locate the catalytic converter.
   3. Inspect for physical damages like cracks or dents.
3. Test exhaust backpressureNote: Refer to the user manual for instructions on how to do this for different modelsNote: High backpressure typically indicates a clogged catalytic converter.
   1. Remove the oxygen sensor before the catalytic converter.
   2. Connect a backpressure tester in place of the oxygen sensor.
   3. Start the engine and check the backpressure reading.
4. Conduct a temperature testNote: A properly functioning catalytic converter will have a higher outlet temperature compared to the inlet.
   1. Use an infrared thermometer to measure the inlet and outlet temperature of the catalytic converter.
   2. Compare the temperature difference

Edited February 14, 20242 yr by ThosL

probably need to test for a vacuum leak not look for one  

check the timing belt belt alignment and pulleys

check fuel pressure

if you have a spare MAF, swap it to test that  

clogged cats can glow red - look at it after driving. 

I'm running it on premium, put in some Lucas and STP fuel treatment and ordered a heavier duty gas treatment.    I'll see if I can get proper help to run those tests.  

The car is running ok generally.  Just a lack of acceleration and bogging when going uphill or trying to accelerate.   I'll figure this out and let others know.  Auto Zone has a lot of free tools as rentals.   The Hispanic guys who've worked on my Forester in the past said they had a Legacy Outback with similar mileage they were willing to make a good deal on.  Look, it hasn't been that many miles since they last worked on the Forester.  They are bad with diagnosis.

Video on diagnosing fuel pressure failure:   

 

Have you tried the classic method of chasing down vacuum leaks by spraying carb cleaner at various vacuum fittings while it's idling? If there's a leak, the car will increase in RPM for a short time if there's a leak near/where you sprayed the carb clean.

As for your other possible issues, idosubaru pretty well covered it.

One other thing I would recommend is pulling the electrical connectors off your injectors and check their resistance across the terminals. If you have one way off from the others, it might be a dead/dying injector.

Twitch

Yeah you can just spray carb cleaner or startintbfouod around all the hoses with the engine running and listen to the engine. If it sucks any in due to a leak you’ll hear the engine respond immediately. im sure you can google it. 

fuel pressure. Just need to hook up a pressure gauge into the rubber line by the drivers side fender well. Subaru doesn’t have an existing port to check it. You just have to Tee into it.
 

 

Changing the fuel filter made no difference.  I spoke with a mechanic at the local Gulf station who kindly gave me his best guesses.   Very likely the MAF sensor is at fault as the vehicle is bogging down as I said and bucking from time to time.  I picked up the Auto Zone free fuel gauge tester and will try to find a couple lines to connect it to tomorrow.

24 minutes ago, ThosL said:

Changing the fuel filter made no difference. 

It usually doesn't.  I've never seen a Subaru have any change of driving due to replacing a fuel filter.  

24 minutes ago, ThosL said:

Very likely the MAF sensor is at fault as the vehicle is bogging down as I said and bucking from time to time.

 

On 2/14/2024 at 6:34 AM, idosubaru said:

if you have a spare MAF, swap it to test that  

It might not be it, but it's the one EJ25D vehicle sensor I've seen cause issues but not throw a code. 
 

Apparently the "MAP" not MAF sensor.   Mass air pressure sensor.

 

Also was getting a code for O2 sensor #1 but have been ignoring it as it has been on a while without performance issues.

Fuel pressure was 36 or more, I assume that is normal from what I've seen online.

I ordered three MAP sensors on Amazon, two were wrong and the third I get tomorrow.   It was a mystery to me where the sensor was, so I disconnected other ones to see if any of them looked like the right one and settled on the one to the right of the coil pack.   I may have messed up the connector as I couldn't get it to snap into place.  Oil pressure gauge was flashing when driving but power came back.  Will hopefully get it fixed tomorrow.

Edited February 18, 20242 yr by ThosL