What are the tips for passing the exhaust emission test for old cars?
3 Answers
Tip 1: Clean the throttle body once before the inspection; Tip 2: Use 97-octane gasoline in advance. Higher-grade gasoline produces less pollution. If ethanol gasoline is available, it's even better. Ethanol gasoline contains 10% ethanol, which can reduce exhaust emissions by 10%; Tip 3: Increase the engine speed on the way to the inspection site. Due to urban traffic congestion and frequent stops, incomplete combustion occurs, leading to pollutants adhering to the surface of the three-way catalytic converter. Tip 4: After the appearance check, rev the engine hard. It is unwise to take a cold car to the inspection line. The reason is that the optimal working temperature of the three-way catalytic converter is between 400 to 800 degrees, and it takes a long time to reach 400 degrees from a cold start.
My old car has been running for over a decade, and the most headache-inducing part of annual inspections is the emissions test. I've found the key is keeping the engine in optimal condition: drive on the highway for half an hour before the test to heat up the catalytic converter for effective exhaust conversion. Don't skimp on regular maintenance—replace spark plugs every 20,000 km, shake out the air filter every six months; use low-volatility engine oil like 5W-30 as specified in the manual. Always add fuel system cleaner to the tank of an old car, the kind specifically for removing carbon deposits in fuel lines, which can reduce CO emissions by 20%. Last year my exhaust pipe turned black, but replacing the oxygen sensor made it pass immediately—this part acts like the car's nose; if it fails, it can't properly adjust the air-fuel mixture.
For old cars to pass the exhaust emission test, the engine's thermal condition is crucial. I usually book the first inspection slot in the morning and let the engine idle for ten minutes to warm up before heading out. Once, I tried testing with a cold engine, and the HC emissions were three times over the limit. Now, during every maintenance check, I make sure to inspect the ignition coils and high-voltage wires—deteriorated rubber insulation can cause incomplete combustion. Setting the spark plug gap to 0.8 mm is optimal; too large a gap weakens the spark and can lead to black smoke. I always use 95-octane fuel for its higher octane rating and cleaner burn. If the catalytic converter gets clogged, a high-speed run might help clear it, but never poke it with an iron wire—my neighbor did that and shattered the honeycomb ceramic, costing him half a month's salary for a new converter.
