Q: Po 301

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Hesitation rough idle missing hard start when warm mis gets worse code po301 keeps coming up already replaced plugs wires cap rotor injectors egr gasket compression 150 no leak down

My car has 120365 miles.
My car has an automatic transmission.

P0301 is a misfire in number one cylinder. I believe you are already aware of this. A common problem on these is the injector assembly. I would do everything below before I condemn it. If I were there in person, I could tell the difference between a cylinder problem and an injector problem.

You need to perform a leak down inspection. It is not necessary to use the percentage type that tells you how much air is leaking out of the cylinder. You just need about 50 psi of air pressure and a hose that will thread into the spark plug hole. With air pressure in the cylinder, listen from the tail pipe, open the throttle and listen there and listen in the crank case with the oil filler cap removed. If you hear a hissing in one of these areas, you have a problem in either the exhaust valve (tail pipe), intake valve (intake), or the rings (crank case).

You can have good compression even with one of these having a problem. A dynamic compression test will build pressure and small amounts of oil will seal small problems, but this changes when the fuel is ignited and the pressure sky rockets.

Keep in mind, you could also have a head gasket failure. Does the coolant keep getting low? Do the spark plugs have a tinge that matches the color of the coolant? Is the cylinder wet from coolant? A good way to test for this is to warm the motor thoroughly and pressure test the cooling system. A cooling system pressure tester will pressurize the cooling system in the same fashion as what occurs when your motor warms up. Once you have pressure in the cooling system, let it sit for at least five minutes. Sometimes longer. I have let it sit for as much as a day of work, depending on the suspected failure. If you have a head gasket issue in number one cylinder, this will push coolant into the cylinder. You should be able to spray air into it and get some wet coolant blowing around. Do what ever you need to see if there is any coolant in the the cylinder.

If you should require more assistance, I recommend getting an engine is misfiring inspection.

Good luck!

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