What are the reasons for oil entering the throttle body?
3 Answers
The reason for oil entering the throttle body is: The engine's exhaust gas recirculation is discharged into the intake manifold, which over time forms deposits on the throttle body, and this is a normal phenomenon. The effects of a damaged throttle body are: 1. Unstable engine idle, persistently high idle speed that does not drop, and difficulty in starting the engine, especially during cold starts; 2. The engine has no idle speed; 3. Insufficient engine power, poor acceleration performance, and unstable operation; 4. The car's exhaust pipe emits black smoke, and fuel consumption increases. The reasons for a dirty throttle body are: 1. Driving environment, poor filtration by the air filter, and high levels of dust in the surrounding environment; 2. Poor quality engine oil with high evaporation rate; 3. Oil vapor and impurities are produced in the crankcase; 4. The throttle body sucks in dust after the engine is shut off.
Last time I was repairing a car, I encountered an owner with oil in the throttle body, which is quite common. It's mainly caused by the backflow of oil vapor inside the engine, especially in older cars that are more prone to this issue. When the piston seals wear out or the small oil mist separator fails, oil vapor can mix into the intake pipe and eventually accumulate at the throttle body. Also, your driving habits play a role—if you frequently floor the gas pedal and brake hard, the fluctuating engine pressure makes it easier to push oil vapor into the intake. I’ve disassembled and cleaned several cars where the back of the throttle body was coated in sticky oil sludge, directly causing rough idling and a spike in fuel consumption. I recommend checking the ventilation lines every 20,000 kilometers—catching issues early can save you a lot of money.
I've studied several cases of throttle body oil contamination, and the root cause lies in inadequate crankcase ventilation. When the plastic tube connecting the engine to the intake manifold becomes aged and cracked, oil vapor leaks in. For turbocharged vehicles, worn turbocharger shaft seals can spray oil directly into the intake pipe. Recently repaired a German car with this exact issue - the owner complained about sluggish acceleration and high fuel consumption. An endoscope inspection revealed the throttle body's backside was completely coated with oil residue. Performance restored immediately after cleaning. During maintenance, always have technicians check the PCV valve and hose conditions. Plastic components should be replaced every five years.
