What is the reason for engine shaking at startup but no shaking while running?
4 Answers
The reasons for engine shaking at startup but no shaking while running are: 1. The spark plug is damaged, the gap between the spark plug electrodes is worn out, resulting in insufficient spark plug ignition and incomplete combustion of the air-fuel mixture; 2. Poor quality gasoline leads to poor atomization effect of the fuel injector, causing incomplete combustion of the air-fuel mixture in the cylinder; 3. The throttle is dirty or the fuel injector has excessive carbon deposits; 4. The fuel pump supply pressure is abnormal or the intake pressure sensor has incorrect values and poor performance. The function of the engine is to convert thermal energy into mechanical energy by expanding gas after combustion in a sealed cylinder, pushing the piston to do work, thereby providing power for the vehicle. The maintenance methods for the engine are: 1. Clean the air filter; 2. Remove sludge from the throttle; 3. Clean carbon deposits from the fuel injector.
I drive frequently and have encountered this issue before. Severe engine shaking at startup that disappears once the car is moving is often due to engine instability at low RPMs. Common causes include ignition system failures, such as worn spark plugs or poor ignition coil contact, which affect the engine's ignition efficiency and cause shaking when the load increases during startup. Additionally, carbon buildup in the intake system or fuel injectors can lead to uneven fuel supply, particularly noticeable at idle; once the car speeds up, improved airflow eliminates the shaking. I've also noticed that slight vacuum line leaks can worsen the problem under low pressure during startup. It's advisable to check the spark plugs first—see if replacing them helps. If not, cleaning the throttle body usually yields quick results. If the issue persists, it's best to visit a professional shop to check sensors or fault codes.
As someone with a bit of interest in automotive technology, I believe that the key reason for initial shaking but smooth running lies in changes in engine load. First, it could be due to excessive spark plug electrode gap or corrosion, which fails to effectively ignite at low-speed startup, causing misfire-induced shaking. Second, a clogged fuel injector due to contaminants may lead to an overly lean or rich air-fuel mixture at idle, especially noticeable during cold starts. Once the vehicle is running, as the RPM increases, the ECU can automatically adjust the mixture ratio. Third, a faulty mass airflow sensor could also be the culprit, as inaccurate input signals cause unstable combustion at low RPMs. I’ve dealt with similar cases before—prioritizing these checks and replacing the faulty parts resolved the issue. Don’t forget to inspect the fuse connections for oxidation; loose connections can expose problems under vibration. Before working on it, ensure the power is off to prevent accidents.
Having driven for many years, I've encountered this issue quite a few times. The shaking at startup but smooth running afterwards is mainly caused by wear and tear on engine components in older vehicles. During low-speed starts, ignition problems like aging spark plugs or damaged wires become particularly noticeable—inaccurate spark transmission causes shaking, which becomes less obvious under high pressure. Additionally, dirty fuel injectors can lead to uneven fuel distribution and an imbalanced air-fuel ratio. Once the car starts moving and the RPM increases, the system self-corrects and stabilizes. I also recommend checking the engine mount bushings; if they're loose, they can amplify vibrations during startup. The inspection cost is low, and simple actions like tightening screws or cleaning the intake system often help. Consistent maintenance can prevent such issues.
