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Troubleshoot Intermittent Engine Problems

Intermittent engine problems can be a nightmare. Problems that come and go or only occur under certain driving or operating conditions can drive you nuts - unless you have a diagnostic strategy to outwit these kinds of problems.

One thing all intermittent problems share in common is they are not a steady-state condition. If an engine dies and refuses to start, you can check for spark, fuel and compression to start isolating the cause. But when you have an engine that runs fine one minute, then dies, bucks, hesitates or misfires, then runs fine again, it is a different story. Something is obviously occurring that is interfering with normal combustion. But what? Is it the ignition system, fuel system, computer or something else? That is where a diagnostic strategy comes in.

The worst kind of intermittents are those that occur infrequently, say once a week or less. Your odds of correctly diagnosing an infrequent intermittent are probably the same as winning the lottery. Unless you are lucky enough to catch the problem when it is occurring, youll have few clues to go on.

You can always guess at a diagnosis based on a hunch or previous experience, but the odds of hitting the target every time are pretty slim. That is how parts changers "fix" cars. They replace the part they think might be causing the problem, and when that fails to cure the problem they replace something else and so on. Eventually they may replace the right part that was really causing the problem. But in the meantime, their poor customer has to pay for a lot of parts they didn't really need.

Adapted from an article written by Larry Carley for Underhood Service magazine


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