Why does the car shake when stationary in P and N gears?
4 Answers
When the engine is idling in D and N gears, if the engine idle vibration exceeds the expected value of ±20r/min, it is not normal. The following are specific explanations: 1. Intake system: The air flow meter or its circuit is damaged, the engine control unit plug has poor contact or the internal computer is damaged, the throttle motor is damaged or stuck, the idle stepper motor, duty cycle solenoid valve, or rotary solenoid valve is damaged or stuck. 2. Fuel system: The injector is clogged, has poor sealing, or sprays fuel in a line, the fuel filter is clogged, the fuel pump filter screen is clogged, the fuel pump has insufficient pressure, the fuel pump safety valve spring has too little tension, or the fuel inlet pipe is clogged. 3. Ignition system: The ignition trigger signal is missing, the ignition module is damaged, the ignition module power supply or ground wire connection is loose or has poor contact, or the primary coil or secondary coil is damaged.
I've encountered this situation quite often in my car repair work. Engine vibration at idle in P or N gear can be caused by several common issues. The most frequent cause is aging and cracking of engine mounts - those rubber blocks are designed to absorb vibrations, but when they deteriorate, the shaking gets transmitted to the car body. Another common reason is misfiring due to air-fuel ratio imbalance, which could be caused by severely worn spark plug electrodes or cracked ignition coils making a cylinder intermittently misfire. Carbon buildup is also worth checking - if the black sludge around the throttle valve edge exceeds 2mm thickness, it can affect intake precision. For automatic transmissions, old and deteriorated hydraulic fluid that hasn't been changed for years can cause valve body clogging leading to jerking movements. I once repaired an old Camry that shook so badly at red lights that phones would dance in the cup holders - replacing all engine mounts immediately solved the problem. A good diagnostic tip: if gently pressing the accelerator in N gear reduces the vibration, it likely points to ignition or fuel system issues. New cars might have slight vibrations during break-in periods, but persistent shaking warrants inspection.
The other day, my neighbor was complaining to me about her car shaking at red lights. This kind of P/N gear vibration is indeed quite annoying. The key is to observe the pattern of the vibration: occasional slight tremors might indicate worn-out engine mounts, where the rubber has aged and lost elasticity; continuous rhythmic shaking could point to issues with spark plugs or ignition coils; if accompanied by slight fluctuations in the tachometer needle, it's likely due to uneven air-fuel mixture caused by clogged fuel injectors. Cars with auto start-stop systems show this more prominently, especially during the restart moment, feeling like a gentle kick. Listening for unusual sounds in the engine bay is also important—a rumbling noise might suggest a failing pulley bearing. Once, when accompanying a friend for maintenance, the technician found abnormal oxygen sensor readings causing an overly rich air-fuel mixture. If the shaking worsens when idling with the AC on, it means the compressor is increasing the engine load and amplifying the problem. It's advisable to first check the mileage—if it's over 60,000 km, prioritize inspecting the ignition system; for older cars, start by measuring the engine mount gaps.
I've encountered idle vibration several times with modified cars. After prolonged use, reinforced engine mounts actually cause more vibration; polyurethane material is harder than factory rubber, resulting in poorer damping. For those who have undergone engine swaps, if the driveshaft angles aren't adjusted precisely, idle resonance can also occur. Straight-through headers weaken exhaust backpressure, causing exhaust pulse imbalance at low RPMs—you can feel the seat buzzing in neutral. Cars with poorly tuned ECU remaps exhibit erratic air-fuel ratio fluctuations at idle. At the last track day, a Civic had a faulty knock sensor, making the engine shake like a massage chair at idle. Ignition coil modifications that overlook heat dissipation can also lead to intermittent failures.
