So, what is decarbonising?
In simple terms, decarbonising an engine involves removal of carbon deposits from the engine, using either mechanical (physical) or chemical methods.
The procedure of mechanically cleaning out carbon deposits entails removal of the cylinder head and then, physically scraping off carbon deposits from the top of the pistons, the combustion chamber and other components (e.g. valves). Two-stroke engines of yore needed frequent decarbonising as the lubricating oil (2-T oil, in common parlance) burnt in the combustion chamber, leaving behind carbon residue.
Opening up the cylinder head of a two-stroke engine is a relatively simple procedure. This isn't the case however, with modern 4-stroke engines and their overhead camshaft(s), injectors, multiple sensors and electronics. There is a lot that can go wrong in the hands of an incompetent mechanic. Therefore, the perceived need for chemical decarbonisation.
Adding certain chemicals like alcohols and terpenes into the conventional fuel supply (petrol / diesel) appears to dissolve and remove at least a part of the carbon deposit formed in various parts of the engine, such as fuel injection systems, piston crowns & rings, combustion chambers, valves, exhaust manifolds, EGR valves, cat-cons and mufflers. The carbon is then ejected out through the exhaust. Various companies make proprietary decarbonising agents, the contents of which are usually trade secrets. A machine, such as the one pictured above, is deployed to meter the decarbonising agent along with fuel into a running engine, where it's supposed to perform its magic.
"Steam cleaning" of engine internals has been put forward at times as a cheap decarbonising method, wherein water is sprayed into the air intake of a running hot engine. The steam thus generated is supposed to dislodge the deposited carbon & remove it via the exhaust. The efficacy of such a method and its positive & negative implications have been discussed at length across the world, but if improperly done, the chances of engine damage cannot be ruled out.
Related Thread: Engine Decarb.
Must-Read Article: Maintain your Car in Top Shape.
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Why NOT Decarbonise?
- The first & main reason could be: Why fix something that isn't broken? There are innumerable examples of cars running perfectly well beyond 200,000 kms WITHOUT engine decarbonisation. A perfectly timed maintenance schedule is key.
- Does your engine really need to be decarbonised? The symptoms of heavy carbon deposit include pre-ignition (aka knocking) and an increase in cylinder compression pressure (above the recommended values).
- 21st century engines & newer technology mean that fuel is burnt inside an IC engine more completely and efficiently than ever before. Complete combustion = no carbon deposit.
- Decarbonising may improve performance temporarily, but it won't address the reason for such deposits - usually over-fueling, or lubricating oil rising to the top of the pistons. You need to fix the injector / fuel pump settings to prevent carbon deposits in the first place, not clean it up afterwards! If your oil control rings aren't doing their job and you have engine oil coming up to the upper cylinder / top of piston, you need to have new rings / an engine overhaul done. Removing the carbon by decarbonising can actually worsen oil control in this situation.
- Fuel additives which are added directly to the tank have some role in cleaning out carbon deposits (they also help in avoiding carbon buildup). These are way cheaper than decarbonising using a machine.

Even if carbon deposits are chemically removed, the following questions remain:
- Are all cylinders / injectors / valves cleaned out equally by decarbonising?
- Did all the carbon go out of the exhaust?
- What if the dissolved carbon clogs the cat-con / EGR?
- What did the dislodged carbon do to the turbo?
- How much of the rock-hard carbon is actually removed?
- Can partial carbon removal cause leaky valves or sticky rings?
- What effects might the decarbonising chemicals have on the fuel pump?
The answers have not been scientifically researched by any independent authority, and it is anybody's guess as to what's happening inside your engine, after it is fed a diet of unknown chemicals for half an hour.
Lastly, how much of the post-decarbonising performance enhancement is psychological? We all know how our cars "seem" to run better when they return from a service, irrespective of what may (or may not) have been done.
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Before you decarbonise, please consider the following:
- Decarbonising a modern emissions-compliant fuel-injected petrol / common-rail diesel engine isn't really warranted and doesn't improve engine health to any great extent.
- Frequent decarbonising is a procedure best avoided, by all expert opinions. Most automobile manufacturers (Tata is an exception) don't recommend decarbonising as a routine maintenance procedure.
- Have you properly troubleshot the real reason behind your engine running rough / sluggishly? Is it really carbon deposit that is causing the problem?
- Is the mechanic performing the procedure aware of what he is doing? An 800cc petrol engine doesn't need as much decarbonising solution as a 3.0L diesel does. Overuse of decarbonising chemicals is harmful to your engine.
- Decarbonising an engine that is 100,000+ kms old could actually increase its fuel and oil consumption. Either way, don't expect a radical boost in FE after decarbonisation, whether your engine is old or new.
- The perceived improvements (post-decarbonisation) are temporary, for just a few thousand kms, before the engine goes back to its old ways.
- There are no scientific evidence-based studies that attest to the effectiveness of these proprietary machines and chemicals.
- The carbon emitted from the exhaust (in the form of smoke) pollutes the air you breathe.
- Decarbonising isn't cheap. It is easier and cheaper to carry out preventive maintenance; pure fuel supply from reliable pumps, timed oil changes, higher quality oils etc. are effective in preventing / reducing carbon buildup.

To conclude, chemically decarbonising an engine isn't the cure-all that manufacturers of such equipment and chemicals tout them to be. There are pros and cons to carrying out the procedure, and I personally do not recommend decarbonisation, especially for an engine that is running in a healthy state. That said, the procedure can be considered as a last-ditch measure after all other options have been explored, in situations where engine performance remains poor, and you are keen to try something a little less 'invasive' before opening up the engine.
In the end, it's your car, and it's your choice.
Image Credits : The pictures of this Article have been sourced from various Team-BHP threads. Thanks to BHPians for shooting & sharing them.