There are a couple of different parts to cleaning the fuel system, and they can be done together or separately. The first is the simplest and involves an additive that is poured into the fuel tank. This can be called a fuel system cleaning service or a fuel injector cleaning service.
For the most part, the fuel system is sealed and stays as clean as the fuel that is put into it. But that’s not always foolproof, and inevitably there is some sediment and contaminants in the system, which can get sucked up into the pump and circulated into the fuel system. The main danger here is that the fine passages in the fuel injectors can clog and the spray pattern and fuel volume can be interfered with. A fuel additive that dissolves contaminants is one way to address that. There are numerous products available, which are all basically solvents, that are added to a tank of gas. The fuel pump itself then circulates the cleaner through the fuel system.
The second kind of fuel system cleaner addresses deposits in the intake system — from the throttle plate to the intake runners, to the intake valves, and the combustion chamber itself. The intake system is prone to sooty or gummy deposits from various sources. Dust enters, depending on how efficient the air filter is; oil enters from recirculated crankcase vapours; and both can bake down to carbon toward the end-point in the combustion chamber where high heat is encountered. Cleaning the intake system is intended to remove gummy deposits and carbon build-up.
The simplest method is to spray a solvent compound under pressure into the intake system while the vehicle runs.
Costs of Fuel System Cleaning
This service is typically done at a fixed price regardless of the make and model of a vehicle. To do a simple gas additive service, the average cost is about £40 to £70. This is often done in conjunction with a scheduled service interval and bundled in with other things, such as inspections and fluid changes.
The more involved intake system cleaning service can cost between £130 and £300. There are various different brands of products available, and various kinds of injection methods. In many cases, an intake system cleaning will include a fuel injector cleaning, which is an additive that is poured into the gas tank.
How Effective is Fuel System Cleaning?
That is a contentious question. A fuel additive cleaner can be effective. There are some more advanced methods of cleaning injectors that have been around for awhile, where the injectors are removed from the vehicle and mounted in a fixture where they can be run and their spray pattern inspected. If there are problems, then solvents can also be run through them until the spray pattern is good and as it should be. This kind of device isn’t common but is the kind of thing that prompted the development of fuel additives to clean and maintain injectors. You can’t see the effects, but there is some objective research that shows a benefit in miles per gallon and emissions. Applied as preventative maintenance, the ideal is that you’d use a fuel system cleaner to avoid problems rather than solve them; done that way, no improvements would be noted.
Cleaning the intake system is a more laborious and tricky thing to do, and less certain to be necessary. In normal circumstances, cleaning the throttle plate is periodically necessary and is a normal part of a traditional tune-up. Most of the time, it’s best done manually (with solvent and a rag), but an alternative is to do it with an intake system cleaner. Objectively, the result is not as good, but it is still better than nothing.
Going farther in, various types of products do a reasonable job of cleaning gummy residue from the intake runners, but there is no real data that says residue in the runners causes actual harm. Furthermore, removing carbon from the valves and combustion chamber is one of the least supported claims. For the most part, based on before and after studies, the one case where intake system cleaners really make a difference is where a heavy gummy build up of burned resinous oil and contaminants is obstructing the valves. But that kind of buildup usually indicates a serious problem with the piston rings or the valve stem seals, which cleaning itself does nothing to address. So it can help sometimes, but it’s more of a stopgap than a cure.
Which is also to say to the initial question, “How effective is fuel system cleaning?”: opinions and results are mixed. The easy and inexpensive part of it may be well justified, while the more expensive part of it may or may not be worthwhile.
What Can Go Wrong with a Fuel System Cleaning?
There isn’t much to worry about with a simple fuel additive that goes into the gas tank. But when doing the more involved work of running solvent compounds into the intake system, problems can occur if care isn’t taken. The vehicle is designed to burn fuel in a specific mixture, and adding a cleaner throws everything out of balance, while also introducing a lot of liquid into the combustion chamber. Liquid doesn’t compress, so if it’s done too quickly, the engine can hydro-lock, which can cause it to stall, and in the worst case, can bend a rod or cause other internal damage.
There are many intake systems where the routing of the runners allows fluid to collect, and an intake system cleaning compound can simply puddle up in the system if it’s not sufficiently atomized. Then, on sudden stops or starts, the trapped fluid can slosh forward into the cylinders suddenly, causing the engine to stall again, or in a worst case scenario, hydro-lock and possibly cause damage.
It is very common for an intake system cleaning to cause isolated misfires as it works and to trigger the engine light, but that most often solves itself and resets within a few miles. Sometimes it’s necessary to stop, turn off the key, and restart the engine to reset if the vehicle goes into limp mode.
While many manufacturers don’t specify an interval for fuel system cleaning, those that do recommend it about every 30,000 miles on average.
It is very easy for any vehicle owner to purchase a fuel system cleaner that goes in the gas tank and pour it in. Cleaning the intake system is more difficult and requires some expertise. It is possible to do it at home, but not often advised.
Following the owner’s manual’s service recommendations is the best way. The second best way is to track mileage; if the vehicle is suffering from poor mileage but has no engine light or other indications of a cause, trying a fuel and intake system cleaning might be the least expensive way to try to improve things.