GDI carbon buildup issue, and a solution

Posted: October 19, 2011 in Tech Info

Here is a slightly technical issue that may be over some people’s head, but I will try to keep it simple.

There are two types of fuel injection used in cars today; port fuel injection and direct injection.  Port fuel injection has fuel injectors that spray into the intake ports, and the air/fuel mixture is drawn in through the intake valve.  Direct injection has injectors that spray fuel directly in to the combustion chamber, so the only thing the engine is drawing in when the intake valves open is air.  The air is mixed with the fuel directly in the combustion chamber.

There is an issue that I see becoming more common as manufacturers begin using GDI (Gas Direct Injection) engines more. This issue will affect turbo GDI engines faster than Naturally Aspirated GDI, but it will be an issue regardless.

All engines have what is known as a “PCV” (Positive Crankcase Ventilation) system.  This system provides a controlled way to relieve the positive pressure in the crankcase of all internal combustion engines.  All engines have a certain amount of “blow-by” that passes the piston rings and enters the crankcase.  This is why your oil gets dirty; because of the small amounts of the  byproducts of combustion passing by the piston rings and getting into the oil while it is running.  This “blow-by” also causes pressure in the crankcase that needs to either be vented to atmosphere or drawn back in to the engine’s intake system to be burned off in the combustion process.  Obviously, the latter is much more environmentally friendly.

The byproduct of drawing the PCV air back in to the intake system is carbon build-up. The crankcase vent air that goes through the PCV contains oil vapor, as well as water vapor and other combustion gases. That vapor normally ends up in your intake track and most of it gets burned. The rest of it collects on the inside of the intake manifold, the back of the throttle body, the intake ports, and the intake valves and bakes on over time to create a very hard carbon build-up.

On a port fuel injected vehicle, the fuel injectors are spraying on the back side of the valves and parts of the intake ports and helps keep them clean. This is from a 180,000 mile Saturn engine that I personally pulled from a junkyard car and disassembled myself (later I rebuilt and put in my 97 Saturn):

You can clearly see the injector spray pattern in the ports and where the valves would be. They would be nearly as clean as the day they were new.

GDI on the other hand doesn’t have that help from the injectors. The gasoline is injected directly in to the cylinder, completely bypassing the intake system.  Therefore, the vapor from the PCV hits the ports and valves, bakes on, and stays there.

Here is a BMW 335i after 40,000 miles:

Audi 2.0T after 28,000 miles:

Mazdaspeed 6 after roughly 36,000 miles:

This is a very well known issue with GDI engines among the people who modify their cars, but this information hasn’t hit the mainstream yet. Some owners report a major loss of power and others report that it runs like a diesel. The only way to remove it is to remove the intake manifold, pour some strong engine cleaner like Seafoam or B-12 Chemtool into the ports, let them soak, then remove the liquid and use a small brass wire brush on a drill to remove it. This is costly if you can’t do it yourself and it’s extremely time consuming if you do have the know-how.

So how do we avoid this issue to begin with? You need a good quality oil catch can. The catch can goes in between the PCV valve and the intake manifold. A good quality catch can has baffles so as the PCV air moves though it, the air swirls around the baffles and the oil vapor can condense and drip to the bottom while the vapor free air exits the can. You then empty it every oil change. The oil in a catch can can not be reused. It contains not only oil vapor, but uncombusted fuel, water, and other byproducts of combustion that don’t belong in your crankcase.  The catch can oil should be disposed of with your used oil.

Why aren’t the manufacturers doing something about this and using catch cans?  Well, there are a few reasons; the first being cost.  For the sake of having an example, let’s say a catch can would cost a manufacturer $50 each.  Take $50 times 20,000 units per year, and that’s an extra $1,000,000 to build the same car, and that’s just one model.  Spread that across 10 models and that’s an extra $10,000,000 per year.  It’s cheaper for them to address any issues on a case-by-case basis without the catch can.  Yes, they could raise the cost of the car, but we could sit here all day and talk about the things that are missing from a car.  The next thing you know, we took a $20,000 car and made it a $25,000 car.  The second big reason is catch cans are a violation of the Federal Emissions law.  Don’t ask me why, it seems silly.  Adding a catch can yourself is a violation of the Federal Emissions law as well, so keep that in mind.  It’s up to you to weigh the risk vs. reward and make the decision.

What about the warranty? I don’t know how this will affect it. They would have to prove it caused the issue that you have your car in the dealer for. It’s easy enough to remove before you take it in (as long as you installed it in an easily removable location).  Any dealer tech that works on your car that knows anything about cars wouldn’t say a word about the catch can.  That’s just me though, dealer techs may be required to report any modifications done to a vehicle that is in for warranty work.


I am not going to provide information on how to install a catch can, what catch can to buy, etc.  I take no responsibility for anything you do to your car or any laws, warrantys, or otherwise that you may violate.  This article is provided for informational purposes ONLY. Modify your car at your own risk.

  1. […] can" to the CCV line going into the manifold. The can had to be emptied once in a while. Related Link With Pic's Hyundi has a 100K mile warrenty on their direct injected engines, including Turbos as I recall. […]

  2. […] Ok guys, some info I researched can help. GDI carbon buildup issue, and a solution drivegeek So apparently an oil catch can, can slow down the process of carbon buildup. Some Veloster guys […]

  3. Michael Colburn says:

    There is not one paper published anywhere on the planet that Identifies the scientific causal mechanism for Carbon build-up in any engine. The current papers published are all speculation and guesswork.

    However the problem is fuel related and is compounded by the use of modern day engine control. Fix the fuel and fix the problem.

    • Doug N. says:

      It seems to me that your statement is speculation and has not basis is research.
      I believe the problem stems from PCV recirculation through the intake manifold. Redirect crankcase venting and you’ve solved the problem. But I do admit this is just my opinion.

  4. […] article with some pics of other brands' GDI intakes, none of whom install a catch can/separator. DGI Carbon Build-Up. Reply   Reply With […]

  5. […] Car Advice – Understand Redlines and Carbon Buildup in this Month's The Fixer – Road & Track GDI carbon buildup issue, and a solution | drivegeek Direct injection and dirty intake valves | Automotive Thinker – Discussing the finer points of […]

  6. […] this issue, just not from the PR staff. Here is an VW technical engineering blog on the subject: GDI carbon buildup issue, and a solution | drivegeek And it does not take much searching to find the same from virtually every automotive brand. The […]

  7. Doug N. says:

    Not convinced a catch can will help. It would be great if you could post photos, and thereby proof, that catch cans eliminate, or at least reduce, carbon build up on the intake valves.
    And it they do help, then why don’t mfgrs use them on GDI engines and just include specs for maintaining them?

  8. […] points. I just don't think they are there yet. At least from everything I have read about it.…nd-a-solution/…e-audi-redline 13Soul 11 200 V6 07GPGXP 07Spectra […]

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