What is the reason for engine shaking at startup but not while running?

[Rudy]

The reasons for engine shaking at startup but not while running are: 1. Damaged spark plugs, where the wear of the spark plug electrode gap leads to insufficient spark ignition, resulting in incomplete combustion of the air-fuel mixture; 2. Poor quality gasoline, causing poor atomization effect of the fuel injector and incomplete combustion of the mixture in the cylinder; 3. Dirty throttle body or excessive carbon buildup on the fuel injector; 4. Abnormal fuel pump supply pressure or incorrect and malfunctioning intake pressure sensor readings. The function of the engine is to convert thermal energy into mechanical energy by burning and expanding gas in a sealed cylinder, pushing the piston to do work, thereby providing power to the vehicle. The maintenance methods for the engine include: 1. Cleaning the air filter; 2. Removing sludge from the throttle body; 3. Cleaning carbon deposits from the fuel injector.

[OPaisley]

I ran into this issue recently - the car body shook like a massage chair during startup. The mechanic diagnosed it as mainly caused by cracked and aged engine mounts. These rubber mounts normally act like shock absorbers supporting the engine. When worn out, they can't suppress vibrations effectively, especially noticeable during low-RPM starts. The shaking becomes less obvious once the engine reaches stable RPM. But don't just focus on engine mounts - cracked ignition coils can also cause specific cylinder misfires, particularly noticeable when the engine is cold. In my case, replacing the right-side engine mount and the third cylinder's ignition coil solved the problem. Also check for aged transmission mounts - my neighbor's car made clunking noises during acceleration due to loose mounts. The repair shop owner warned that prolonged vibration from such issues could even crack exhaust pipe connections.

[JoannaMarie]

With 20 years in the auto repair business, I've found that 7 out of 10 cars with startup shaking issues suffer from carbon buildup. The throttle body's backside looks like it's coated in black sesame paste, and the fuel injectors get clogged with gunk, causing uneven spray. During cold starts, while the ECU is still adjusting the air-fuel ratio, the engine wheezes like it's got tuberculosis. After driving 3-5 kilometers when fuel injection stabilizes, the shaking stops. Last month, a Nissan Sylphy owner had this issue from using substandard gasoline—cleaning the fuel system fixed it immediately. But pay special attention to engine mounts on older Camrys; just half a centimeter of rubber sag eliminates their damping effect. Nowadays, we use borescopes to inspect cylinder carbon deposits—way easier than engine disassembly. Don't believe fuel additives can cure it; stubborn carbon requires physical removal for reliable results.

[ArabellaFitz]

Last time I drove the old Tiguan, it had the same issue—shaking like going over speed bumps when starting cold. The mechanic hooked up the computer to read the data stream and found intermittent misfires in cylinder three. After replacing the spark plugs, it smoothed out immediately. The original spark plugs had run 80,000 kilometers, and the gaps had burned wider. Keep an eye on the ignition coils; rubber aging and leakage can also cause specific cylinders to misfire. My colleague’s Focus had cracked ignition coils, and it shook like crazy on rainy days. With modern direct-injection engines, pay attention to the fuel injectors—sticking solenoid valves can lead to poor fuel atomization. The mechanic said this kind of shaking usually happens at low RPMs under high load, like when starting on a slope with the steering wheel shaking, but it’s fine when driving fast on flat roads.