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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.

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Engine Decarb

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Maintain your Car in Top Shape

Most explanations about the necessity of decarbonising any engine unsurprisingly come from manufacturers of the chemical decarbonising machines! The benefits are said to be as follows:

• "Spring cleaning" of the engine from the inside, without opening it.
• Decarbonises the entire fuel system, including the O2 sensor and catalytic converter (sic).
• Restores / improves fuel efficiency.
• Maximizes vehicle performance.
• Reduces harmful emissions.
• Reduces engine noise & vibration.
• Repairs driveability problems - Eliminate engine hesitation, hard starts & rough idling.

http://www.youtube.com/watch?v=sKTp2DjugOA

• 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.

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.

Many thanks for sharing, SS-Traveller! I’ve learned a lot through your article. *Rates 5 stars*

As petrol-heads, we love our cars and like to pamper them. The marketing spiel would have us believe that decarb is indeed a magic solution that'll make our engines run smoother, faster and more efficiently. Personally, I think it’s all rubbish. Shom, do you also suppose there is the possibility of leaks on an old engine (similar reason to why engine flushing isn’t recommended for 6-digit motors)?

I’m wary of feeding my engine with suspect, untested and uncertified chemicals. Why fix something that isn’t broken? I’ve owned several engines that have run well over 150,000 kms, delivering optimum levels of performance and efficiency (WITHOUT decarbonisation). I'm hopeful your write-up prevents folk from getting fleeced; have seen service advisors convince owners of 20,000 km hatchbacks into decarbonisation!

As you rightly stated, perfectly timed maintenance (oil changes, good oil) are far more important. Case in point : The Vtec producing each and every one of its horses at the 7 year mark. Main reason = Mobil 1 + regular oil changes (link to full thread).

http://www.youtube.com/watch?v=HdsCul-rtmE

Thanks for an excellent overview.
I agree with the various considerations you list. Interestingly enough, my Jaguar dealer in Kansas City used to promote a fuel additive. Cant remember what is was, but it was supposed to do the usual, reduce carbon build up, keep injectors clean etc. etc. Never believed them.

Only once did I have an engine decarbonised. Many years ago I owned a Volvo 340DL. This was a carburator engine, no catalytic convertor. It kept suffering from pre-ignition and knocking. The Volvo dealer brought in an expert and even he couldn't fix it. But he ultemately suggested decarbonizing. They added something to the fuel and left the car running for 30-60 minutes or so, maybe we even drove a bit, cant remember. By that time the engine was definitely running very differently. Volvo than managed to adjust and tune the engine properly and the problems never returned.

So very likely that the knocking was being caused by considerable carbon built up in the cilinder and cilender head. Somehow this treatment managed to get rid of sufficient quantity, so the engine could be returned to normal.

Again, this was probably more than 30 years ago. These days with modern diesel/petrol cars its hardly a problem.

I generally don't believe in any add ons to fuel or oil. With the exception of fuel stabilizers which actually do work. (if you buy the right one!)

Jeroen

That decarbonising can cause a slight loss of power or increased oil consumption is well-documented for older engines. This is usually attributed to the breaking down of the oil seal between piston edge and cylinder bore, by the removal of the ring of carbon that had formed on the piston edge. However, in time a new oil-seal may form and the compression would therefore improve.

New generation engines (e.g. BS-III, BS-IV) do not form so much carbon during combustion. Removing whatever little has formed to compensate for cylinder bore wear, MAY leave you with leaky pistons after decarbonising.

Flushing can cause oil seals in the lower half of the engine (such as big-end bearing seal) to leak - decarbonising can do the same to the fuel pump.
I don't suppose you tried an Italian tune-up before that, did you? :D

Super write-up SS Traveller. This is a very informative article. I only wish I had read something like this some 4 years back:D when I was conned into doing engine decarbonisation for my swift diesel which had run only 26,000 kms:Frustrati. Hopefully, this prevents others from getting these done.

Rated 5 stars

To be honest, back home, my normal driving style is pretty much default Italian tune up mode. I drive all of my cars hard all the time

Although on modern cars driving them very gently and at low rpm isn't a big thing, its also a myth that reving (under load) and generally giving it the beans is bad for your engine. It is not. Car engines are designed to work well with little wear and tear across a wide range of performance, rpm and loading. Still, rule of thumb is that from a wear and tear point of view car engines do best at around 60 - 80% of maximum power ratings.

On a slighly lighter note: When I was in the merchant navy I spent many years on ocean going tugs and supply and anchor handling vessels. It meant often going along side an oil rig for extended periods of time. For safery reasons we always kept all engines going. But that could mean we had two main diesels of say 6-8000HP each, running at idle for 24-48 hours at a stretch. By the time, we did cast of and run up to full power you would not believe the amount of smoke and dirt that came out of our smoke stacks! Whole placks of soot and dirt were spewed out for minutes.

Also, on merchant ships there is a preventive maintenance activity called washing or flushing the turbo's. On the exhaust turbine, there is carbon built up, because there is obviously carbon particles in the exhaust gas. Large turbo have a washing/spray system. The engine load is run at roughly medium rpm or loading and then water is sprayed into the exhaust turbine. The water droplets are extremely fine. It's really the mechanical impact of the water droplets hitting the carbon built up and knocking it off.

No need to do so on your little car diesel. If anything, please do not spray water into the turbo!

Jeroen

Italian tuners, eat your heart out!

Jeroen

On a related note, found this article:

http://blogs.menshea...opsy/2012/04/19

A guy in America has a 1.8T engine'd Jetta which has clocked up 500k miles (804672 kms). Even more impressive is the lack of wear in the engine. All the components were still in factory specification. His explanation - Use of synthetic oil, use of fuel system cleaner every 10000 miles and platinum spark plugs.

Hi SS Traveller,

I had a few queries:

How about Engine Flushes? They are known to clean out your engine of carbon deposits without a lot of expense- Cyclo, Abro, Xado etc are available in 400-500 range.

Are these useful?

Best,
Tapish

I remember dismantling my old Bajaj Pulsar 180 classic a lot during my college days in a quest to reduce its engine mechanical noise(Its a single cylinder carburetted motorcycle)

The issue was with the timing chain guides which were damaged accidently and the original ones were not available for replacement. So that meant frequent removal of engine head to install handmade prototypes made out of teflon.

When i dismantled the engine for the first time, i spent a good time scraping off the carbon deposits on the piston crown. but whenever i dismantled the head after that, be it a week or even 10 minutes after running the engine, they were back. scrubbed them off again for a few times, but stopped bothering when i realized that they are destined to be there :D. The engine was in a good state of tune and the plugs were a beautiful brown color indicating a good mixture.

I think carbon deposits are bound to happen as the carbon particles crystallize on the combustion chamber surfaces during cold starts and low engine temperatures (richer mixture) and the harsh environment inside the engine (violent air movement) will prevent the layer to grow by blowing off any peaks on the deposits surface. Hitting the redline once in a while would help here. Over all, i dont see any point or logic in decarbonising in modern 4- stroke engines

Even im interested to know if these are any good by experienced members. But these are supposed to remove deposits and sludge from a different section of the engine- the internals except the combustion chamber. They are mixed with the engine oil and circulate throughout the engine along the lubricating lines and are supposed to dissolve and wash away sludge and carbon deposits in the sump, oil lines and head(which wouldn't be there if you change your engine oil on time anyways)

AFAIK, they wont help with carbon deposits in the combustion chamber, intake and exhaust system.

Thanks for the great article SS-Traveller!

Each time I visit the service station for my periodic servicing the fellow keeps on insisting that I get an engine flush and Engine de-carb done. My Ritz has done only 30,000 kms! I went through the whole service manual and did not find a word about this. Thanks to your article now I know what exactly happens and would personally never get it done.

I was thinking of driving down to NCR for a Decarb job.
Now i realized its not necessary,my car is running fine with a dosage of Delvac1 every 8K kms,and its primarily used on highways,so no chance of clogged internals.