Piston Slap: A Solution to Our Coking Problem?

Sajeev Mehta
by Sajeev Mehta

Kurt asks:

I’ve heard from maintenance shops and oil additive producers that DI engines, especially Audi and BMW, have severe problems with carbon buildup in their valve systems. Might be good to chat about this and also poll readers to see if other vehicles have the same issue. Thank you.

Sajeev answers:

Luckily a previous editorial in our “Ask an Engineer” series discussed this problem, and it agrees with your assessment. It also agrees with what I heard before GDI (Gasoline Direct Injection) motors were released in significant numbers here in the USA: my mechanic friend in the UK was the first to tell me about the walnut shell blasting method, discussed here. Using all his subtle British charm in describing his true feelings, you can say that he rather hates GDI engines. And he’s probably not the only wrench to feel that way.

Which rather blows. Because GDI (and Diesel DI) is a fantastic concept that makes perfect sense. It is the next logical step in the evolution of the internal combustion engine. Hopefully we can find a way around engine coking, aside from the obvious answer: running at wide open throttle a lot of the time…that kinda defeats DI’s advantages over port-EFI, ya know.

I wonder if the latest GDI motors, especially the non-turbo versions in many a mainstream GM/Hyundai/Ford sedan, shall meet the same fate of the coke-happy BMW and Audi products you mentioned.

So my question to the B&B: how will technology overcome our coking problem?

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry.

Sajeev Mehta
Sajeev Mehta

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  • Definitely do a FI service about every 30K. I use a particular two-letter brand in our shop and it works very well on just about every engine we apply it to. It will set you back around $100.

  • Andy D Andy D on Jul 26, 2012

    Coupla old BMW and furren car tricks to de-carbonize valves. First and funnest is the Italian tune up. An hours run at high RPM in a low gear Next is a hot soak used as instructed on the can. Inducing SeaFoam or if you're really brave , water through a vacuum line

  • Kurtamaxxguy Kurtamaxxguy on Jul 27, 2012

    I asked Audi about the known intake coking problem on A$'s and they claimed their switching piston rings (using rings from the A3) solved the problem. Perhaps it also lowered the A4's oil consumption as well? German cars for some reason consume a great deal more engine oil than other brands.

    • DeadWeight DeadWeight on Aug 10, 2012

      You may have heard me mention it before, but our brand new 2006 VW Passat 2.0T was burning no less than 1 quart of synthetic oil- from the get go- every 1,000 miles. When I took it in for service under warranty, the tech I dragged off to the side to speak with admitted it was a problem, but after two days with a loaner, the service manager called me to tell me my car was "ready" and that their testing revealed no problems whatsoever. When I asked if it's normal for a new car to burn at least a quart of synthetic oil every 850 to 1,000 miles, his canned response was that VW has specified that's within normal parameters, and subsequent calls to VW North America HQ only elicited the same canned response.

  • DIYer DIYer on Aug 01, 2012

    At every oil change interval, pour half a can of Sea Foam into the brake brake booster vacuum hose. Rev the motor, there will be lots of white smoke, and then turn the motor off and let it sit for 10 minutes to soak in. Then drive the car hard for 5 miles until the white smoke goes away. This will clean most of the carbon off the intake valves. Pull one of the plugs to see if carbon has been deposited, and clean the plugs as necessary.