Intermittent low power issue

[3viction]

Hi there, I have an automatic Subaru Legacy 2000. About 6 months ago I took it into the mechanic because the engine light was on - turns out it was because of the knock sensor and o2 sensor, they replaced both of them, the fuel consumption decreased a lot but then I started having low power issues.

 

Around 1500 rpm the power would be dead and the car would crawl along until I put my foot flat down and then it would sort itself out after a few seconds and eventually take off. This doesn't only happen when turning a corner and accelerating but also if I'm going a speed where the revs drop to around 1500, then it'll often go sluggish and start losing speed until I push my foot quite far down again. I took it back into the mechanic who said it was the air flow meter (or sensor?) and they cleared it and it seemed to fix it but only for a week so they ended up replacing this too. This issue is now still there and they don't know what to do to fix it and neither do I... already spent far too much money - they assure me its none of the parts they replaced as they're all genuine new subaru parts.

This issue is intermittent and seems to come and go randomly, will often disappear after driving for a good 10-15 mins. When the problem is happening often it smells a lot like burning when I get out and turn the car off.

 

I have read a few other threads, some which mention the knock sensor and others say stop throwing money at the car and try cheaper fixes first. What should I do?
 

Thanks

[lmdew]

Get the code # and let us know what they are!

Find a good Subaru shop or USMB member in your area!

 

Yes changing parts is not a way to fix a symptom.

[3viction]

There is no longer any fault code. After they replaced the first 2 sensors the fault code went away but the low power issue came about.

[3viction]

Any suggestions? Thanks

[Legacy]

I had a similar intermittent low power/hesitation issue, though for me the vehicle is a 97 Civic. (I used to have a 91 Legacy, but that's besides the point...). This started in Aug/Sept 2014. Sometimes it would happen several times in a trip, sometimes not at all.

 

The symptom for me was that I would try to accelerate, and it would feel as though I had taken my foot off the accelerator. If I kept the accelerator pedal in the same position, I could feel the RPM bleeding off (not sure of the exact numbers, the car doesn't have a tach), then it would suddenly start accelerating again, then bleed off, etc in a "sawtooth" pattern. This was always once the car was warm, so it suggested the O2 sensor was faulty (O2 sensor is ignored when engine is initially warming up).

 

To diagnose I disconnected the harness from the O2 sensor. It ran a bit rough at idle like this but I never got the low power/hesitation with the O2 sensor disconnected. One thing though is that I never got a check engine light or any codes, so I was still not sure the O2 sensor was the culprit. I changed it anyway as this was a driveability issue and it was getting progressively worse. It was happening more often and eventually on every trip. I changed out the O2 sensor at the beginning of January and it has solved the problem for me. In my case I got a NTK sensor rather than going to Honda but I had looked at the old sensor and identified it as NTK so stuck with that.

 

But you've already changed the O2 sensor so most likely it's NTK or Denso, who both make the sensors for the Japanese makes.

 

The other possibility I read about when researching my issue is the throttle position sensor. It may have one or more "dead" spots throughout its range, where it puts out no voltage rather than what it should be for that position of the throttle valve.

[3viction]

I was wondering if it was an issue with a sensor they replaced as the problem only started after they replaced the first two sensors.

Is it common for a throttle position sensor to be playing up occasionally?