What causes the thud sound when starting the car after it has been parked for several days?

[SophieMarie]

This occurs due to ignition delay, where unburned gasoline is expelled into the exhaust pipe just as the spark plug ignites, causing the atomized gasoline to combust within the exhaust system and produce the sound. The primary cause is prolonged parking, which drains the battery and leads to this abnormal noise. Precautions for long-term vehicle storage are as follows: 1. Disconnect the battery's negative terminal: Even when the vehicle is locked with the remote, the anti-theft system remains active. Although the voltage is low, extended periods can still deplete the battery, preventing the car from starting next time. 2. Protect the paintwork: Use a car cover to shield the vehicle. This prevents rapid oxidation of the paint due to exposure to sun and rain, and also avoids excessive interior temperatures from direct sunlight. 3. Check tire pressure: Ensure the tire pressure is adequate. If it's too low, inflate to the standard pressure to prevent insufficient pressure after long-term parking, which could hinder driving upon next startup.

[MasonLee]

Once I left my car parked outdoors for several days, and when I started it again, I heard a clunking sound. This is usually due to rust on the brake discs because prolonged parking in humid conditions causes rust to form on the discs, making a noise when the brakes are released. It typically goes away after driving a few meters. It could also be due to rusted suspension ball joints or bushings sticking, which release and make a popping sound when starting. It's advisable not to leave the car parked for too long after rain, regularly lubricate chassis components, or lightly press the brakes a few times before driving to warm them up and prevent rust, ensuring the brake system operates smoothly. If this happens frequently, it might indicate aging components, so timely inspection is recommended to prevent potential hazards.

[HopeLynn]

As someone who often maintains my own vehicle, I've noticed that a clunking sound when starting after the car has been parked for a few days is often due to issues with the chassis connecting components. Parts like suspension arms, stabilizer bars, or the brake system can get rusted and stuck due to exposure to wind and rain, causing metal surfaces to rust and adhere together, then suddenly release when the clutch or brake is released. Short trips may not be affected, but over time this can accelerate wear. I recommend parking on dry, level ground to avoid water accumulation and regularly applying grease to lubricate the joint areas to ensure durability. If the noise becomes louder, it's essential to inspect the suspension to ensure driving safety.

[Jeffrey]

When I first started driving, I encountered a similar situation that gave me a scare. After asking around, I learned the cause was either tire tread deformation or brake disc adhesion. Parking for extended periods can cause localized deformation of the tires due to pressure, resulting in noise when adjusting posture during startup. Alternatively, the brake discs might rust and form an adhesion, causing a clunking sound when released. The lesson is to avoid parking for too long or to test-drive a few steps after moving the car. Rust prevention measures, such as keeping the parking space dry, can prevent most of these issues.