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10 Common Diesel Vehicle Repairs

If you own and drive a car, then you will know that feeling of dread when your check engine light comes on. As much as you may wish to avoid the expense of a mechanic, with diesel cars especially there is much that can go wrong if you don’t get the problem checked by a professional – the best option is always to always entrust the reliability of diesel garage services.

If you’re unsure whether an issue with your diesel vehicle is worth a trip to the mechanic, our guide will help you to understand common faults that show up in the diagnostics of diesel issues and when these require reparation.

Diesel Engine Fault Diagnosis

If you drive a diesel car, there are some faults that your engine is prone to. Below, we have listed 10 common issues you may encounter with your diesel engine.

Oxidised Oil

Oxidised oil occurs when oxygen reacts with the oil in your engine. This causes a chemical reaction, resulting in changes to your oil’s efficiency.

This can include:

  • Increased viscosity – Reduces efficiency.
  • Formation of sludge and deposits – Pose risk of engine failure.
  • Accelerated degradation – Requires more frequent oil changes.
  • Depletion of additives – Makes the oil less effective.

Unfortunately, this cannot be fully prevented, however measures can be taken to limit the effects of oxidised oil and prolong the quality of your oil before this happens. The way to do this is through the use of synthetic based oils.

The base component of these oils reduces the rate of oxidation, which in conjunction with the antioxidants they contain (that have a higher resistance to oxidation) prolongs the life of your engine. Speak to a diesel specialist for advice on the best oil to use for your vehicle.

Hard Starting

As the name suggests, hard starting is when your car is having trouble firing up. There are a number of issues that could be the cause of this, including:

  • Defective fuel pump
  • Contaminated fuel or the wrong fuel being used
  • Clogged fuel filter
  • Clogged fuel injector
  • Faulty ignition switch

If your car is hard starting, it is best to seek diesel garage services to identify the exact cause and fix it accordingly.

Contaminated Fuel

There are two types of fuel contamination that can affect diesel in particular.

The first of these is water contamination. Diesel is known to be hygroscopic, meaning it attracts and holds onto water particles – this particularly presents an issue when fuel tanks are left at a low level for a prolonged period of time.

There are different types of water contamination that can affect your diesel tank:

  • Free water – Water that is present in the tank and sinks to the bottom, sitting separately under the fuel.
  • Emulsified water – Water that becomes completely amalgamated with the fuel (usually due to heavy pressure and agitation), turning your fuel milky looking.
  • Suspended water – Water that binds to fuel molecules, making your fuel appear cloudy.

The other form of fuel contamination is microbial contamination. This happens as a result of the presence of free water in your diesel tank.

Microorganisms present within the fuel are able to grow with the presence of oxygen from the water, whilst hydrocarbons in the diesel offers them food and energy. As a result, they will grow where the fuel and water meet, depositing a layer of sludge as a byproduct in your fuel tank.

Contaminated fuel will stop your car from running smoothly and may result in further issues. It should be replaced by a specialist.

Defective Glow Plug

Glow plugs are the component that heats the fuel and air that comes into your engine to encourage efficient fuel combustion, which is required for your car to start.

A defective glow plug can impact your car’s ability to start and run efficiently. If your check engine light comes on, you’re experiencing hard starting, or you are seeing white or black smoke leave your exhaust, then there is a high chance your glow plug needs replacing.

Humidity Reactions

Increased humidity results in the presence of vapour in your engine, which has a roll-on effect on your engine’s injection efficiency.

When starting your vehicle, air needs to be combusted to a certain pressure before a fuel injector sprays fuel into the combustion chamber and is ignited to run the engine. The presence of vapour increases the duration of this combustion rate and reduces the ignition rate, resulting in lacking fuel efficiency.

If your car consistently misfires, you may need diesel injection repairs.

Black Exhaust

If excessive amounts of black smoke are coming from your exhaust, this is an indicator that you most likely have something wrong with your fuel delivery system.

This could be due to various causes (all of which need to be assessed by a professional):

  • Faulty injector – Faulty or blocked injectors can spray more fuel into the cylinder than required, resulting in an excess of carbon residue.
  • Faulty MAF sensor – If your mass air flow (MAF) sensor cannot properly calculate the air flow in your engine, it may result in other internal miscalculations, causing your vehicle to jolt or stall when driving.
  • Dirty air filter – A dirty air filter stops your car from receiving the amount of air that it requires, which will impact its performance when driving. Whether it needs to be cleaned or replaced depends on how dirty it is.
  • Clogged valve – Carbon deposits from your exhaust fumes can cause your valve to be blocked open or closed, reducing power and efficiency.

Lack of Power

When a diesel engine lacks power, there are several forces that could be to blame.

As well as any of the reasons on this list, other causative factors could include overheating, malfunctioning pedals, low fuel pressure, or multiple internal engine failures.

Due to this volume of potential causes, and considering that diesel vehicles are supposed to be powerful, if you feel that your vehicle is starting to lose its power it is highly recommended that you seek out a diesel mechanic to identify what could be going wrong and stop it from getting worse.

Failing Lead-Acid Storage Batteries

Your car’s battery is what sets off the glow plug, allowing your car to turn on; it is then recharged as you’re driving your car. Healthy batteries should last you around five years, although this is not always the case; signs could be showing that your battery is starting to fail.

If it takes longer for you to start your engine, for example, this could be an indicator that your batteries are losing power and will need to be replaced. Likewise, if your car fails to even attempt to start when you turn the key in the ignition, there is high potential that this is due to battery failure.

Another obvious indicator that your battery is failing is if the electrical features in your car stop working, such as windows or heated seats.

If you notice that your battery is failing, it may need to be replaced or repaired.


There are many noises from a diesel engine that are not atypical if they are only heard every now and then. In saying that, though, there are other noises that should be taken as a sign that something is wrong with your vehicle – some more seriously than others.

The not-so-holy trio of noises to listen out for are rattling, ticking and knocking.

  • Rattling: That distinct rattling noise often associated with diesel cars is due to the air and fuel combination in the compression cylinders. If ignited prematurely, this can cause an increased pressure that can damage other components within the cylinder, such as the valves or pistons. If you notice an intensified rattling sound in your engine, then it is best to have it looked at to check the condition of said components.
  • Ticking: Ticking noises tend to be indicators that the components in your engine are not getting enough lubrication. Checking your engine oil levels should be the first step you take when you hear your engine ticking; if your oil levels are okay, then it is best to take your car to a diesel mechanic.
  • Knocking: Knocking noises in diesel engines are commonly due to the fuel injectors. It could be that they are not well lubricated enough, or that there is a fuel build-up at their tips. If this is the case, or knocking persists despite using good quality fuel, then it may be best to get diesel injection repairs.

Higher Compression Ratio

A higher compression ratio is used in diesel vehicles compared to those which use petrol. This is because diesel is more volatile and able to combust at a higher rate, without the need for a spark plug.

If the compression ratio is too high, however, the components in the engine are under greater stress. Overtime, this can lead to damage that may require you to have a diesel engine rebuild or replacement.

How to Find a Quality Diesel Mechanic

If you are experiencing issues with your diesel engine and require maintenance and repair, finding a quality diesel specialist is crucial.

When looking for a mechanic, ensure that you find someone who is qualified, experienced and reputable. Do your research to see what other people have said about their services, or speak to family and friends to see if they know of a trusted diesel mechanic they can refer you to.

If you are looking for an expert diesel mechanic and reside in the East Midlands, Welham Group are who you need. Taking pride in our services, we specialise in diesel engine rebuild, restoration and replacement services across all types of diesel vehicles. We even offer vintage truck and bus repair, and vintage tractor repair.

Get in touch with us today with your query, a member of our team will be more than happy to help.