Skip the auto shop - Our mechanics make house calls

Q: Po 301

asked by on

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!

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Car is bogging and losing power - 1996 Chevrolet Blazer

It is not unusual for there to be multiple failures when there is an engine management problem, particularly on a high miles engine, so the issue you had with the defective MAF sensor may simply have been just one of...

Q: Air suspension system flat

There are many components involved in air suspension. Some basic failures could have occurred such as a fuse, a faulty compressor, or an air leak. Each component needs to be checked individually. Check the switch in the trunk, check you...

Q: What causes the truck not to go?

Hi and thanks for contacting YourMechanic. Check the throttle position sensor (TPS) for voltage and resistance. There should be a 5 volt reference voltage to the sensor with a ground. There should be a resistance on the sensor. If there...

Related articles

P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the...
P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...
What are the Car Pool Rules in Hawaii?
Hawaii is widely regarded as a land of vacation and relaxation, and as such, its scenic roads and routes are far better known than the state’s freeways. But, as with all...