A 2000 Chevy Blazer with a 4.3 engine and about 165,000 miles on it came into the shop, and the owner was complaining of a check engine light, poor fuel mileage, and a code P0300. P0300 is a random misfire code. The PCM, or powertrain control module, calculates engine misfires by closely observing the speed of the crankshaft through the signals it receives from the crankshaft position sensor. Code P0300 means the PCM has detected a misfire or misfires, but the frequency has not reached a point where the PCM can call out a particular cylinder or cylinders as the cause.
Our first step in this diagnosis was to determine which cylinder, or cylinders, were misfiring. An ignition, fuel, or engine issue could have caused the problem. The easiest thing to check on this vehicle is for an ignition or spark problem, so that was where we started.
After starting the engine we could immediately feel an intermittent misfire. An easy way to check for an ignition or spark misfire is by using a timing light or spark tester. We connected a timing light to each of the ignition wires and observed the light. If the cylinder is firing, the light should consistently flash in time with engine RPM. When the light flash skips you have found a cylinder with a spark issue.
All the other cylinders appeared to have a good spark, and it looked as if cylinder number two was the only one having a problem. We removed the spark plug from cylinder two and it looked old, but it looked as if it could still fire. We then tested the number two ignition wire. The spec for suppression wires is no more than 8,000 ohms per foot. The wire going to cylinder two is approximately two feet long, which means it should not read more than 16,000 ohms. When the wire was moved around, the resistance was going far above the spec.
We replaced all the ignition wires and the plugs on this engine. We also did an ignition tuneup, which included inspecting, and possibly replacing, the distributor cap and rotor.
After inspecting and removing the distributor cap we found another interesting issue. There was corrosion on the terminals inside the cap. This corrosion occurs because of the moisture and excessive amounts of ozone trapped inside the cap.
When the gap between the rotor blade and one of the cap towers is jumped or bridged by an electrical flow, ozone gases are created inside the cap. This is the primary reason vent holes are incorporated in distributors.
A service bulletin for this vehicle said we should remove and discard the screens in these vent holes and blow compressed air through the holes to ensure they are open for proper ventilation. When we looked closely at the bottom of the rotor we saw that there were fins around the outer edge to promote ventilation and ozone removal. As the engine runs and the rotor turns inside the distributor, these fins are constantly pulling fresh air in through a vent hole and pushing harmful ozone gases out through the other.