I have a 2008 Subaru Outback, 2.5L non-turbo engine, 4 speed auto transmission. The car has just shy of 200k miles on it.

About 3-4 months ago, I thought I noticed a slight lack of power while ascending a highway mountain pass. I only noticed it on the steepest part of the highway, so I thought nothing of it. However over time it got worse, and now my car doesn’t accelerate if I press the gas pedal more than a tiny bit. This happens regardless of the engine speed.

My fuel consumption has also gotten worse. I used to get 20 city and 27 highway mpg, and now I get about 10 mpg across the board. There is currently no check engine light.

Probably unrelated, but the car has been loud, and I think there’s probably a hole or crack in the muffler causing that.

I recently had the following repairs done (or done them myself); all of these were done after the problem first started (some of them were regular maintenance things, not necessarily because I thought they would fix the problem).

  • Replaced leaking head gasket (including replacing all the coolant, oil, and some other gaskets). When the shop put the engine back together, they did a compression test of some sort for all the cylinders and it was apparently “normal”.
  • Replaced variable valve timing switch because they were leaking and had caused a check engine light.
  • Replaced the timing belt and worn idlers
  • Replaced the spark plugs (the old ones were pretty bad and had been occasionally misfiring)
  • Flushed the transmission

I took it to a mechanic, and they found a dislodged baffle in the airbox (it’s been temporarily screwed back in place), but they said the lack of power is because the transmission is slipping.

This doesn’t make sense to me because if the transmission is slipping, I should see a discrepancy between the road speed and the engine speed when I try to accelerate, right?

Could my lack of power be caused by the transmission, or should I be looking for something else? Is there anything else I could try first before I put a lot of money into a new/rebuild transmission?

  • Welcome to Motor Vehicle Maintenance & Repair! Commented Jul 7, 2019 at 21:36
  • Check that the exhaust remains free-flowing. If, for example, you had a progressive physical failure inside a catalytic converter, or a muffler within which the packing had begun to melt and move, the exhaust path can be obstructed. A progressively-plugged exhaust could generate the symptoms you report. Commented Jul 7, 2019 at 22:38
  • The open source project @ romraider.com (laptop and cable required) allows you to check the long term fuel trims. We recently used this on the 2006 model of your car that showed similar symptoms to find airflow restricted by 20%.
    – ajayel
    Commented Jul 19, 2019 at 17:44

1 Answer 1


The problem was an exhaust restriction.

I determined this when all 4 cylinders misfired at the same time. The freeze frame data indicated that the intake vacuum at the time was only ~7 inHg and the long term fuel trim was around -11%. Disconnecting the exhaust between the front and rear catalysts allowed the engine to run normally, confirming that it was an exhaust restriction.

A 2008 Subaru Outback (and many other Subarus) has 2 catalysts. The front catalyst on my car is fine, which is why the O2 sensors didn't detect a catalyst failure. It seems like it is the rear catalyst that was clogged up, causing an exhaust restriction.

It turns out that there were some front catalysts that Subaru recalled. When the defective front catalysts failed, they would blow out and clog the back catalyst. I don't know many details of the recall, but for anyone experiencing a similar failure, you should contact Subaru to see if you can get your back catalyst replaced as part of the recall. (Otherwise it's ~$1000 for that part in the USA.)

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