Reader Question: Direct Injection Defense?

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Here’s the latest reader question, along with my reply!

Mark asks: I just learned that my car has direct injection. What does one do to prevent carbon buildup on the valves?

My reply: Many (soon, probably almost all) new car engines have direct injection, a system in which misted fuel is introduced at extremely high pressure (several thousand PSI) directly into each cylinder.

In port fuel injection systems, the injectors are located adjacent to the intake valve, where fuel is sprayed at much lower pressure (35-40 or so psi) and drawn into the cylinder.

The gasoline has a solvent effect – and cleans off the backside/stems of the intake valves.

There is no such effect in a DI system, which is why these engines are vulnerable to carbon building up over time. When excess carbon builds up, the intake valves may no longer seat properly when closed and now you’ve got blowby and that’s bad news. Sometimes the carbon can be cleaned off via flushing; sometimes, it requires top end disassembly.

A number of manufacturers have added a port fuel circuit to their DI-injected engines. For the sole purpose of avoiding the carbon build-up issue.

You may ask: Why not just stick with the PFI system and forget the DI – which is also far more complex than PFI, including two fuel pumps? One word….


Government pressure to extract MPG gains – however modest and regardless of the cost to the buyer – have incentivized increasingly over-the-top engineering solutions; DI is just one of several examples (others include cylinder deactivation, “mild hybrid” drivetrains, transmissions with as many as three overdrive gears).

DI tempts the engineers – and the car manufacturers – because it offers an efficiency gain over PFI and so a way to make Uncle happy without down-powering or downsizing engines, which would make buyers unhappy.

DI also increases engine output, which is a definite benefit for the buyer.

But, it comes at a cost – and not just up front (DI cars are more expensive to buy than a car equipped with PFI).

There are several potential down-the-road costs which the buyer will have to absorb  – including carbon fouling but also the possibility of the various additional/more expensive parts in the DI system failing and having to be replaced.

Back to your question!

The first thing to do is check whether your car has the extra PFI circuit; if it does, you’re all set.

If not, the second thing to do is hope your engine doesn’t become carbon fouled and the third thing is to hope that if it does, it can be cleaned by flushing rather than disassembly.

Got a question about cars – or anything else? Click on the “ask Eric” link and send ’em in!

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  1. Hey Eric: I’ve heard guys/seen Youtube’s about adding an oil catch can to the PCV system. This helps in preventing some of the oil mist/particles from making it all the way to the (top sides of) intake valves. Empty the catch can when you change your oil. You could also Seafoam (e.g. solvent) the intake system, injecting/having it get sucked into the intake plenum so as for the solvent to make it to the valves. I think this gets called for about once a year.