Products of this type include Cataclean, 10k Boost, Powerboost, Redex, STP

This is slightly "off topic", but since many makers of engine cleaning / decoke products claim improved economy and emissions, it is perhaps helpful to discuss them here.

It is of course entirely true that deposits of carbon and other combustion products tend to accumulate in engines - particularly in the cylinders, on pistons and valves, and sometimes on the injectors. These deposits can at least in principle have negative effects on economy, emissions and performance, and as a result there are many products on the market that claim to clean up these deposits. Some are added to the petrol tank, others sprayed into the intake system.

The first thing to say is that, unlike most fuel "saving" devices, I believe that the majority of these products do approximately what they claim, ie they clean up deposits. However, I personally would not generally use them - certainly not as a regular matter, and probably not even occasionally - for the reasons given below. I would not claim to be any sort of expert on these products so what follows are my impressions rather than the definitive facts I present on my other pages; nonetheless I believe them to be accurate.

• Car makers do not recommend their use, indeed in many cases the handbooks specifically warn against use of such additives. This is probably because some may be positively harmful to components such as lambda sensors or (in the case of products sprayed into the intake) air meters or manifold pressure sensors. From the 2003 Ford Fiesta handbook: Do not use supplemental additives or other engine treatments. They are unnecessary and could lead to engine damage. (Anything strong enough to dissolve years of deposits in just a few seconds may well also attack sensitive components such as manifold pressure sensors, which are not generally designed or tested to be resistant to such cleaning products)

• They should not generally be needed on modern vehicles. Car makers test their engines over thousands of hours to check for durability and ability to maintain emissions; indeed it is a legal requirement that the emissions after 50 000 miles of typical use must still be within strict limits. Modern fuels contain sophisticated detergents that should be quite sufficient to keep deposits under control - certainly no durability fleets I have been involved with have needed additional cleaning products to be used. Even spark plugs last 30 000 or even 60 000 miles without replacement or cleaning on many modern cars.

• Cars that are always used for short journeys may develop excessive deposits due to never reaching full temperature, but these can be cleaned by simply taking the vehicle for a long run (at least half an hour) at high speed and load - say a fast motorway cruise or a twisty country road with plenty of accelerating and decelerating. This will also heat the oil up sufficiently to drive off any fuel that may be dissolved in it, which can otherwise cause engine wear. Interestingly, some "decoke" products ask you to take the car for a good run after applying them to "clean out the deposits" - sounds a bit like the Soup Stone to me...

• A sparklingly clean engine is not necessarily a good thing anyway. The fuelling and ignition for an engine are optimised on an engine that has had time to "run-in" and develop some deposits. The ignition timing and mixture control will probably therefore be slighly away from optimal if the engine is entirely free of deposits.

However, a quick study of some dyno figures in reviews for this sort of product suggests typical power gains of just 2 - 5%, and even then only on vehicles that have already been driven 100 000 miles or so. Benefits on newer vehicles are likely to be much less, and it is in any case debatable whether a typical driver could notice 2% more power.

There is also no particular theoretical reason to expect lower carbon monoxide emissions or fuel consumption as a result of having a "cleaner" engine, unless it was previously exceptionally "dirty". Reports of this occuring are probably due to the placebo effect (see comments here on fuel economy in general) or because other conditions have changed. For example, the effectiveness of a catalyst is very strongly dependent on how hot it is, so just running the vehicle hard immediately before the test can give very significant reductions in emissions.

Since the "official" fuel consumption and CO₂ figures for cars are now so important from a marketing point of view, car makers are keen to ensure that the test car used for these measurements is as economical as possible (tyres correctly inflated, brakes not binding, etc). If a quick squirt of "engine cleaner" could significantly improve economy, you would expect that this would be a standard part of the prepararation routine - strange, then, that I have never come across this being done...

Claims to clean the catalyst seem particularly implausible, as this implies that the cleaning fluid can survive the extremely high temperatures (in excess of 1000 celcius) found in the combustion chamber. Since most cleaners are based on organic compounds, which are generally entirely destroyed under such conditions, this seems unlikely. It is also rare for a catalyst to be damaged by being "dirty" - usually the cause is either overheating or contamination by lead, silicon, etc. The catalyst is generally self-cleaning of carbon deposits, at least providing it has reached operating temperature. Determining the effectiveness of the cat also requires that both the "pre-cat" and "post-cat" emissions are measured - a reduction in tailpipe emissions may be because the engine itself is producing lower emissions, not because the cat is working better. Finally, catalyst effectiveness - especially with "aged" catalysts - is highly dependent on temperature. Raising the cat temperature through a brief period of hard running may lower emissions significantly compared to a cool-ish catalyst that has sat at idle for several minutes.

A couple of other points to note:

1) Users often report a large cloud of smoke from the exhaust when these products are used, and comment "look at all the dirt being removed from my engine". But of course the smoke could just be the "cleaning" product itself burning...

2) Dyno power measurements do suffer from test-to-test variability anyway, so claims of up to 5% performance improvement should be taken with a pinch of salt. A change in intake air temperature of just 10 celcius can easily change performance by 5%, and the conversion from measured dyno horsepower to calculated flywheel power is fraught with difficulties.

(As an aside, one of the best-known "cleaning" products in the UK is Powerboost from Ecotek plc. The Advertising Standards Authority recently ruled that Ecotek did not have proof that Powerboost improved power, economy or throttle response, and as a result they are effectively banned from making these claims in print advertising.)

If I had an older (say five years or 50 000 miles) car and suspected problems with carbon build-up, my first action would be to take it for a good "thrash" as described above.

If the problem persisted, I would probably take the £20 or so that these cleaning products typically cost, and spend it on buying a "premium" fuel such as Shell Optimax instead of normal petrol for a couple of months. Optimax has enhanced detergent qualities compared to normal fuel and has been proven to clean up even already dirty engines over the course of a thousand miles or so. To put it bluntly, I trust Shell's boffins more than I do the (generally very small) research teams of additive companies.

Finally, if all else failed, I would resort to an additional fuel additive or engine cleaner - but this is something I would do perhaps once every five years, not once a month as some sellers of such products seem to recommend. Certainly my own car, after 3 years and 35 000 miles, is not showing any sign of degraded economy due to deposit buildup.

Other products are specifically marketing as cleaning injectors rather than the whole engine. Again a good quality fuel should keep injectors clean anyway, and there are potential risks from "over-agressive" cleaners. Having said that, I know that in some parts of the world fuel does not contain as much detergent as it probably should, and so an appropriate cleaner used occasionally may be of benefit in keeping the extremely small metering holes in the injector clear.

Please also read the general comments on fuel "saving" devices, if you have not done so already.

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