I am having problems with my 2001 Legacy L (2.5 liter/manual transmisson). It developed a bucking sometimes in the 1500-2500 rpm range and the CEL stayed on. After some research I thought it was the TPS. I tested the old one and it was bad. I took the battery cables off to get the CEL to go off and replaced the TPS. It ran smoothly with the CEL off for about five miles then the CEL came back on, however the car was still running smoothly. Then about 150 miles later the car started bucking again. The bucking happens the majority of the time but not always. Could I possibly need a new computer? Should I just give up and take it to a mechanic?

Get the PXXXX codes read and come back and let us know what code is posted. I doubt you need a new ECU.

Agreed. We need to see the actual code or codes that was/are being set.

 

This "bucking" can be caused by any number of things, most commonly it's attributed to misfires caused by old spark plugs or wires.

When it first started doing it (over a year ago, this has been a long time battle im afraid) I did have the code read. If I remember correctly it was p420 and I was told it was most likely a catalytic converter. However, I did get it smogged a few months later and it wasn't the cat. The shop did replace the pigtail that hooks up to the TPS and it ran fine for a few days before starting the bucking again. I am just getting sick of the bucking and want it fixed once and for all. Replacing plugs and wires is next on my list, hopefully in the next week or two. I have read to only use Subaru made wires because they are made differently than regular spark plug wires. Is this correct?

If you take your car to a major auto parts store like Autozone or Advance Auto, they will come out to the parking lot and read your codes at no charge. You really need to know the result of a code read to guide you in repairs.

 

It is possible that your coil pack is bad, that along with bad spark plugs, and plug wires can cause bucking.

 

OEM Subaru wires and plugs are great, however, I have had great service from Bosch wires and Bosch plugs. Avoid cheap plug wires sold by auto part houses, as they can be junk.

P0420 is catalyst efficiency code. If the car smog tested fine then the front O2 sensor may be bad.

 

When the codes are reset the computer resets the fuel trim and ignition timing curves that it learns as it is being driven. It takes a few drive cycles for the ECU to "relearn" the settings that work best. This can take 2 hours or a week depending on how far you drive.

 

You can do a search of the board here for P0420. There are literally hundreds of threads concerning that particular code. You should find some good info about what causes it and what to address to fix the problem that is causing it.

So I replaced the plugs (NGK platinum) and wires and it's still bucking. I replaced the fuel filter as well. Someone suggested it may be the O2 sensors. Any suggestions?

Any suggestions?

 

 

Is there a code?

 

If so before you throw any more money at the car, bring us the code so we can guide you. You're shooting in the dark if you have a check engine light and haven't bothered to get the codes read.

Have a 2001 Subaru Legacy sedan that juts and bucks, seemed like transmission issue, Check Engine Light came on, got a reading of PO420 and a PO122 PO328 replaced the knock sensor, replace o2 sensors. it seemed all right for 4days and then it did the jut and buck again and then CEL PO328 but all ready replaced the knock sensor. At a loss on this. took it to a mechanic and he thinks maybe the brain box ECU? he said something about voltage not changing. he quoted like 500 bucks but if it is the ECU i see them on ebay from 50 to 200 bucks. any other thoughts would be helpful.

I have heard of difficulties for some reason on the slightly older cars is the pigtail of the knock sensor is routed wrong.

 

The torque spec on the knock sensor isn't much.

That could be, I remember Subaru calling the routing of the pigtail the 'cord extraction angle' or something like that. I assume that has to do with the translation. I have an image of that somewhere...

 

that is so it doesn't get broken when tightening it down.

 

sounds like you have a plugged catalytic converter. I had a 98 forester that did this. occasional P0420 and a sort of cyclic bucking, especially under load. the more load or the more I put my foot down, the worse it was. It started out very occasionally, then after a while became more frequent.

 

I pulled the cat off and the "comb" inside was melted and blocking about 3/4 of the flow.

i had thiss recently on a 2001 legacy is a problem with that year of mass air flow was not that good the car whould hesatae surge lug and just not get any feul changed the mass airflow and fixed the standard ones were realy not good it took me forever to find one and 3 bad ones before got one good one