What you need to know about diesel injectors

Unlike petrol injectors, diesel injectors aren’t that prone to developing build-ups. 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 often not related to injector deposits.

Diesel injectors operate at very high pressures, up to 1800 bar (26,000psi) is common, and this helps keep them clean, but it also means that, in time, they wear out.  This means they must have their nozzles replaced at regular intervals. 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 dirty injectors and worn injectors? It’s simple. Take them to a diesel injection specialist who will test them. 

As unpalatable as this may sound, the point is 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.

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. They will not restore a worn injector or overcome a mechanical problem in the engine. If you choose 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.

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.  This can cause rough running, poor performance, and increased exhaust smoke. 
  • Low or uneven compressions can produce the same symptoms. (Diesel compression tests are quite different to those carried out on petrol engines and are not something the average DIYer will have the equipment for) 
  • 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 for a blocked fuel filter.
  • 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. This requires specialist knowledge and test equipment to diagnose. 
  • Frequent diesel particulate filter regeneration or DPF warning light illuminating frequently. Possible DPF system fault or vehicle is not being driven far or fast enough to allow regeneration to occur. 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. 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).  There is risk of severe electric shock. 
  • Common Rail fuel systems typically incorporate single use fuel pipes. Once they have been loosened, they must be replaced as they cannot be reliably resealed. 
  • 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 must be recalibrated using special equipment. 

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.
  • Don’t buy 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 quickly destroy an injection system. Later Common Rail systems are particularly sensitive to fuel quality issues. 
  • Always keep the receipt for your fuel purchases.  That way if you do get a bad or contaminated batch you’ll be in a better position to negotiate some assistance from the supplier. 
  • If this does happen, ensure you keep at least a 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 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 damage. 
  • If you fill from a bulk storage tank (farm tanks etc.), regularly drain off any water condensation that may form in the tank. 
  • Ensure storage tanks prevent rain and dirt entry which can promote algae growth. See our fact sheet on diesel fuel sludging for more information. 
  • Avoid filling from drums 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. Summer blend fuels can cause starting and performance issues in the cold weather. All fuels have a definite shelf life which is dependent on storage conditions. This should be discussed with your fuel supplier. 

Suggestions from the Internet

Not surprisingly, the internet has all sorts of advice about injector cleaning. But we certainly couldn’t recommend some of the ‘home-grown’ solutions we’ve seen.
  • Many involve chemical cleaning products.  Try them if you like but don’t be surprised if they don’t fully resolve the problem, if at all. 
  • Don’t add petrol to the tank.  It can do damage.
  • Diesel injectors are pieces of precision equipment. Dismantle and clean them at your own risk.
  • Don’t attempt to replace nozzles unless you have the equipment needed to calibrate opening pressures.  You might get away with it, but you may also wreck the engine. 

More information

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