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Pastimes : Auto Repair & Maintenance Tip
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To: Jon Khymn who wrote (63)11/5/1999 7:29:00 AM
From: rustyjack  Read Replies (2) of 134
 
Knocking....

High mileage cars can begin to knock due to a carbon build-up in the cylinders, which increases the compression ratio, and thus the need for a higher octane fuel. But this is not as common as it was years ago.
Knocking can also be caused by over advanced ignition timing ,but this too is not as likely due to electronic controls which automatically adjust timing. A knock sensor detects the knocking and sends signals to the computer to retard the timing. (It is possible the sensor has failed but not too common).
Knocking is also a symptom of the engine running too hot.

But the most likely cause IMO would be a problem in the EGR system (Exhaust gas recirculation). This system is used to dilute the fuel/air mixture with a small amount of exhaust gas, to lower the combustion temperatures in the cylinder. This helps prevent the formation of oxides of nitrogen and lowers harmful emissions. A carbon build-up can restrict the passages and reduce the flow of exhaust gas into the intake system. Or the system could be malfunctioning due to broken vacuum hoses or a faulty EGR valve or switching solenoid.

It would help to know what kind or car you are driving as many models have problems specific to them only and would make it easier to surmise the cause.
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