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Throttle Body Cleaning


Usually there should be no problems with the throttle body, so no symptoms. However, when I purchased this particular B5, it had problems keeping a stable idle (wide fluctuation), as well as odd behavior when releasing the accelerator during drives. The usual tune-up did nothing to alleviate the problem - spark plugs, fuel injector cleaning, air filter replacement, vacuum hose replacement, etc. Finally, during a round of carbon cleaning, I found the throttle body had accumulated quite a bit carbon, preventing the throttle plate from seating fully closed. The dealer the car had been serviced with was probably using some fairly terrible oil to cause such a buildup (after 30k miles at the time). As the car has now reached 60k miles, I was curious to see if there was still an accumulation in the throttle body - see for yourself. (illustration)

The most likely difference is the use of fully synthetic oils with an occasional engine flush added prior to draining old oil. I can only hope that this combination is also keeping the turbo's oil supply line in good shape - if I get around to pulling the supply line to finally just take a look at it, you'll find the update here.


Oct. 2004 - The jury is still out on whether using a flush is a problem - the idea being that the thinned oil isn't capable of being a good fluid bearing for the turbo. I'm on the fence on this, as the turbo isn't in much use when the car is at idle. In the case of the '98, there was enough carbon around that there was justification for using a flush, but the current 2000 B5 has very little carbon build up. In this case, I haven't found a need for a flush - synthetic oil alone is very capable of keeping the oil pathways clean.

July 28, 2005 - Updated procedure to include DBW throttle bodies (pretty much the same procedure sans cruise control cable).

Part Information & Sources:

Note - always confirm part numbers by looking at the actual part mounted in the car! Mistakes will happen sometime, somewhere.

Part number:

1.8t throttle body gasket (all model years):    028 129 748

OEM Pricing: $2


  • 5mm socketed hex (Allen) driver

Throttle body cleaning:

The throttle body is connected to the intake manifold, with the air intake hose from the intercooler connecting on the opposite side. (illustration)

  1. Start by unscrewing the air intake hose clamp. Pull the intake hose from the throttle body. (illustration)
    Note that the intake hose is thoughtfully marked with the correct mounting orientation.
  2. Pull the throttle position sensor connector from the throttle body. (illustration)
  3. On non-DBW (drive-by-wire) cars (with the AEB engine), the connecting rod on the throttle body allows the cruise control system to keep you comfy on the freeway. The easiest way to free the throttle body of the car is to pop the lower end of the connecting rod out of its bracket. (illustration)
  4. Using the 5mm hex driver, remove the four screws holding the throttle body to the intake manifold.
  5. You should now be able to pull the throttle body and gasket from the intake manifold.
  6. On non-DBW cars, remove the throttle cable by moving the cable through the maze on the throttle body. (illustration)
  7. With the throttle body removed from the car, take a look inside. There will probably be some carbon accumulation where the butterfly plate meets the throttle body walls - usually nothing substantial, but in my case there were solid chunks of carbon keeping the plate from closing fully. So much for wondering why ECU was going nuts with the idle.
  8. To clean off the throttle body, use some throttle body cleaner on cotton/paper towels to get rid of the accumulation. Try to avoid using a lot of throttle cleaner, and don't just spray a bunch into the throttle body - the throttle position sensor may be damaged (but shouldn't be if you're using actual throttle body cleaner. Take care if using carb cleaner you have laying around). Just take it easy with a little elbow grease and the throttle body will shine once again. Be sure to clean the edges of the butterfly plate, as well. (illustration)
  9. When you're done, remove the old gasket and clean off the mating surfaces of the throttle body and intake manifold.
  10. Reinstall in the reverse order, tightening the throttle body mounting bolts to 84 in-lbs / 10 Nm. Note - not 84 ft-lbs!

After the first throttle body cleaning at 30k miles, the idle immediately stabilized to the normal 800-850 rpm range and has stayed that way since. Lesson - use fully synthetic oil! If you notice any substantial carbon buildup in the throttle body (more than just a thin coating of carbon on the throttle body walls), you probably have carbon buildup on your valves and piston heads.

For the valves, toss in a bottle of Techron or BG 44K with the next tank of gas. For the piston heads, search around for information on combustion chamber cleaning. My piston heads had quite a bit of carbon build up at 30k miles, and after cleaning were pretty much spotless. Now at 60k miles, I've noticed some carbon accumulation again, so a combustion chamber cleaning write-up may be in the near future.

Good luck!