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!
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).
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?
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.
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.
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.
Take a close look at this:
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.
I was original poster - When the 02 sensor was finally replaced, car ran fine and runs fine.
Thanks for the reply!
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.
By the way. Just wanted to recap my Subaru?s symptoms:
- Engine Surge while in Drive and stopped at stop signs and lights. Shifting into neutral stops the surging and idle stabilizes.
- Engine occasionally stalls or almost stalls when driving very slowly like through a parking lot.
- 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 + ).
- 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.
As others have already suggested I would suspect the MAF or O2 sensors. I know others have fixed this kind of trouble by replacing at least one of the two sensors. I would try monitoring the power and signal voltages to those sensors to see if that would help pin the correct sensor down. You could first try replacing the O2 sensor if that is the orignial one since it is not real expensive to do that.
Posting back on my 2000 Subaru Outback Sedan with the issues I mentioned above. Last night I swapped out the O2 sensor and the car now runs GREAT! Problem Solved!
I should mention that the O2 sensor is not easy to get at in this car. You can’t just put a wrench on it because you can’t even see the base of the sensor and it’s in a very tight spot. I purchased a O2 Sensor socket (has a “U” shaped cut out for the wire, about $10) for my socket wrench and even though there was not room to put the socket wrench on the end to turn the socket… this tool saved the day. The “U” shaped groove that runs down the length of the socket means that the profile of the socket is not a circle but instead a “D” shape. So I was able to use the sliver of space I had to put a crescent wrench right in the middle of the socket to turn it. Once loose I removed the socket and used a set of pliers to turn it the rest of the way. The biggest help was having someone on the top side holding the wire and turning the wire with the rotation of the sensor so it didn’t get all wrapped around itself. From start to finish, it took about 30 minutes.
During the fix, I noticed that one of the rubber boots was torn on the drive shaft… so on to the next repair! Ugh! 
So i also own a 2002 subaru forester and am having very similar problems. It now has 95000 miles on it and about 1000 miles ago it started this surge stall issue. Mostly at low speeds, it stalls when you take off and then will either sputter or surge quickly. Sometimes driving at speed it will start jerking with the engine surging then dropping in rpm’s creating a jerk. Sometimes it idles great, sometimes it idles really low and horribly and occasionally dies.
I started by having the spark plugs replaced. That had no effect. I just had new belts put on as they were squealing and could have been related but again no fix. So I just got the coil and wires replaced and yup you got it nothing fixed. I am interested in any info about possible sensor problems. I took my car to subaru dealer, but they seem to not have run any diagnostics and are really only guessing on my dime. The check engine light has not come on so there are no codes for a specific problem… Does anyone have any ideas… you help is greatly appreciated…
I had the same problem with my Subaru Outback 2002 (141,000 miles).
Surging, stalling, no error codes or check engine light.
Replaced dead temperature sensor – but that was no help.
Replaced the front “Air Fuel Sensor” – that fixed the problem.
Total cost to me: $900 for various troubleshooting adventures.
Hope you get by cheaper.