Nothing causes more anxiety when the engine of a gleaming classic begins to vibrate and knock, most often without warning.

All the warning lights in the world cannot prepare the driver for such an alarming experience.

The reasons why an engine will suddenly begin to knock cannot be ignored – as they are often a strong indication of more severe problems not too far down the road.

There are a few reasons why an engine will begin to knock, cough and splutter, with the most common occurring fuel starts to burn unevenly in the engine's cylinders.

Cylinders in the engine should remain set at a level where they provide the correct balance of air and fuel. Fuel is consumed in small and concise quantities in this mode instead of in large and uncontrolled packages.

As each fuel pulse burns, it generates a minute electric shock, igniting the next one, thus continuing the cycle.

When fuel is not regulated evenly within the cylinder, it will burn faster, causing that unwelcome knocking noise that, if left unattended, will soon wear an engine's pistons and cylinder walls .

While this scenario is the most common among the shortlist of factors that can cause engine knock, the others are no less problematic and noisy, so proper diagnosis to find the exact cause should be the first order of the day,

One problem which should never happen but often does, especially in engines that have been tuned for performance, is if the fuel in the cylinders is at a lower octane level than it should be. The higher the octane, the more uniformly fuel will burn, and the risk of engine knock will be eliminated.

Classic car owners should always ensure that the correct octane rating is always used to fill their tank.
Knocking can also raise its head even when the car's engine is being turned over. This situation will occur when one or all of the spark plugs fail to fire in sequence, causing multiple detonations in the cylinder that will rapidly lead to "engine knock".

Another possible source of engine knocking can be caused by Belt Tensioners/Pulleys gradually coming loose. If the tensioner begins to malfunction, the belts become stretched and loose, or a pulley becomes bent out of shape, the engine's rhythm will be disturbed and will display its distress by sending out rattling, clicking and slapping noises that might be mistaken for engine knock.

The good news is that all of these problems are simple to fix

While issues with cylinders, fuel belts or spark plugs are the most common and most readily solvable, others that can cause knocking are rarer and more expensive to repair.

The first of these is when the engine's bearing have begun to wear, causing a painful sounding syndrome known as "Rod Knock."

Rod Knock affects the vehicle's pistons, which turn the crankshaft which transmitting power to the wheels. Crucial parts known as rod bearings will eventually wear out or prematurely fall out position after extended use.

Worn bearings will mean pistons rattling against the crankshaft, causing a loud knocking sound. Repairs to worn bearings, pistons or crankshaft will almost always mean an expensive repair bill.

When experiencing "engine knocking", the first reaction is to rush off to the nearest repair shop to get the problem solved. A cool head is called for before taking this step as, the truth is, most of the issues that cause an engine to knock may be repairable "in house."
Using a simple and inexpensive piece of equipment known as a mechanics stethoscope should be called in to play.

A mechanics stethoscope is used to isolate noises within a car engine. For example, in cylinders, the stethoscope is so accurate that it can detect which particular cylinder or cylinders are malfunctioning.

Checking spark plugs is more complicated, requiring each plug to be removed individually to insert the stethoscope to detect a change in tone. The same procedure can be used to check the fuel injector harness.

Another invaluable and inexpensive aid that is handy to have around in such a situation is the simple compression tester, which will give an accurate but sweeping picture of the engine's state of health. Compression testing alone will not be sufficient to diagnosis more specific problems.

In order to more specifically trace the type of problems that will cause an engine to "knock", access to a vacuum gauge or a leakage tester ( or both) will make for a thorough final analysis.

Time spent finding the source of the" engine knock" will be well spent, especially if the necessary repairs can be rapidly and inexpensively attended to either in-house or at a local garage.


All Categories

Got a question, a comment, a suggestion or an offer??? - FEEL FREE TO CONTACT US ANYTIME!!

Take me back to the homepage.

All you need to know about restoring and maintaining UK or European cars of the Fifties and Sixties- as well as the history of the great cars of the era and the people that produced them.