What you should know about engine knock

Knocking combustion causes major engine damage, and also reduces the efficiency. Photo by Roland D. Nasasira

What you need to know:

  • An overhaul comprises replacing parts such as gaskets and piston rings.
  • The cost of carrying out an engine overhaul depends on the car brand.
  • Peter Amadi, a mechanic at Dalas Auto Limited in Bunga, a Kampala suburb, observes that before your car experiences an engine knock, it runs smoothly.

Occasionally, an engine will develop noises that are difficult to diagnose. A “knocking” sound is the most typical engine noise that is an indication of a mechanical problem that can be fixed if diagnosed correctly and early.

At the start of September 2018, Emma Miriga’s Subaru Legacy Turbo suddenly stopped. As a result, he towed it to a garage. On reaching there, he was honest and told the mechanic that he had last serviced it in January 2017. The service tag indicated that he had driven 5,000 kilometres above the recommended service schedule.
“When the mechanic checked for engine oil, there was about one litre left in the engine block. He also found out that some sections of the engine block had changed to brown,” Miriga says.

Peter Amadi, a mechanic at Dalas Auto Limited in Bunga, a Kampala suburb, observes that before your car experiences an engine knock, it runs smoothly. When you start hearing a clicking or knocking sound, it is an alarm that something is wrong. It could be a problem with the oil, oil filter, oil pump or oil sump trainer. When you hear these sounds, Amadi recommends visiting a nearby garage.
To be safe, park the car in a safe place and check your oil level. It could be that oil pressure is low and the oil pump is not picking up enough oil.

Do not ignore noise
“Do not ignore the noises. Be patient, park the car and find a way of getting a mechanic to come to you. Inside the engine block are the piston and main bearings where the piston and crankshaft and other parts run. The moment these stop working, the engine does not rotate. If you continue driving and the engine is spoilt, you risk spending more money to get new parts such as the crankshaft, bearings and cylinder bolts where the piston sits,” Amadi advises.

“In case of a worn out piece bearing, your mechanic opens the engine and replaces the piece bearing. But when you keep driving, you damage more parts such as the piece and main bearings, copper bush bearings and can end up breaking the piston. In some cases, the car may require a car carrier because some mechanics can tell what the problem is by just describing to them the kind of noise made,” he ads.

Amadi says the moment your engine experiences a knock, it has to be opened first to assess the extent of damage. The damage then helps you and the mechanic to determine between replacing the entire engine or repairing the one you have. Most times, repairing the one you have is cheaper than buying a different engine. Alvin Kidega, a mechanic, says normally a new or used engine costs Shs3.5m.

“You may buy another engine thinking it is perfect but when you are not sure of how it works. One month or a short time down the road, it breaks down not due to an engine knock, but something else and the dealer will not refund your money. You may end up spending another Shs3m to fix the new engine, yet you could have spent approximately Shs1.8m to carry out an engine overhaul on the one you had,” Kidega explains, adding that overhauling an engine is not as expensive as it used to be in the past because parts are readily available.

To buy new spare parts for an overhaul, Amadi strongly advises visiting a minimum of four to five dealers for price comparison. “For example, there are dealers who sell piston rings at Shs150,000 while there are those who sell the same piston rings at Shs300,000. Some dealers get old piston rings and keep them in new boxes and when they open them before you, you are quick to pay and conclude that they are new, yet they are in actual sense old,” he warns.

Engine overhaul
The cost of carrying out an engine overhaul depends on the car brand. The cost of overhauling a Mercedes Benz or BMW is different from one for a Toyota. However, at Dalas Auto Limited, overhauling a Toyota engine of 1500cc to 2200cc is approximately Shs2m. This includes changing the oil and most parts as recommended in damage assessment. For brands such as the Audi, BMW and Mercedes Benzes, the charge increases.

What engine overhaul entails
An engine overhaul is carried out after experiencing an engine knock or when your mileage has reached a certain mark, say 150,000 kilometers as advised by the manufacturer.

An overhaul comprises replacing parts such as gaskets and piston rings. But because car owners do not trust mechanics, for price comparisons, a car owner makes his own requirements and so does the mechanic to avoid cheating. Nevertheless, it is advisable to give a mechanic time to do a thorough job.

“If you ask me to fix a certain part on your car yet I have confirmed that your engine experienced a knock, which is risky, it is as good as doing it yourself. If you want me to fix a given part, I ask you to put it in writing so that in case anything happens, I am not blamed,” Amadi says, adding that when you take too long without servicing your car, the engine compartments turn brown, which is also considered engine knocking.

Causes of engine knocks

Using adulterated is one of the causes of engine knocks. On the market, look for the right oil quality. It is possible to replace your engine because of something as small as on oil filter. Before the oil goes up to the engine chamber, it is supposed to go through the oil filter to rid it of any dirt.
The oil filter is always under the oil pump. If the oil filter is chocked, oil does not go up. Some oil filters have paper elements inside them. Due to pressure and poor quality, the paper element leaves its position and blocks the oil from going up. Amadi recalls a scenario where a car owner paid him Shs7m to overhaul their engine because they had bought a poor oil filter at Shs15,000.

Oil pumps
Sula Kilimani, a mechanic in Kisekka Market in Kampala, says if oil overstays in the engine, it becomes dirty. This makes the oil pump become weak due to thickening of the oil. This also causes an engine knock. He advises that you make it a point to change your engine oil at least every 5,000 kilometres covered.
Engine oil and the oil filter must be changed at every service. In some cars whose gear boxes come with oil filters, if you replace the Automatic Transmission Fluid (ATF), replace the oil filters as well.

Overheating also causes an engine knock. If you drive the car and the AC is on and the temperature is normal, you could be out of water in the radiator due to a leakage, or when the fan has stopped working. As you drive, the engine will just. Signs of an overheated engine include a clicking sound.

Amadi advises against permanent use of your car AC to allow you look at the temperature gauge to determine if the engine temperature is okay. When the AC is on, the fan is on. This means that it cools the radiator and when you check the gauge, you think you are okay, yet you have no water. You are losing water but you do not. Amadi says overheating can even blow the engine block, something he says is worse than losing engine oil.


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