After replacing the engine in a 2007 outback 2.5l. The engine starts fine warm and cold. The engine has plenty of power cold but loses power as the car warms up. I've been swapping sensors and even the throttle body, one at a time. No changes. No codes Help?

EDIT: (Additions from comments)

Replaced the EJ253 engine in my 2007 Outback with a known-good running engine from a 2006 Legacy EJ253 mated to a 4EAT. I switched the keeper on the timing belt, and added the flywheel, and mated it to a 5MT.

The engine starts fine warm and cold. The engine has plenty of power cold, but loses power as the car warms up. The power loss seems to get progressively worse the warmer the engine. No codes.

What I've tried:

  • Swapping Sensors
  • Throttle Body
  • Knock Sensor
  • New Spark Plugs and Wires
  • IAT Sensor
  • Dropped Exhaust Manifold
  • Welcome to Motor Vehicle Maintenance & Repair! Jan 24, 2018 at 1:21
  • Did you replace the engine with a used, rebuilt or new one
    – CharlieRB
    Jan 24, 2018 at 1:46
  • The motor was used. It was a automatic I switched the keeper on the timing belt added the flywheel for my manual transmission. Jan 24, 2018 at 10:38
  • Did you have any way to verify the engine was good?
    – CharlieRB
    Jan 24, 2018 at 12:54
  • 1
    Wow a lot of variables. Including trans differences and starting with a used motor. And so many other points of failure. But: compression test each cylinder. Check fuel spark. Look for codes, check all harness connections three times. Check timing, can't actually set it but look at it. Maintenance 101. Come back here with data. Swapping so much stuff-you might be out of your depth. I'm experienced and engine swaps are daunting even for me!
    – geoO
    Mar 28, 2018 at 14:31

1 Answer 1


Pay particular attention to the temperature sensors, the IAT (intake air, likely part of the mass-airflow unit and the ECT (engine coolant temp).

A decent code reader will allow you to monitor these and see if they go out of whack.

Also, see if the problem occurs when the vehicle changes from "open loop" (cold baseline) and "closed loop" (using feedback from the O2 sensors). You can also see this on a good scanner that has "PID" parameter display capabiliity.

Subarus also have a hyper-sensitive piezo knock sensor that can cripple timing (and power) when going into closed loop, without throwing codes. This is often caused by overtightening. Mere inch-pounds are sufficient. Cranking down on this sensor makes it edgy and causes a huge retard of timing along with an associated loss of power. It's a two-wire donut thing with a 10mm head bolt on the engine block, under the throttle body area on the driver's side.

  • I changed the knock sensor ,no change. Put new spark plugs and wires no change . I replaced what I believe is the iat sensor. Mounts on top of the throttle body, two 10mm bolts. No change. I even dropped the exhause manifold . Loud but no change or codes. The readings seem in prameters I'm a weekend warrior . The power loss seems to get progressly worse the warmer the engine. Jan 27, 2018 at 19:51
  • The IAT is most likely part of the MAF unit. Rather than replace things, get a cheap diagnostic scanner that can display the PIDs (parameters like coolant temperature or intake air temperature. Looking at these may point out something that is obviously wrong, without lifting a wrench. Also, see if the problem happens as it transitions from open loop to closed loop on the ECU. Pay particular attention to temperatures, timing advance, air flow volume, and fuel delivery. The explanation is probably fairly simple.
    – SteveRacer
    Mar 27, 2018 at 3:55

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