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Random misfire from 800-1200 RPM under load


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I know this isn't a Legacy question, but a Subaru question for Phase 2 motors.

 

My 02 Forester has an intermittent issue where I will get a nasty misfire from 800 on up to 1200 RPM.

 

Head gaskets were replaced 2,000 miles ago. The engine was never overheated, coolant was topped off every other day to make sure it still was reliable until I fixed the motor. All measurements specced to normal tolerances and new head bolts and updated MLS gaskets were used. I've tested and retested again and the head gasket isn't leaking internally or externally, nor am I losing coolant. All the carbon buildup was removed from the heads and the pistons before reassembly. I experienced the issue before when I discovered my head gasket was leaking and it presumably went away after the head gasket job was done, or so I thought. I am thinking the head gasket and compression leaks as a result were not the cause of the misfire.

 

The vehicle has 208,000 miles on it, throwing no codes, AFR sits steady at 13:1 at idle, the vehicle idles beautifully and pulls hard in the higher RPM range. Timing belt and water pump done at 195,000 by the Subaru dealer.

 

Any idea where to start?

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Coil Pack dying

Spark Plug(s) fouled or gapped too wide

Pinched or damaged wiring harness

Loose or faulty wiring connector on coil pack or injector

 

You would typically get a CEL for a misfire or "too lean" for these though.

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My initial thought was the coil pack dying. On cold starts or after it has been sitting a few hours, there is no misfire in that range. The misfire also isn't consistent. For example, I will be driving home from work and at 3 of the 5 signal lights I stop at it won't do it when I accelerate from a stop. I tend to rev the engine to about 1300-1500 RPM to coordinate letting out the clutch and that is when I will notice it, before I start to let out the clutch. Humidity also seems like it plays a factor. After a monsoon storm rolls through it hates the humidity and will misfire every time I step on the gas (but only in that range of 800-1200).

 

I think I'm going to do a full tune up on it (coil, plugs, and wires). Funny you say that a loose or faulty wire would throw a CEL. My wife's 01 Outback 3.0 never threw a code when one of the COP wires came loose. We almost parted out the car because the misfire was the final straw after replacing the entire engine.

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