RACQ Motoring

Petrol injector cleaning

Injector cleaners are commonly available and widely promoted additives that claim to cure or prevent a range of fuel system ills.

Fuel system, and more particularly, injector cleaners encompass a great number of products and systems that range from simple DIY, pour into the fuel tank additives, to complex, purpose built cleaning and testing machines.

Why do we need to clean injectors?

Injectors become clogged with fuel residues (something similar to varnish) due to the evaporation of fuel in the injector nozzle when the engine has been stopped.

Residue build up

Every time you stop a hot engine a small amount of residue adds to the already present build up. The more frequent the stops, the more rapidly the build up accumulates. This accumulation eventually reduces the flow of fuel through the injector and disrupts its spray pattern, causing the engine to run lean. This can manifest itself in a number of ways, although the most common is driveability problems such as flat spots and hesitations.


Most, if not all, fuels contain detergents to clean the fuel system and injectors, with premium fuels often containing higher concentrations. However there is a limit to what they can achieve and it is sometimes necessary to use a more severe cleaner to remove long-term accumulations.

Cleaning methods

DIY in-tank additive

There are a number of common methods for cleaning injectors, the most basic of which is the DIY in-tank additive. Like many additives, the content of fuel system treatments is a mystery to those not in the industry. While some fuel manufacturers produce their own fuel system cleaners that they openly promote as being a more concentrated form of the cleaning package used in their fuels, the make-up of the majority of products is not widely known. Nor in many cases is their effectiveness.

While in tank cleaners may offer some benefits, they have a couple of drawbacks. Firstly their strength, and therefore their ability to remove heavy deposits, is limited by the fact that some fuel system components may be damaged by very aggressive cleaners. Secondly, the effectiveness of the process is unknown, as apart from removing and testing, there is no way of checking the injector's flow rate and spray pattern - two of the critical aspects of an injectors operation. In fact sometimes the only measure will be that you think the car goes better.

Rail Flush Kits

The second form of injector cleaning falls under the general heading of Rail Flush Kits. In these systems the fuel pump is disabled. The fuel lines are then removed and replaced with a system that delivers a concentrated cleaner directly to the fuel rail and injectors. The engine is then run until the cleaning agent is used up. This system has the advantage that a more aggressive cleaner can be used, however like in-tank cleaners, the outcome may still be uncertain.

Removal of the injectors

The most certain and effective cleaning method is to remove the injectors from the vehicle for cleaning in a specialist injector cleaner. Such specialised cleaning equipment often utilises ultra-sonic cleaning in addition to special cleaning chemicals. Such systems also have the advantage of being able to test the injector's flow rate and spray pattern before and after cleaning to determine the effectiveness of the procedure and the serviceability of the injectors. The disadvantage of such systems is the added labour costs involved in removing and refitting the injectors.

Other common problems and causes

Blocked injectors are not the only cause of drivability problems. An increasingly common problem with later vehicles is the build up of organic material in the throttle body and intake manifold, and carbon on intake valves and ports - all of which can cause a range of problems that can be difficult to trace. Generally these problems are dealt with by spraying the affected areas with a quite aggressive cleaner to soften and dissolve the build up.

How often should injectors be cleaned?

There really is no definite time to clean injectors as it will very much depend on how the car gets used and whether it is a model that is sensitive to the problem. If it is a car that develops unexplained flat spots or hesitations dirty injectors may be responsible, but it may be worth investigating for throttle body deposits as well.  Injector cleaning won't correct a dirty throttle body - cleaning will be required.

Prevention V Cure

Some vehicle manufacturers specify definite intervals and provide their own branded injector cleaners, and these recommendations should of course be followed as it is likely that experience has shown the vehicle to be prone to injector fouling.

Vehicle manufacturer recommendations

You should ensure that it is actually a vehicle manufacturer’s recommendation rather than that of the servicing dealer. If it is a manufacturer’s requirement it will be listed in the vehicle’s service schedule.

Where a manufacturer specified service interval isn't provided it becomes a question of how regularly should fuel system cleaner be used.  There are certainly many workshops that encourage customers to have such procedures carried out, as often as every service, as preventative maintenance. 

Fuel system cleaners

There is no doubt that an effective fuel system cleaner can provide benefits by removing deposits before they become a problem.  However it is questionable if, given their often significant cost, their regular use is economically justifiable. Actual problems arising from such deposits occur fairly infrequently and based on that alone it would be difficult to justify frequent servicing. Ultimately however it is up to you, as the vehicle's owner, to choose when such servicing is performed.

A word of warning

If you do choose to use a DIY in-tank cleaner, pay particular attention to the instructions for use and do not exceed the specified dosage. While in-tank products may not be particularly strong in the overall scheme of things, some are perfectly capable of damaging fuel system components if incorrectly handled.

More information

This fact sheet deals exclusively with petrol injectors. For information on cleaning diesel injectors see our “Diesel injector cleaning” fact sheet.

Should you require further assistance please phone our Motoring Advice Department on (07) 3666 9148 or from outside the Brisbane area on 1800 623 456.

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The information provided in this fact sheet is intended as a general guide only. You should carry out further research and investigations into the topics outlined above prior to implementing any actions or recommendations