Check Engine Light Code P0300 On GM 2.2L and 2.4L
We have been seeing quite a few problems with the GM 2.2L and 2.4L engines. Mostly on model years 2005 through 2009. This engine is very common in the Chevrolet Malibu, Cobalt and HHR. As well as the Pontiac G5 and G6. This is the 4 cylinder. The problem I am going to talk about today is a check engine light and misfire codes.
Check Engine Light
Now, there are many, many things that can cause an SES light. Anything from a loose gas cap to any number of failed sensors. But this particular problem is with a computer trouble code P0300. This is a general engine misfire. For a particular cylinder, the code would be P0301, P0302, P0303 or P0304, as the last number refers to which cylinder is misfiring. On a scan tool we can actually watch the cylinder misfire. This problem can be caused by a faulty spark plug, malfunctioning ignition coil or ignition module, a plugged or restricted fuel injector or an internal mechanical problem.
The problem we are seeing with this engine is the misfire on a warmed up motor, only at idle. If you press the gas pedal just a little, the fault goes away. There is also no problem at higher speeds or steady driving or accelerating. Normally we mechanics would first look for a vacuum leak. This would cause the problem at idle any any motor. When no vacuum leak is found, the ignition and fuel system components are tested and shown to be working properly, then the next step is to look into an internal mechanical fault.(See Pictures Below)
There are a few test that can be performed to test the overall health of the engine. First would be a cylinder compression check. That is done by removing the spark plugs, disabling the fuel injector and ignition systems, and cranking the motor over with the tool installed. We watch the gauge for several things. First, how high the pressure jumps on the first revolution, and then what is the highest reading after a few seconds of cranking. Cylinders are compared to each other looking at both these readings. This will show the difference, if any, of compression between cylinders. That can give an indication as to what cylinder to concentrate on. But since we are using a scan tool to watch live data stream, we know which to look closely at. A compression test can be a little misleading. Low total compression can be caused by worn piston rings, incorrect valve timing, or valve and seat problems. Depending on the condition, this test may not show any abnormal readings.
In this case, the compression on one cylinder is a little low. Squirting a little motor oil in the cylinder and running the test again shows no improvement. If it had, we would suspect worn piston rings, as the oil would have helped seal up the piston rings and raised the compression, which is not what is happening in this case.
So the next test on this 2.2L or 2.4L, which is much more comprehensive, in what is called a cylinder leak down test. This is a little more involved. This entails removing the valve cover, loosening the camshafts (this is an overhead cam engine) and removing the rocker arms (also called followers) to let the intake and exhaust valves come all the way up and seal the combustion chamber.
The the tools is installed to pressurise each cylinder with a regulated amount. What we do is look for a percentage of leaking of this pressure. A certain percentage of loss means there is a problem. So now we have to find what is leaking. Listening in several different places is what is needed to locate the bad part. Opening the throttle to listen for the leak in the intake side. Listening in the exhaust pipe will show a problem on the exhaust side. Either way, the cylinder head must be removed.
After removal, the valves and valve seats are inspected. A great test to find where the leak is, is to turn the head over and fill the combustion chamber with water, or even better, washer fluid. The blue color makes it easier to spot the leaking valve and or seat. See Pictures below.
The fluid is leaking past the exhaust valve seats and into the exhaust port. This is causing a sort of vacuum leak inside that cylinder. More specifically a lean condition. This is why there is a misfire at idle. Above idle, at faster engine RPM, there is no time for the cylinder to leak, so no engine misfire. The repair for this particular engine misfire is to replace the cylinder head and bad valves. This aluminum head is not repairable, so replacement is required.
Again, this problem is specific to the 2005 through 2009 2.2L and 2.4L. So if you are having an SES light and misfire or rough idle, be sure your mechanic does a thorough inspection and diagnosis to avoid guessing and replacing parts. A simple tune up or sensor is not going to fix this one.
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