Why does my engine misfire?

What does it mean if your car is misfiring?

The most common culprit for engine misfire is worn, damaged or faulty spark plugs. Engine misfiring combined with a lack of power could also be a sign of a worn-out timing belt.

The cylinder is the part of the engine where combustion takes place. This combustion is what powers your car. Depending on the make and model of your car, an engine may have as few as two or up to as many as 12 cylinders.

A misfire is the result of incomplete combustion inside one or more of these cylinders. This can cause a proportional loss of power. For example, if one cylinder misfires in a four cylinder engine, the car will lose 25 percent of its power. This incomplete combustion, which leads to an engine misfire, can be caused by faulty or damaged spark plugs - if there's no spark, there's no combustion or ignition in the cylinders.

A worn-out timing belt can also affect your engine fire rate since it is attached to the pulleys that drive the crankshaft. Sometimes, the belt will slip on the camshaft drive causing the engine cylinder to open and close earlier than it should, resulting in a misfire.

What does an engine misfire feel like?

You may be wondering how to identify an engine misfire. There may be a noticeable loss of power, or a brief hesitation in the delivery of power when accelerating. The car will still be able to run and you'll be able to drive it, but you'll start to notice the engine jerking or stumbling a lot. This could be followed by exhaust smoke coming out of the tailpipe and the smell of gas.

What does an engine misfire sound like?

During a misfire, the engine will make a loud popping sound or a sudden bang, sometimes described as backfiring. Backfiring occurs when unburned fuel leaves one cylinder on the exhaust stroke and is then ignited further in the system by the spark of the next cylinder.

What are the common causes of an engine misfire?

Worn, faulty or damaged spark plugs

Risk level - Medium

What to do - Inspect the spark plugs for signs of damage. Use a spark plug socket to remove the plug so you can get a good look at it. If the spark plug is just old or damaged, replacing it may solve the problem. You should get replacement spark plugs fitted as soon as possible. Each car requires a particular type of spark plug made from specific materials and with an exact spark plug gap that is set by a mechanic during installation.

It's important to note that with each misfire you could be dumping unburned fuel into the catalytic converter. The catalytic converter will burn up the fuel, but it will get very hot in the process and so you risk damaging it. A catalytic converter is a very expensive item to replace, therefore it's important to diagnose and resolve your engine's misfiring problems as soon as possible.

Timing belt issues

Risk level - Medium

What to do - In most cars you can visually inspect your timing belt. Be sure to look for a slipped belt and signs of wear, such as shredding or tearing and check for cracks in the belt as this can be a sign that your belt is old, dried-out and needs replacing.

A worn or failing timing belt can cause an array of serious issues, therefore it is vital that you get a replacement timing belt as soon as possible.

The fire rate of the engine could be jeopardized from a worn, faulty or slipped timing belt. If the timing belt were to slip away from the gears and fall onto the camshaft, the cylinders will open and close too soon. If that happens, an engine misfire could occur which means the belt needs to be replaced right away. If you don't replace the belt soon, the engine could be permanently damaged.

How do you fix a misfire?

The cause of your misfiring engine will determine the best course of action to fix the problem. If the misfire is being caused by failing spark plugs, these can be replaced fairly easily and affordably, either at home (if you're confident performing DIY car repairs) or by a trained mechanic.

If your timing belt is the cause of your engine misfiring you will need to book your car in for a replacement. Fitting a new timing belt requires a decent understanding of engines, so if you're not that experienced working under the bonnet we'd advise you to leave it to the pros.

Is a misfiring engine dangerous? Can I drive with a misfiring engine?

A misfiring engine is likely a symptom of failing spark plugs. You'll probably be able to continue driving your car, however driving with worn out or damaged spark plugs can ultimately cause engine damage, so don't put it off.

If your engine keeps misfiring, it could indicate a worn or failing timing belt. You cannot drive your car if the timing belt is broken. The timing belt is one of the most important parts of your entire engine though not every driver is even aware of what it does or where to find it. While it looks like a simple part of your engine, and really it is, the job it does is integral to the operation of your entire car. Continuing to drive without getting your timing belt replaced, may result in a heavy damage to the engine with broken or bent valves, damaged pistons and, possibly, destroyed cylinder head and block.

How much does it cost to replace spark plugs?

The most likely reason your engine is misfiring is due to damaged or failing spark plugs. The cost of replacement spark plugs depends on the made and model of your car. Costs can range from £40 to £350, with the average price in the UK working out at £97.

How much does it cost to replace a timing belt?

Frequent engine misfiring could be related to issues with your timing belt. Depending on your car's make and model, the cost of a new timing belt and water pump can range between £210 and £620, the average cost in the UK is £369.