---
title: 'Why Direct Injection Engines Get Carbon Deposits'
source: 'https://youtube.com/watch?v=AvLU5EfhNI8'
video_id: 'AvLU5EfhNI8'
date: 2026-07-28
duration_sec: 847
channel: 'Regular Car Reviews'
---

# Why Direct Injection Engines Get Carbon Deposits

> Source: [Why Direct Injection Engines Get Carbon Deposits](https://youtube.com/watch?v=AvLU5EfhNI8)

## Summary

This video explains why direct-injected engines accumulate carbon deposits on intake valves, tracing the issue back to emissions regulations and the shift from port to direct fuel injection. It covers the history, mechanics, and maintenance solution known as walnut blasting.

### Key Points

- **Carbon deposits on intake valves** [00:02] — Direct-injected engines foul intake valves because fuel no longer washes over them, allowing oil and crankcase fumes to caramelize and form deposits.
- **Historical context: pre-emissions cars** [00:14] — 1963 Ford Galaxy had no EPA, no seat belts, and a ventilated oil cap that released crankcase gases directly into the air.
- **Crankcase fumes and pollution** [01:09] — Engines are not sealed; crankcase fumes escape even when off, causing old cars to smell. Emissions regulations led to sealing the engine and routing fumes back into the intake.
- **Solution: burn off fumes** [03:09] — Crankcase fumes were routed into the carburetor to be re-burned, but this introduced oily gunk that fouled spark plugs and coated intake valves.
- **Effect on intake valves** [04:35] — Hot intake valves cause oil to caramelize into sludge. With port injection, gasoline (a solvent) cleaned the valves; direct injection removes that cleaning effect.
- **Direct injection adoption** [05:57] — CAFE standards pushed manufacturers to direct injection for better fuel economy and tuning, moving injectors inside the combustion chamber.
- **Direct injection problem** [07:38] — Without fuel washing over intake valves, carbon deposits build up slowly, leading to power loss and engine issues around 100,000 miles.
- **Walnut blasting solution** [09:15] — Crushed walnut shells are blasted at high pressure onto valves to remove deposits, then vacuumed out. Leftover shells are harmless if consumed by the engine.
- **Toyota's dual injection** [10:28] — Toyota uses both port and direct injection, cycling between them to keep valves clean while maintaining fuel economy.
- **Cost and DIY** [11:11] — Walnut blasting costs $1,000–$3,000 depending on labor. DIY is possible with patience, but scraping is less effective than blasting.

### Conclusion

Direct injection improves fuel economy but requires periodic walnut blasting to remove carbon deposits. Budgeting for this service is part of ownership, similar to Subaru head gasket maintenance.

## Transcript

Why do some cars foul their intake valves with carbon deposits while others don't? Well, to understand this is to understand automotive history, the EPA,
the US government, and physics. Going back to 1963, which is what this is, this is a 1963 Ford Galaxy, this car predates the Environmental Protection Agency.
It predates most emission controls. A lot of people like to say, "YEAH, THAT'S WHEN A CAR WAS REALLY KNOW HOW TO make cars back then." Well, what they're really saying when you hear old-timers like beat their
chests and make noise about that is that automotive manufacturers had the freedom regulations, and in fact, this car at the time of manufacture was not required
to have seat belts. Is it legal to drive? Yes. A car must meet the safety do with emissions regulations? Well, this is an engine
that pollutes even when it's off. What are you talking about? Well, engines are not sealed units. There is combustion,
but there also is crankcase fumes and oil. It's a big slushy mess that goes around and around and around. This particular car, check out its oil cap.
Yeah. It doesn't even seal. The oil cap on a 1963 Galaxy is ventilated, and gases from inside the engine, not the combustion chamber, but inside the
the oil is sloshing around, just kind of get hot, and as you're driving along the engine will
ex- expel these gases into the air. And then when you get back the engine's still hot it's still gassing out. That's why old cars smell
like that. And of course there's no catalytic converter. During the 1970s, early 1970s emissions regulations started to take into effect after people figured out how ugly Los Angeles was. Those old pictures
of 1970s LA lots of smog. Most American cities in was all those gases that just normally escape through the oil cap, through the
gas tank, various other orifices well we're going to seal the engine completely up. But that creates a problem because the pressures inside the crankcase are always fluctuating.
And if you seal though and if you seal it up the pistons coming down will face resistance and the engine won't run right. Oh but we have a solution. Let's burn them off. [snorts]
We're going to take some We're going to allow some of the crankcase fumes to travel into the carburetor. This of course is not a
carburetor this is now a throttle body fuel injection unit but same principle Looks the same that's what I'm saying. The fumes will come in. They will go into the engine along with the fuel that's being sprayed
and be re-burned. And that worked okay. And later on as emissions became more thorough they started putting little hoses like that from the exhaust and taking some of the exhaust fumes,
routing them back into the combustion chambers or somewhere along the intake leading to the combustion chambers, to be re-burnt again. Well, okay, fine. It fouls up our spark plugs a little bit doing that, having like oily gunk and
stuff go into the engine, but at least you can take the spark plugs out anytime brake clean, maybe some very light sand papering. I used to do that with my motorcycles. Put them back in, you're fine. Now, what does that have to do
with the valves? Well, I'm getting to that. All that oily and crud has to travel over your intake valves before they go into the engine. Now, those intake valves are very, very
hot. And if there's one thing that oil likes to do when it gets very hot is to caramelize. It gets thicker. It's almost like a reduction in cooking. It gets gooey, nasty. You ever spill
soda on something, like a counter top or a plate, and then forget about it, and What's next? This ooey, gooey, sludge, snot-like stuff.
stuff. But, it doesn't collect on the valves with an engine with throttle body fuel injection or with something like port fuel injection. Because all that goo is getting mixed with gasoline because
the gasoline is being sprayed in along with it. And gasoline is a wonderful solvent. It cleans stuff really well. It's almost as good as brake clean. And
modern fuels have extra detergents in them to help keep your valves clean and your intake and everything else. It counteracts the necessary emission controls that cars have. And that was all well and good until the early 2010s
when new CAFE standards were coming along and manufacturers were searching for ways to get more fuel economy out of engines, run leaner burns, and still make the kind of power they
want to. And the solution was to move the fuel injectors from outside the combustion chamber inside the combustion chamber. Now, what that allows you to do is control where inside the combustion
chamber the explosion takes place. That opened up all kinds of possibilities for tuning, especially on performance machines like this Golf R that is a giveaway. And forgive me for plugging my giveaway again, go to
fanofem.com/regularcars, buy some limited edition merch, and you're automatically entered to win this very fast 2013 Golf R with direct injection. Engines with direct injection can run
leaner, use less fuel, make more power, and you can get microscopic with the tuning. Sure, the fuel injectors cost a little more because now they have to
withstand the crazy cylinder pressure inside the engine, but if you look at them, this is a direct injector, and it was in this engine right here. This little John is meant to withstand pressures thousands
of like 150 to 180 PSI, 250 PSI, and explosions and fire and deliver fuel all at the same time. It's a wonderful piece of engineering. It's delightfully Is
this Is this cast? No, I think this is machined. They are fantastic. While direct injection solved the problem of fuel economy, solved the problem of tuning,
it created a new one. All that gasoline that was cleaning the intake valves from all the gunk and stuff that was flowing in based on emissions regulations, now all of that hot gunk
was traveling over the intake valves without the aid of fuel. Without the aid of gasoline to keep those intake valves clean, to wash all that gunk and have it go into the engine and get burned and passed out
the back. The fuel injectors are now inside the engine and those intake sticks to them. Little bit. Like a little bit of stubble in your bathroom sink.
Like some leaves in your gutters. Not a problem. But over time, little bit more gunk sticks, more gunk And then suddenly, ever so slowly, and it's like what James
Sexton said about uh divorces. How do divorces happen? Very, very slowly and then all at once. Very, very slowly you start losing power and then all at once your engine's not running right and you don't know why.
And that service is called walnut blasting. The intake manifold comes off and a little hose,
let's use this air pump as an example. Little hose is pushed down the intake manifold until you can see the valves and it sprays crushed up walnut shells at very high pressure down around the valves and everything else. You ever
dentist has sometimes? It's kind of like that. And while they're doing it, they have an attachment for their shop vac that goes right next to it that sucks the walnut shavings back out. So, walnut shavings are being blasted. It's
cleaning off all that gunk and then getting sucked out at the same time. Is that a problem? No, because walnut shavings are softer
than all the metal that's around it. If a few are left when you start the engine, they're immediately consumed by the engine, shot out the back. Kind of They pass right through you. The problem is a lot of consumers didn't
know about this problem. And after about 100,000 mi, they started to get engine codes and their engine was behaving weird and they thought they had a lemon. Toyota solved this by using both port fuel injection and
direct injection. And it cycles between the two. Every now and again, spraying down the intake valves with a s- with a fresh batch of gasoline to make sure nothing sticks to them and then it
switches back to the direct injection for good fuel economy. But this car from 2013 predates that. This is direct injected only. So, every 100,000 mi the walnut blasting service has to be
done. Was it done to this car? Oh, yes. Did I notice a difference? Oh, yes. This engine is fresh. It's good for
another 100,000 mi. Right now, there's 107,000 mi. Should that deter you from getting a direct gasoline injected car knowing that that service has to be
done. Depending on your shop, it could be a thousand dollars, it could be three thousand dollars, labor rates vary. Can you do it yourself? Absolutely. You just have to take your time removing the intake manifold and a lot of the
extra stuff that's in the way. Can you do it without the little walnut blasting kit? Yeah, you can get in there with a little pick and scraper and scrape it all out. Will it be as good as blasting? Not
really. Frankly, I wouldn't be deterred from this as long as I budgeted that extra thousand dollars to three thousand dollars. Subaru guys know this immediately. You have anything with an EJ engine? Well, got to think about
those head gaskets every hundred thousand miles. EJs Boy, I sure do love composite head gaskets. So, at a hundred thousand miles with any Subaru with an EJ engine that doesn't have the multi-layer steel you
The problem is you really have to remove the engine of are people with single cam engines that can get them out in there. It's a
complete pain in the butt. So, it's one of those Subaru things. I know I'm getting off topic, but you drop the engine to do the head gaskets and you go Well, long as I'm there, let's do timing belt, water pump, spark plugs,
everything else. Soon that's a five thousand dollar job, alternator, everything else since you have it out. Does that deter Subaru people? Not really. Just cost of ownership.
So, this giveaway car that we have is all set to go. Whoever wins this giveaway is getting a Golf R that well, I don't want to claim any sort of guarantees, but as far as the engine goes,
perfect. Did I close that all right? Yeah. See, I'm just I'm just used to American cars. The hoods close like these.
Slam a jama. That was just very nice European. Just clicks like, "No, it's fine." I hope that answered your question about walnut blasting and why direct injected engines develop carbon deposits. We're going to get some more
shots of this driving out there and I'll show you how much fun this is to drive once that service has been done. Drives like a new car.