An engine knock can mean trouble

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IOL mot pic may11 Engine Cutaway 1 . A cutaway of a four-cylinder pushrod engine showing the oilways.

Most of us are a little nervous when buying a used car, especially when it is more than a few years old and perhaps has covered a high mileage.

Service history, if available, is a generally good guide, but when that is not available it pays to have the vehicle checked by a reliable mechanic or the AA.

We all dread the heavy knock that suggests wear on the big-ends and the expense of a complete engine overhaul to put things right.

However, there is a great difference between a bottom-end knock and a light tapping from the top-end; the latter is normally no cause for concern.

Nevertheless, I was asked to give my opinion on an engine producing such a noise.

The car was being sold privately and it seemed that both vendor and potential buyer were not very knowledgeable about such things.

The noise occurred only on start-up and disappeared after a few seconds.

There was, in fact, nothing to worry about. The initial light tapping is quite normal with hydraulic tappets.

The whole point of these is to do away with the need for regular tappet adjustment. Oil pressure fills the tappets and they then expand to the correct setting.

Naturally, when an engine has been left standing for a time, the oil tends to drain from the tappets and may also take a few seconds to get to the cylinder head; hence the initial light rattle on start-up.

Not all engines do this - even the same make and model could prove entirely different.

If the noise persists after warm-up, it’s possible the tappets are worn or dirty and so are letting oil escape. If it sounds worse after warm-up, there could be camshaft wear.

When engine-oil change intervals have been irregular or so rare that they are unrecorded, change the oil and filter.

Use only top quality oil and filter.

Economising on oil will eventually cost you dearly. Flush the system to make certain the engine is really clean.

As for the oil change, this can be done either by removing the sump drain plug and flushing afterwards, or the far cleaner vacuum-hose method.

A car that has been regularly serviced may quite safely use the latter; it saves time and is not messy and helps prevent possibly dangerous skin contact.

But on a car with a somewhat erratic service history I would remove the drain plug, or even the sump itself, and still use flushing oil to clear the rest of the system.


If on sump removal there are pieces of metal, rather than the very fine residue caused by normal wear, then further inspection is called for because this means serious wear and immediate action is necessary.

Light top-end tapping may largely be ignored. If in doubt seek a second opinion. Heavy knocking at the lower part of the engine requires immediate attention.

Some people do not trust the vacuum hose, but when you consider that most of the dirt will be in suspension and not resting on the sump bottom, the hose is capable of doing a good job.

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