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1998 Subaru Forester PO301-PO304 Cylinder Misfire: Symptoms, Causes & Solutions

Model: SUBARU Fault Code: P0300 Posted: 2009-01-28 18:59

My 1998 Subaru Forester (2.5L DOHC, manual transmission) is showing intermittent cylinder misfire codes: PO301, PO302, PO303, and PO304 — appearing one at a time in sequence but not all simultaneously. The idle is slightly rough, and when the engine is cold, it 'bucks' under acceleration. Once warmed up, there's a slight hesitation during acceleration. These misfire codes appear occasionally at idle or highway speeds. I've already replaced spark plugs (platinum), ignition wires, coil pack, throttle body, TPS, EGR, injectors, PCV system, intake manifold, and gaskets. The intake vacuum reads 15 hg — which I believe may be low. Fuel pressure is 35 psi when the engine is off, 30 psi while running, and increases to 39 psi when the vacuum hose from the fuel pressure regulator is disconnected. The head gaskets were replaced with updated units and are performing well. The car sometimes runs smoothly for the entire drive before suddenly becoming rough at a stop sign or idle. I'm unable to find a repair manual specific to my model year, and I have limited knowledge about Subaru technical specifications. Can anyone confirm the correct manifold vacuum and fuel pressure values for this 1998 Subaru Forester? Any insight into why misfires occur intermittently and how to resolve them would be greatly appreciated.

Related fault codes
P0300P0301P0302P0303P0304P0306
Comments (3)
Anonymous 2009-02-12 19:13

I don't have a definitive answer, but one forum user suggested using original equipment (OE) spark plugs and ignition wires. My recommendation is to visit your local library and locate a repair manual — those thick 3-inch books contain all the technical specifications you're looking for. Good luck with your diagnosis!

Anonymous 2009-02-15 12:10

Hello! I found that the correct fuel pressure specification for my 1998 Subaru Forester is 43.5 psi. After replacing both the fuel pump and regulator, the engine now runs smoothly without any misfire issues.

Anonymous 2010-02-13 05:12

I recently performed a head gasket replacement due to cylinder 2 misfiring. After the repair, the car started exhibiting rough idle and would occasionally stall when stopped at traffic lights — followed by a check engine light reactivating for cylinder 2 misfire. This behavior wasn't present before the repair. Does anyone know what could be causing this stalling after head gasket work? Was it possible that the installation or sealing caused an underlying issue?