This article is from the VW Technical FAQ, by with numerous contributions by Jan Vandenbrande others.
A: Check the following:
- Vacuum hoses
- Fuel Filter
- Ignition timing
- Valve timing (belt may have slipped)
- Spark plugs
- Valve clearances (older cars)
- On G60's: Some batteries have an overflow tube that dumps acid on the
intercooler tubes => holes => loss of power.
- You may be running to hot, inducing knock, retarding timing in new cars.
- Other defects...
 
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