When Should You Replace a Fuel Injector?

22 September 2019

The demands of consumers for better fuel economy and lower emissions have led automotive technology to evolve from the carburettor to mechanical injection at the manifold and today’s electronic direct injection systems that feed fuel straight into the combustion chamber.

Both diesel and petrol engines now use direct injection, which is not just cleaner and more efficient: it provides improved driveability, better cold starting and more. However, like any component, injectors can wear over time. They can also be subjected to injector fouling fuels – particularly if the vehicle does a lot of short stop-start journeys – that clog the injectors and harm performance. So how do you know if an injector needs to be serviced or replaced?

Signs of Trouble

There are a number of indicators that your injection system may need attention. Poor starting is one, as is rough idling. Problems may also manifest themselves as a drop in fuel consumption or a hesitation under acceleration. On diesel engines you may notice black smoke from the exhaust due to over-fuelling.

It may be that to cure these symptoms all your injectors need is a clean. The problem is that you can’t see the business end of the injector because it’s inside the cylinder head. To strip down the injection system and clean the injectors therefore can be a costly and time-consuming task.

Modern premium brands of petrol and diesel usually contains additives to help negate the effect of injector fouling fuels. Even so, a life of short trips can lead to problems. Many people advocate an ‘Italian tune-up’ occasionally – filling up with premium fuel and taking the car for a motorway run of 20 miles or more using higher engine revs.

When to Replace?

Direct injection systems are very precise, and therefore only a small amount of restriction due to clogging can cause a problem. In turbo-charged installations this problem is greater because the demands of the engine at high revs are higher. Since many manufacturers are now turning to small petrol turbo engines to meet emission and fuel consumption targets, this is a problem we’re likely to see more often.

Another issue is that when the engine is switched off the injectors experience heat soak, which can cause deposits to be baked on to their surfaces. Again, this is more of a problem with turbos, where under-bonnet temperatures are higher.

The car’s engine management electronics will, to an extent, try to compensate for clogging by opening the injector for longer. But there comes a point where it can’t do this – if the injector is badly clogged or if fuel pressure is low due to a filter or pump problem.

Injectors are operated by solenoids, and these rely on electrical resistance to operate the injector spring. As they age, the resistance can drop and this leads to replacing the injector. A general lack of maintenance, such as not changing oil and filters on schedule, can also have a knock-on effect on the injection system.

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