Subaru Legacy 1999 2.5L piston misfire code p0301, 02, 03 & 04

Parts salesman have responded that the most likely cause of this is the Cam Position Sensor is malfunctioning. I’d replace it if it was cheap or knowing it would solve the problem, but an auto mechanic instructor responded it is most likely not OEM spark plugs. I’ve put in two sets of plugs for this problem before I’d heard this, the second set was certainly not cheap, but I don’t recall the type without taking them out. I also don’t know what the OEM is other than NGK. Suggestions?

There should be a sticker on the underside of the hood saying what the OEM plugs are.
Likely NGK or Denso.
I’m not familiar with the details of this car.
Does it have a distributor? If so, have the cap and rotor been replaced? Coil?

Did you look in your owners manual??

Misfires can have lots and lots of causes. So you are right to not just go along with someone’s guess.

Misfire codes do often just come from issues with spark plugs & wires. You’re stuck on the plugs. How are the wires? Then there’s the rest of the ignition system - coil/distributor, for instance - however it is equipped.

But then there are the other 3 issues - air, fuel & compression. Poor fuel pressure, injector problems, intake leaks, running too rich, compression issues. Lots of people think that misfire codes are mostly about spark. Often they are - but they usually just mean that the computer saw a “hiccup” in the rotation of the crank shaft that comes from combustion not taking place in a cylinder at the right time. Spark is only one part of combustion.

People also assume that the computer will “tell” you if you have some other kind of issue - such as an intake leak or fuel injector problems. The car’s computer isn’t nearly as “smart” as many people want to think.

So take care of the plugs & wires - perhaps other parts of the ignition system. While the plugs are out a compression check is not a bad idea. If that doesn’t help move on to checking fuel pressure & for vacuum leaks.

Its free to pull the cam sensor and inspect it and its wiring.

Interesting that you mention fuel pressure. I have a hard time filling my gas tank, the nozzle likes to click off. I have verified that both the filler tube and its companion return tube are unobstructed but the spagetti of tubes to the charcoal container, where most of the excess volume of air is sent when filling, is probably where the obstruction lies. I presume that since gas in the tank goes down much slower than filling it up, that the relief of its pressure is much less obstructed; however, maybe not and therefore the engine codes?

There’s a lot of things that can cause multiple misfires. Just my opinion, but I think the possibility of any and all mechanical faults should be weeded out first.

This means a compression test of all cylinders. Parts can be thrown at a vehicle with a compression fault until the end of eternity without solving anything.
While it would be unusual to occur with all 4 cylinders a misfire can occur if the valve lash on any valve lifter is too tight. Valve lash should be inspected every 30k miles but seldom is this ever done. I will also add that I’ve personally seen tight lash on multiple valves on a number of occasions so having a problem with all cylinders is not unheard of.

Assuming compression is good (180 PSI give or take) and there are no codes present then the number of possibilities increase.
Any history of head gasket problems, cylinder head removal, etc on this car?

I’m not sure about the head lash – had to look it up, I presume you’re talking about rod to cam distance. This is something I recall on cars from my childhood.

In answer to the last question, the 2.5 liter Subaru engine had a design flaw with the block. This engine had it’s short block replaced just over 100,000 miles ago meaning the head gaskets had been replaced but the heads are original. No discussion suggests this engine has issues with the heads that I’m aware of.