Why do direct injection engines have carbon build up issues


Pro Tuner
Staff member
2002 Clio 172
I've read quite a bit on here about direct injection engines having carbon build up issues.

Why is this? What causes it? Do all direct injection engines have this problem and can it be prevented?

If you have a carbon build up issue what can you do about it?
On conventional engines, the fuel is injected into the engine along with the intake air, so it effectively mixes on the way in. This wets and helps clean the intake valves as fuel is very corrosive.

To aid emissions and help meet CO2 emissions some exhaust gases are recirculated into the intake, this helps warm up the engine, forces a cleaner burn of the partially burnt fuel and the heat generally helps the combustion process.

This oily, carbon riddled intake gases come into the engine via the valves, where the carbon and soot builds up on the intake.

As the fuel is injected directly into the cylinders there is no cleaning process going on, so if the engine is used at low inefficient burning speeds the carbon build up will become evident. Driving it hard, keeps things going cleanly and fewer dirty particles are fed back into the intake.

The larger engines and lower rpm driving bands are the main causes. As fuel is generally all burnt in a fast flowing high revving burn cycle there is nothing to go back into the engine.

Makes have added catch cans, filters and similar to help mitigate the problem, and some are even reverting to a little fuel injection into the intake to help clean the valves.

Drive it hard and use a good quality fuel and fuel cleaner from time to time and you'll have little problems with carbon build up.

The more modern an engine is the better designed it will be, older direct injections are more susceptible to the problem.
An engine that has not been run in properly will have more blow by due to the rings not sealing properly and allow oil to bypass the oil control rings and go into the combustion chamber OR is worn will send more oily fumes back into the inlet manifold via the PCV valve no matter what type of injection is used.

Carbon build up also happens due to the cam timing when the inlet valve is starting to open before the exhaust stroke is finished and some of the gas is pushed into the inlet tract.

IMO an injector cleaner can only help clean /reduce carbon build up in port injected motors.
IMO an injector cleaner can only help clean /reduce carbon build up in port injected motors.
Yes an injector cleaner will not remove any carbon on the intake at all.

BUT it will help the engine burn fuel more cleanly, keeping the injectors clean and removing carbon deposits from the rest of the engine and this will help mitigate and reduce further carbon build up.

Sorry I wasn't very clear about that in my post.
Obi I like to think we are a team and nobody can get everything 100% correct 100% of the time :) |B

A cleaner will remove or prevent carbon build up in a port injected motor downstream of the injector.

This is a quite lengthy article on the subject BUT IMO there is an error in it where it mentions the thickness of the carbon build up .

IMO the decimal point is in the wrong place .V(

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