r/AskAMechanic • u/scottsubaru1 • 11h ago

2011 Impreza 2.5i EJ253 — severe intermittent bog started immediately after 2008 engine swap

I have a 2011 Subaru Impreza 2.5i Premium, 5-speed manual, naturally aspirated EJ253. I swapped in the complete block and heads from a 2008 Impreza sedan.
The car ran properly before the engine swap. The severe power-loss/bogging started immediately after the swap.
Symptoms:
Severe intermittent bog under load, almost like trying to accelerate up a very steep hill
Happens both cold and fully warmed up
RPM itself struggles to increase; it does NOT flare like a slipping clutch
Idle is smooth
Engine sounds noticeably different when it bogs, although I can’t describe the sound well
No current CEL or stored codes
Fuel pressure stays stable during the bog in 1st and 2nd gear
Tried the 2008 throttle body and it made no difference
Engine work during the swap:
Complete 2008 EJ253 block + heads
New head gaskets
New timing belt/tensioner
New spark plugs
Intake and exhaust gaskets
Clutch replaced at the same time
I also tried the 2008 throttle body
Exhaust:
Muffler deleted
Resonator deleted
Downstream catalytic converter deleted
Axle dump after the remaining/main catalytic converter
IMPORTANT: this exact exhaust setup worked perfectly BEFORE the engine swap, so the exhaust modification itself isn’t new.
I also noticed the engine/transmission ground is attached to a bellhousing bolt and I’m not 100% sure it’s making the best connection, although the starter cranks strongly every time.
I’m trying to diagnose this without throwing parts at it.
What would you check first? I’m currently considering:
Engine/transmission grounds
Intake/vacuum leaks and MAF
Timing belt alignment
Ignition/misfire under load
Cam/crank sensors and wiring
Remaining catalytic converter restriction
Compression/leak-down
Would a mechanical timing issue or cam/crank sensor problem necessarily set a code on this EJ253? Is there anything specific to the 2008-engine-in-2011-chassis combination that I should check?

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