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2001 subaru outback 3.0

cylinders 4,5,6 misfire. idles rough, occasionally when you throttle up, it will run smooth, and other times goes back to misfiring. Seems worse under load. Where do I start? New spark plugs and coil packs? They seem expensive if they arent the problem.

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4-6 are on the driver side of the block. It is unlikely that all 3 coils are failing.

Check your fuel filter and/or fuel pump. The driver's side of this engine gets fuel last in the fuel flow chart. The pressure regulator is also on this fuel rail and should be checked. It would be unlikely that the regulator is bad, but it could happen.

It sounds like it is vacuum or fuel related to one side. Improper fuel delivery causes a misfire. Too rich or too lean. So if its been a while since your last fuel filter change, start there, you'd be due anyway. After changing the filter, when you start it to check for leaks, listen for a whistle which would indicate the engine is sucking in air outside of the intake tube/box. There are seals at the base of the injectors that seal off the equalizer port and the intake gasket itself. Also, a few vacuum hoses connect on that side for fuel regulator, pressure equalizing ports and evap system which get hardened over time and may crack.

If you can get your hands on a fuel pressure gauge, one that ties inline to the fuel hose is available relatively cheap at a parts store, the pressure should be about 40 psi with the key on not running after the initial burst. You can tie in at the fuel feed hose right at the filter. Check the pressure idling and it shouldn't be lower than 38. You could then pull the vacuum hose from the regulator to check its operation, but this requires the removal of the injector cover which on the driver side is tight in. If you get this far, pulling the hose from the regulator will increase the pressure. If it doesn't jump to about 48, then the regulator is not functioning and has an internal leak.
 
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