Feeling your car judder for the first time can be an unsettling experience - the sense you've lost full control of your car. Any kind of judder poses a risk to you, your passengers and other road users, should be addressed as soon as possible.
Finding out why your car is juddering is often complicated because there are so many possible causes.
If your car has relatively low mileage (say, 30,000 - 50,000 miles), it’s unlikely to be a serious problem. The easiest and quickest way to diagnose the issue is to take your vehicle to a garage.
Getting an idea before you visit the garage
If you're concerned your garage may carry out excessive tests or even exaggerate work done, you may wish to understand the possible causes first. This way you can at least sound like you understand the likely issues.
Is your car under warranty?
It goes without saying that if your car is still under warranty, you should contact your dealer to have it looked at.
Is your Check Engine Light on?
If your car is juddering, your 'check engine' light may well be on, or come on intermittently. However, it may not come on at all even though you are experiencing a judder.
Some issues can perplex even the most experienced mechanic. However, the cause is probably down to a faulty component - the trick is finding out which one!
Is the noise coming from the engine, transmission, suspension, or driveshaft?
It's important to identify if the juddering is related to the engine (engine speed issue), or the transmission/suspension/driveshaft (road speed issue). If you have little mechanical knowledge then this will of course be a big challenge.
Here are some of the most common (and less common) causes of a juddering car:
Do your spark plugs need to be cleaned or replaced?
A faulty spark plug could cause the engine to misfire. Any halt in the ignition of the fuel-air mixture could cause the engine to halt momentarily - resulting in a judder.
High tension leads (aka spark plug wires) may need to be replaced
These cables connect distributors to the ignition coil. If a fault develops then ignition failure can occur.
Ignition coil failure
The ignition coil amplifies the battery's voltage, creating the surge of electricity required to fire up the engine. If a coil fails, it won’t ignite the air-fuel mixture and a cylinder will stop operating momentarily, interrupting the engine's power production. The more coils and cylinders your car has, the less noticeable any judder will be.
Lambda/oxygen sensor: Air-fuel mixture may be 'too lean'
Your car is almost certain to have at least one oxygen sensor or 'lambda' sensor. These reside in your exhaust system and tell the car's computer how much oxygen is in the fuel. The vehicle then adjusts or compensates based on this. However, if the sensor is faulty, it may be giving an inaccurate reading, prompting your engine management system to operate on incorrect data, leading to a fuel mixture which is too 'lean'. This could cause a judder.
Oxygen/lambda sensors come into contact with very high temperatures and various contaminants, so will deteriorate over time. They should last for around 100,000 miles, but like all auto parts they will eventually fail.
A high RPM may send too much power through the transmission, so that when you drive off the car jerks forward. A high idle rate could be caused by a faulty engine management system, or a vacuum leak in one or more of the hoses.
Faulty CV axle joints
Faulty CV (constant velocity) axles on the drive train could cause heavy vibrations or juddering. CV axles wear out over time. Other signs your CV axles are under-par include loud clicking noises while turning and grease on tyre edges.
Poor quality petrol
Although unlikely these days, poor quality fuel could cause problem with your engine.
Visiting a garage
Unless you understand the mechanics of your car very well, or are prepared to spend a lot of time learning, your best bet is to visit a reputable garage. The above information should, however, provide some indication as to what the issue may be in the first instance. It may be something as straightforward as a faulty spark plug - which you may be able to address yourself - or it could be something more complex, requiring the diagnostic tools and skills of an experienced mechanic.