What you need to know about diesel injectors

A quick Internet search will come up with a plethora of products and methods that claim to clean dirty diesel injectors. However the reality is that unlike petrol injectors, diesel injectors aren’t that prone to developing build-ups that block their nozzles or affect fuel atomisation. In fact the symptoms that many attribute to dirty injectors – loss of power, missing on start up, poor running, increased fuel consumption and exhaust smoke are more often than not related to issues other than injector deposits.

Diesel injectors operate at very high pressures, up to 1800 bar (26,000psi) is not unusual, and this helps keep them clean, but it also means that, in time, they wear out. Depending on the type of injector and the vehicle’s use, this can mean they will need to have their nozzles replaced anywhere from around 100,000km. Certainly most injectors that have done 100,000km won’t be at their best and no amount of cleaning is going to change that.

So how do you tell the difference between a dirty injector and a worn injector? It’s quite simple really. You take them to a diesel injection specialist who will test them. If the injector fails the test the nozzles are most likely worn and need to be replaced. But if diesel specialist thinks they only need to be cleaned, and can be cleaned successfully, they’ll do it.

As unpalatable as this may sound, the bottom line is that there are no user serviceable parts in diesel injectors and pumps. If you are going to do any DIY fuel system repairs, limit it to replacing the fuel filter. Even most general repair workshops don’t get too involved in such specialised repairs. Instead, they will often send the whole vehicle to a diesel injection specialist for repair, or at most they’ll remove the fuel system parts and send them out for repair.

As for the solutions found on the Internet, you need to make your own decision as to their effectiveness – and safety.  We certainly couldn’t recommend some of the ‘home-grown’ solutions we’ve seen, and have certain reservations about most of the others as well.

Chemical cleaners may assist in cleaning a dirty fuel system, or they may just loosen build-ups resulting in on-going fuel filter blockages, or other problems, but they will not restore a worn injector or overcome a mechanical problem in the engine. If you chose to follow the cleaning route, be prepared that it may not fully resolve the issue and that a proper repair of some sort may still be needed afterwards.

Adding a quantity of petrol into the fuel tank, as suggested on a number of Internet forums, is very risky and is certainly not recommended. For one it significantly lowers the fuel’s flash point - the temperature at which it will combust. This presents a potential safety issue, particularly given that diesel is regarded as a relatively ‘safe’ fuel that can be in some types of hazardous environments. It also changes the way the fuel burns in the cylinder, which may not be good for the engine.

But it is also potentially hazardous to the fuel system components as it can affect the fuel’s lubricity. Diesel fuel system components are made to very fine tolerances and rely on the diesel fuel for lubrication, but petrol is a solvent which can strip the lubricant from the parts resulting in expensive damage. This is much more of an issue with later Common Rail diesel fuel systems (i.e. most later small diesels) that operate at very high pressures. Like chemical cleaners, this will not overcome mechanical and wear related problems, and in fact it’s questionable if it can even effectively clean an injector.

Other methods, usually found on DIY forums, purport to show step by step removal and dismantling of injectors for cleaning, or even how to replace injector nozzles. However these guides overlook a very fundamental issue – the need for specialised equipment, facilities and knowledge to determine if the injector is serviceable and to reset its opening pressure. Quite simply, the job isn’t done properly if injection pressures aren’t set – and there’s a very real risk of causing engine damage if this isn’t done properly.

Bear in mind also that injector nozzles have very fine spray holes and a well blocked nozzle may not be recoverable at all.

Other issues 

Even a reconditioned fuel system won’t be able to give its best if the rest of the engine isn’t right. Many engine performance problems are caused by basic engine maintenance issues or normal wear and tear. So before blaming the fuel system it’s important to check the basics first.
  • Check tappet adjustment, if applicable. Can cause rough running, poor performance and increased exhaust smoke.
  •  Low or uneven compressions can cause rough running, poor performance and increased exhaust smoke. (Diesel compression tests are quite different to those carried out on petrol engines and are not something a DIYer will be able to do).
  • Consider the general condition of the engine. A worn engine won’t perform properly. (A compression test may help determine this, however be aware that a worn engine could still have satisfactory compression)
  • Check for blocked air cleaners. A common cause of poor performance and excessive smoke.
  • Check glow plug operation if hard to start or if the engine doesn’t start cleanly. (Not all glow plugs may be working causing the engine to start initially on only some cylinders) Similar symptoms can result from fuel drain back.
  • Engine management system faults. The Check Engine light may or may not illuminate. Requires specialist knowledge and test equipment to diagnose.
  • Frequent diesel particulate filter regeneration or DPF warning light illuminating frequently. Possible DPF fault or could relate to the type of use the vehicle is subjected to. DPF equipped vehicles typically must be driven regularly at higher speeds to allow regeneration to occur. Vehicles that spend all their time at city speeds may have ongoing issues that result in the need for a manually initiated regeneration. This is not necessarily a vehicle fault and may relate to the vehicle’s suitability for that type of use.  See our fact sheet on Diesel Particulate Filters for more information.
  • For some engines equipped with a Common Rail diesel injection system the correct engine oil grade is vital. The use of the wrong oil can produce all sorts of performance problems.

Safety warnings

  • Diesel fuel systems operate at very high pressures. Injection pressures of 1800 bar (26,000psi) are not unusual. There is a risk of serious injury or death from contact with high pressure fuel.
  • It is dangerous to remove an injector and start the engine to check its spray pattern.
  • Some electronic diesel injectors work on high voltage (around 150 volts) and there is risk of severe electric shock.
  • Common Rail fuel systems typically incorporate single use fuel pipes. That is, once the pipes have been loosened, they must be replaced as it is not possible to reliably reseal them due to the very high pressures involved.
  • Electronic injectors are often individually calibrated to the engine management computer and must be refitted into their original place. If mixed up, or the injectors are overhauled, the system will need to be recalibrated.

Avoiding fuel system problems 

While you can’t avoid the normal fuel system wear and tear associated with regular use, here are some steps you can take to avoid unnecessary problems.
  • Rather than buying fuel purely on price, buy only a well known brand from a reputable source, even if it costs a bit more. Dirty, poor quality or contaminated fuel will destroy a diesel injection system very quickly. Later Common Rail systems are particularly sensitive to fuel quality issues.
  • Always keep the receipt for your fuel purchases, and that way if you do get a bad or contaminated batch you’ll have proof of where it came from and may be able to negotiate some assistance from the supplier.
  • If this does happen, ensure you keep a sample (at least one litre) of the suspect fuel. The company involved may want to have it tested. Collect samples in clean, dry, sealable and fuel safe containers. Used plastic oil, fruit juice or similar containers are not suitable.
  • The fuel filter is there to protect the fuel system so be mindful of the need for regular replacement. If operating under adverse conditions more frequent filter replacement may be desirable or necessary. Use only original equipment or high quality replacement filters as the life of the system depends on them.
  • Clean dirt and dust from around fuel fillers and avoid filling in the rain to reduce the possibility of introducing dirt and water into the system.
  • Regularly drain water traps, where fitted. Don’t wait for the warning light to come on.
  • Take care with fuel storage. Never use empty chemical drums or other unapproved containers as residual chemicals could cause extensive fuel system damage.
  • If you fill from a bulk storage tank (farm tanks etc), regularly drain off any water condensation that may form in the tank. Also ensure the tank is designed to prevent rain and dirt entry which could promote algae growth. See our fact sheet on diesel fuel sludging for more information. 
  • Avoid filling from drums if at all possible as the chances of introducing contaminants is greatly increased.
  • If buying bulk diesel, consider how long it will need to be stored and the season. While summer blend fuels won’t cause any long term damage they can produce significant starting and performance issues if used in the cold of winter. Also, all fuels have a definite shelf life which will depend greatly on storage conditions. Shelf life should be discussed with your fuel supplier.

More information

Should you require further assistance please contact our Motoring Advice Service or email us a question now.