Gasoline direct injection has quickly become the most popular way to burn fuel. Over the last decade, GDI has worked its way into most new cars, both cheap and expensive ones. With new tech comes new servicing demands, and in this case, we have the dreaded carbon cleaning to contend with. Luckily, it’s actually not all that bad.

We discussed GDI in general terms in a previous post you can see here. This writeup will focus on my experience actually going through the cleaning process.

My 2010 VW GTI suffers from some of the worst carbon buildup issues of any direct injection car. The early (and current) Volkswagen TSI engine is infamous for the frequency and aggression of the carbon gunkery that builds up on its intake valves. 

My intake manifold happened to break, so I figured this would be a great time to see how the cleaner-in-a-can I’d tried 20,000 miles ago, and learn how to walnut blast the valves.

If this all sounds like gibberish, a quick rundown:

Most cars up until about 2012 have what’s called port fuel injection. This means that gasoline is sprayed by the fuel injectors before the valves, somewhere in the intake. It isn’t sprayed far from the valves, but crucially, flowing fuel cleans the valves regularly. It’s a mostly unproblematic system for delivering fuel to the cylinders, maybe a little imprecise relative to direct injection, but still carefully metered.

Direct injection moves that injector from the intake, directly into the actual cylinder. Some cars position it up top next to the spark plug, some (like my VW) position them near the intake manifold so that it sprays fuel sideways into the cylinder. Direct injection is really great stuff for fuel economy and emissions, and helps engines make great power to boot. The downside is that now there’s nothing naturally cleaning the intake valves as the car’s used.

Combustion is a naturally filthy process, creating lots of carbon compounds and gunk that gets pushed into the rest of the engine, past the pistons. For emissions, that gross stuff gets recycled by the PCV (Positive Crankcase Ventilation) valve into the intake to burn off.

Like I said earlier – port injection cleans the valves of those carbon deposits constantly. Direct injection lets it build up indefinitely. Some manufacturers have gotten around this by doing dual injection; both port and direct injection. In that setup, the car is primarily fueled by the direct injectors, and assisted/maintained by the port injectors.

But generally, if your car has GDI, you’re ideally cleaning the valves regularly. Sure, you can let it gunk up forever, but there will be a point where the car is undrivable, misfiring, and may even refuse to run. There is no getting around it, if you have a direct injection only car, this is a mandatory service at some point. Get it done, or do it yourself! I say do it yourself, because a walnut blaster runs about $500, not counting the need for an air compressor and shop vac. Most shops will charge upwards of $700 to do the service once. 

Your car’s owner’s manual (you should be able to look it up online if you don’t have one) should tell you how often you should optimally do this cleaning, and in the spirit of Car Bibles, we’ll run you through the best way to take care of it yourself if you’re so inclined. Usually, it’s something to think about every 40,000 miles. Sometimes, the manuals won’t mention it at all, so we’re here to help anyways!

My car, away from it’s natural habitat, amongst many Subarus. – Image: Chris Rosales

An unusually stormy SoCal sky rumbled above as I made the zero-boost trip to my friends at Yimisport Tuning in Valencia. Paul Leung, tuner and owner, was gracious enough to let me borrow his walnut blaster for my impending service. Yes, the best way to clean an intake manifold is to force-feed it crushed walnuts. Well, sort of. Walnut blasting almost resurfaces the intake ports, like sand blasting but more gently. It uses walnut shells to pummel the carbon off surfaces. This is done with a special tool that uses pressurized air, a shop vac, and an adapter to stick onto the intake ports. Such a tool runs about $200.

Also, thanks to the techs (my friends) for letting me steal some of their excellent pro-grade tools to get my intake off quickly!

Excuse how filthy this is, I do not believe in engine detailing. – Image: Chris Rosales

About an hour after arrival I had everything removed, and ready to get walnut blasted. The valves were filthy, especially considering the time I took with some CRC GDI cleaner and a pick about 20,000 miles ago. It was a good call to clean these instead of slapping my new manifold on and calling it a day.

The experiment went like this: I walnut blasted two intake ports, left one alone as a control, and flooded one with an off-the-shelf valve cleaner. I soaked the valve cleaner port, grabbed some lunch, came back and blasted the other two valves.

Spraying the semi-magic stuff in, with dubious results. – Image: Chris Rosales

To put it crudely, the spray didn’t seem to work. The walls of the intake runner were cleaner for sure, but I let it marinate for two hours, soaked it up, gave it a few scrapes with a pick, and the big carbon chunks were as stuck on as before. You could also apply this spray with the engine running via the intake, but I didn’t have a way of showing a real before and after without spending the whole day screwing around with my intake. 

The valve cleaner in a can after two hours of sitting. There was still a ton of gunk stuck on the valves that a pick wouldn’t move. – Image: Chris Rosales

Either way, I don’t believe the spray will do the necessary work to clean the valves. The can directs the user to apply the spray every 10,000 miles via the intake, which shows that it doesn’t really clean the valves, it just bandages them up and sends em on their way to get gross again. The spray doesn’t kill the fine layer of carbon deposits on the walls of the intake port, allowing carbon to redeposit easily.

Usually, you would use an adapter specially shaped for your engine. Seeing as Yimisport is a Subaru shop, they only had a Subaru FA20DIT adapter that I made work on my VW TSI engine. It’s super easy to use. The adapter has a port for a shop-vac to suck up all the walnuts, and the walnut blaster is inserted into a small port. Voila, you’re walnut blastin’.

Using a Subaru adapter to blast my VW valves! – Image: Chris Rosales

All it takes is about a minute of blasting per port, making sure you have ear and eye protection (take my advice), and not letting the flying walnuts sting your arm too much. Easy peasy! Take a look at the results:

This took about 30 seconds of walnut blasting, unbelievable. – Image: Chris Rosales

Literally sparkling clean. The treatment looks really aggressive, and gives the ports a dull texture. I could swear it looks like it blasts material away, but it just looks strange because it’s so clean. 

Reader, I wouldn’t waste my time with the cleaner-in-a-can. Sure, it’s cheaper, but it doesn’t really fix the issue. I cleaned these valves a mere 20,000 miles before this with a similar cleaner, and I spent five hours picking and hacking at the carbon, for nothing. The usual interval for buildup this bad is about 40,000 miles, and it got this bad in half that.

The performance difference was notable for my car, too. Immediately the VW felt like it made more power, and ran a bit smoother. After a few days of driving, the car is actually self-tuning more boost into itself. It used to peak around 15 psi, and now the car is happy to peak at 18 psi like the APR tune advertises. All around, this was a big win for the GTI.

Factoring in the cost of your time (trust me, it’s valuable), and assuming you’re going to own your direct injection car for a good long while, I think it’s wisest to invest in the walnut blasting rig for a long lasting, and easier clean. It’s a bit more money up front, but you’ll save your back, and your time over the ownership period of your car. Especially considering most shops will charge north of $500 to do the same service.

Arm up and DIY it people, it’s easy and honestly fun! Get to blastin’!