Many factors could cause a cylinder to misfire on a 2007 PT Cruiser. Apart from the rough running engine, the computer will set a code identifying the cylinder in question, as evidenced by a check engine light on the instrument panel. A code scanner, when plugged into the OBD access port, supplies the code number and description of the fault. A code P0304 indicates that the No. 4 cylinder is experiencing a misfire.
Problems Caused By a Misfire
A misfire caused by the ignition system exhausts unburned fuel into the exhaust system that enters the catalytic converter and causes its temperature to rise dramatically. When the severity is such that the converter integrity will soon be compromised, the check engine light will cycle on and off to warn the operator of an impending converter failure.
The catalytic converter temperature, under these circumstances, will turn the converter red hot, which can easily start a fire if it is located over any flammable material. Additionally, the unburned fuel will wash the engine cylinder walls and pass the rings where it will contaminate the oil supply, reducing its viscosity.
Causes Of an Ignition System Misfire
Oil infiltration into the spark plug well in the cylinder head is a common problem this type of engine faces. The cause is contributed to rubber gasket leaks at the base of the spark plug tubes in the valve cover. The metal tubes used to access the spark plugs need to be sealed at the top and bottom to prevent the oil within the valve cover from seeping through and onto the spark plug and wire boot.
The oil saturates the spark plug boot and causes a short circuit the tube. The remedy to this problem is replacing the valve cover gaskets, being sure to include the circular spark plug well gaskets on the base of the tubes. Once this is done, you must replace the spark plugs and wires.
If the spark plug wire boot for the No. 4 cylinder isnt saturated with oil and the valve cover gaskets are intact, the problem is elsewhere. Inspect the spark plug gap and test the wire and coil for a malfunction.
Fuel Injector Related Misfire
A bad fuel injector for the No. 4 cylinder can cause a rough running cylinder, which appears similar to a misfire. This can be determined by unplugging the fuel injector electrical connectors momentarily one at a time. Note the rpm drop when the injector is disconnected and reconnected on adjacent cylinders. If the No. 4 injector rpm drop is significantly less than the rest, the injector is the problem.
Ensure that the injector electrical connector has a good connection and that you can hear the injector ticking as it operates. In order for the injector to work, it must have power and must not leak. If none of the above problems exist, the injector is fouled and must be replaced.
Vacuum Leaks
A vacuum leak in the intake manifold near the No. 4 cylinder will also cause a misfire. The leak will originate at the base of the intake manifold or at the O-ring at the base of the fuel injector. Propane or carburetor cleaner introduced in these areas will cause the engine to smooth out, confirming a leak. Replace the injector O-ring or intake manifold gasket, if an intake leak is present.
Mechanically-Induced Misfire
If all else has failed to identify the problem, you need to perform a compression test. This test is used to determine the condition of all the components within the cylinder. Each cylinders compression must be within 5 to10 percent of the average compression on all the cylinders for a good balance. This is to ensure that all cylinders are producing an equal amount of power. A compression test checks for burnt or poorly seated valves, piston ring sealing, cracked cylinder heads and head gasket problems. Any of these will cause a misfire through lack of compression within the cylinder.
The camshaft lobes, which are responsible for opening and closing the intake and exhaust vales, are another source for a misfire and rough running engine. The cam lobes for the No. 4 cylinder should be inspected to see if they have been worn down due to lack of oil or other mechanical problem. A damaged camshaft lobe will not allow a valve to open properly if at all.
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