Jim Grant's Tech Tips
‘96 Chrysler Town & Country, Phantom Misfire
Q: My wife's ’96 Chrysler Town & Country gave a P0300 Multiple Cylinder Misfire code. The first time this happened we towed it home. When the car arrived home, it cranked up and drove normally as if nothing had happened. As a precaution I changed the suppression cables, spark plugs, and the MAP sensor, after 2-3 weeks same thing. I brought her the other car on the Turnpike and it drove as if nothing had happened. First, I turned off the check engine light with the scanner. We're afraid to bring the car to a Dealer or mechanic shop because of the way those guys crank up the bills is hard to deal with and asking for bankruptcy. Why aren't there any books that can tell you exactly what to change when a certain diagnostic code shows up? What is the best we can do to make this P0300 code disappear forever?
First, there is no exact repair or parts replacement procedure for a multiple misfire code. Too many factors can cause that code to set. That is why the repair/diagnostic procedure has the technician perform several tests to determine the needed repair. To make matters worse, your vehicle only exhibits the condition/problem once every 2 to 3 weeks. From the diagnosis and repair side of things, that is one tough problem to solve. After all, if it is not broken, what do you fix? Because the problem is intermittent, it does help in narrowing the possible causes. For example the ignition system. If the spark plugs, plug wires or ignition coil packs were the cause, the problem would be more constant. Why? Once an ignition related component starts degrading the condition becomes more pronounced. Apply the same thought process to the fuel system. If the fuel pump was starting to fail causing a drop in fuel pressure it would be very unlikely that the pump would get better for 2 to 3 weeks and then act up again. If the engine had a mechanical problem, it would be just as unlikely to get better for weeks at a time and then go bad. It would be a unique situation for any one of these systems to act up like that. This leaves computers and emission controls. Either one of these systems can do what you’re describing. It is unlikely the computer is bad, but it is possible that there is a poor connection. A faulty power supply or ground connection can cause the computer to do some bizarre things. Lastly and what I believe to be most likely is the emission controls. I would target the EGR system. The EGR valve can stick open causing the engine to run rough resulting in a random misfire code to be set. Because the computer’s program prioritizes, a misfire code would go to the top of the list and the computer would not be running any test on the EGR system at that time. If the EGR valve, EGR solenoid or the Pressure Transducer for the EGR valve were to stick, they could easily return to normal once the engine was turned off. Restart the engine and the problem would be gone and each component would test just fine. In the event, you determine the EGR valve is the problem “do not” buy an aftermarket EGR valve. Get a factory EGR valve; it comes with a new solenoid and pressure transducer as well.