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. saw this the other day, will paste it in since you need to register to view the topic. May be of some help to anyone with a DI car…


    begin quote;
    “UhOh,, You did it now. You got me up on my soapbox so get ready to do some reading. This issue has angered me for many years. Here it goes..

    There is plenty of room for R&D in this area but, DI has some serious issues! Of and by itself, its a wonderful fuel management system and it has great promise for the future charge stratification combustion. Nothing wrong with DI of and by itself. When used it in conjunction with (EGR) retarded cam timing to reburn exhaust gases , you get a total fricken disaster of monumental proportions. Disaster I say! These OEMs are retarding the cams 20-30+ degrees in order to reburn exhaust gases during cruise. During CRUZE! How stupid is this you ask? Well, from an engineering perspective its just monumentally stupid especially from a financial aspect.

    Its tantamount to running your engine so far out of tune that it carbons up everything, fouls the plugs, contaminates the crankcase, reduces engine life and increase and increases both maintenance schedule and cost. Of course this is not what actually happens, but what actually happens is just as bad.

    These OEM DI engines retard the cams and recirculate the very hot exhaust gases full of soot particulates and a sparse amount of contaminated but unburned fuel. This EGR increases fuel economy by about 1 mile to the gallon or maybe more. This is a manipulation in tuning in order to conform to, and satisfy the out of control CAFE standards. The effect of this manipulation in tuning has DIRE consequences for both power and,,, fuel economy! Over time this “out of tune” EGR situation cokes up the valves and shuts off the air flow into the engine. If you run a DI engine for 20000 miles, you will have reduced power due to the valves being coked up and shutting of the air flow into the engine. Its not uncommon at all to see 10-20%+ power loss after 30000 miles!. After 30000 mile, fuel economy suffers as well. That’s nonsensical at best. Your instigating a situation to increase fuel economy but that very situation causes decreased fuel economy for more than half of the life of the vehicle. Valve Coking not only shuts off the air flow after 15000-20000 miles, its reduces the longevity of the engine over all. Now, does that sound like common sense engineering. Not in your wildest dreams would you or I ever design an engine that did this. If you did, I would venture to say the great majority of people would call you a monumental dumbass for doing something that stupid.

    Now for the most nonsensical part of this ordeal., This situation is “false economy”. This is going to make you mad,,,, I hope.

    Modern DI engines utilize EGR for increased fuel economy. This results is coked valves that shut off air flow to the engine greatly reducing power and eventually affecting the very fuel economy it was supposed to help. Now for the “false economy” part of this diatribe. If you drive the car for 100000 miles at an average of 29 miles to the gallon you will use about 3448 galls of fuel. With EGR active you will use about 3225 gallons of fuel at the very best. You save about 223 gallons of gas. At 3.50 a gallon your cost saving over 100000 miles will come to about $780.00. That’s not a lot of saving per individual. The grand scheme of all this is to save about 9 million barrels of fuel every year in the US. At what cost though? What does this actually cost the consumer. Over the life of the vehicle it will cost you about $4000.00. Why, because you have to take your vehicle into the dealer and get the coked valves blasted clean at a cost of $1200,00 every 3000 miles. Every 50000 at the very least. That’s right, they have to pull the intake and BLAST the coked valves clean using crushed nut shells as an abrasive. This is what OEMs now call, ” Routine maintenance”. Routine for any DI engine that is. By implementing EGR they have made the consumer pay dearly!

    Bet they didn’t tell you that when you purchased your new Corvette with the LT1 DI engine, did they.

    I purchased Chevy Cruze for a back and forth work vehicle last moth. Cheap car with great fuel economy and a peppy turbo 4 banger. I have heads off some of these engine in the shop. I see what happens to the ports and valves after only 15000 miles. After about 15000-2000 miles the port cross section is reduced by 20-30%! The VERY first thing I did was tune the cams to standard positioning and take all the EGR out of the system. You can tune the cam position with EFI live or HPTuners as I do. Its very simple. I have tuned the engine relative to load and engine speed. It gives me plenty of latitude to tune the power curve as I like it. It allows one to keep the cams at 2-3 advanced and retard the cam timing as engine speed increases for a broad power curve. It also allows me to tune out that 28 degrees EGR retard. The engine is much happier operating in a tuned situation. How would have thunk it…..

    You can find articles on this issue all over the net.

    Darin Morgan
    Mast Motorsports
    Induction research and development
    Custom porting, CNC programs, Repair ”
    end quote.

    • Hi Dirty Bob,

      Thank you for posting this; it reminds of the Air Injection Reaction (AIR) pumps used for similar reasons back in the late ’60s and into the ’70s; also the Mopar “lean burn” system… remember those?

      We are in the nuthouse – but it’s the wardens who are crazy!

      • Yes, welcome to Crazy World. Sponsored in part by crazy regulators, crazy corporations, and just plain crazy (among others). Hope you enjoy your stay.

        I have seen but am unfamiliar with the lean burn systems. I always thought the AIR pumps to be unnecessary but fairly innocuous (aid in faster light off of the catalytic converter). Plenty more in the useless/counterproductive emissions compliance category.

        link at bottom of orig. quote broken, hope this fixes it…
        If it isn’t, you’re not missing much. Most of it was already covered.

  2. 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.


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