Engine Misfire

Misfire – What Is It?

When an engine is running properly, each cylinder is “firing”. For the cylinder to fire correctly, it needs certain variables. It needs fuel, oxygen, heat and pressure.

Fuel – Petrol (gasolene), diesel or LPG.
Oxygen – Atmospheric air contains 21% oxygen.
Heat – Either from a spark from a sparkplug (petrol) or the heat from high compression (diesel).
Compression – From the piston moving along a closed cylinder.

Removing any one of these factors will cause a misfire. In other words, the engine doesn’t fire correctly. Also, varying the variable can also cause a misfire. Low compression, incorrect fuel or air etc.

What Does A Misfire Sound/Feel Like?

A misfire can range from the engine sounding different to the engine not running. It can sound like it is ticking louder than normal but this can also be the  spark jumping from the HT lead to the body if the HT lead is damaged. Viewing the coils and HT leads in total darkness can show you where it’s arcing.

The engine can shudder like it’s coughing each time it misfires.

The misfire that most people initially recognise is a loss of engine power under load. The car may tick over perfectly well but misfire when driven or when driven hard. This is generally due to a lost spark because the spark plug can’t deliver the sparks quickly enough. It could be a bad connection at either end so the electricity has to jump 2 gaps instead of 1.

If the engine runs fine until it heats up and then it stalls, it’s probably the crankshaft rotation sensor however, it could also be where the engine block is expanding with the heat which opens cracks or damage.

How Is A Misfire Determined By The Car?

The ECU knows the position of the pistons and the firing order of the spark plugs so it knows when there should be combustion noise in the engine. It listens to the engine with a knock sensor. This sensor is normally used for monitoring pre-detonation (the fuel igniting early) but it can also tell when there wasn’t a suitable combustion noise. It is also possible that the ECU monitors the rotation of the cranks shaft. Even at constant RPM, there will be 3 distinct surges in power for every 2 rotations of the crank as the combustion cycle fires in each cylinder. If the ECU uses this method, it can see when a cylinder isn’t creating power because of a misfire.

What Are The Main Causes For A Misfire?

Incorrect fuel ratio
Incorrect air ratio
No heat
No or low compression
Failing knock sensor

Can You Be More Specific?

Oh for fu… yeah of course.

Incorrect Fuel Ratio

Incorrect fueling can happen for the following reasons.

Injectors blocked. The holes in the end of the injectors are very small. Any particles that make it through the fuel filter can easily block these holes causing a loss of flow. Many fuels contain detergent that break down any build up in the injector nozzles, so this issue is not as wide spread as it used to be. Putting injector cleaner into the fuel tank can help but if the injectors are totally block, it’s unlikely to help. They’ll need to be removed and cleaned.

If you decide to visually check the spray pattern of the injectors, be very careful. The fuel comes out at very high pressure and very fast.

Poor injector spray pattern. Instead of being fully blocked, it is more likely that the injector nozzles will partially blocked up. This reduces fuel flow and alters the spray pattern. The pattern of the spray is important to the effectiveness of the fire in the cylinder. A fine spray will ignite better than a jet of fuel will.

Running injector cleaner through the fuel tank can break down the deposits in the injector nozzles. If you decide to visually check the spray pattern of the injectors, be very careful. The fuel comes out at very high pressure and very fast.

Fuel filter blocked

Reducing the fuel flow will affect the fuel ratio.

No power to the injector. An injector is a high speed valve that uses an electric pulse generated by the ECU to open the injector to allow the fuel to flow. When the electric pulse is off, the injector springs closed. If there is no electrical pulse getting to the injector, it won’t open so no fuel is injected. The issue could be a poor connection to the injector, a damaged wire, poor connection to the ECU or ECU failure.

Incorrect fuelling. The rate at which the fuel is injected is determined by the ECU using data from a range of sensors. Air flow meter, air temperature sensor, pressure sensor and lambda sensors. If any of these sensors has failed or is giving an incorrect reading, the ECU could under or over fuel. The ratio of fuel to air is important for the correct running of the engine.

Incorrect Air Ratio

Incorrect air ratio can happen for the following reasons.

Faulty sensors. The rate at which the air is allowed into the engine is determined by the ECU using data from a range of sensors. Air flow meter, air temperature sensor, pressure sensor and lambda sensors.

If any of these sensors has failed or is giving an incorrect reading, the ECU could under or over airate. The ratio of fuel to air is important for the correct running of the engine.

Most smart engines have a turbo which is also regulated by the ECU. The amount of boost and the position of the throttle body valve changes the amount of air forced into the engine. If the amount is incorrect, the ratio will be wrong and the engine will not run properly.

Faulty throttle body. If the throttle body isn’t in the correct position as expected by the ECU, not enough air will get to the engine.

Faulty turbo components. The amount of boost is controlled by the wastegate, which is controlled by the wastegate actuator, which is controlled by the cycle valve, which is controlled by the ECU. So, as you can see, that’s a chain that can easily be broken. However, if you get a failure in these, you rarely get a misfire (and if you do, it’s not for long).

More often than not, the car will go into safe mode or just reduce the fueling to maintain the correct air/fuel ratio.

No Heat

No heat can happen for the following reasons.

Damaged spark plug. You have to remember that the tip of the spark plugs are small pieces of metal in a very harsh environment. It’s not unusual for the positive or negative electrode to get burnt or damaged which can reduce the power of the spark, or even stop the spark altogether.

Incorrect length or incorrect heat range spark plugs will cause you issues too.

Remove all of the plugs and visually inspect them for damage. Replace as necessary.
450 and Roadster have 6 spark plugs.
451 fortwo, 453 fortwo/forfour have 3 spark plugs.
3 cylinder 454 forfours have 3 spark plugs.
4 cylinder 454 forfours have 4 spark plugs.

Poor connection from the HT lead to the spark plug. The HT lead is the fat cable that connects to the end of the spark plug. It can be tricky to get these connected properly to the end of the spark plugs do to the poor aceess.

Check that the HT leads are on securely and the connection isn’t damaged. Visually check the HT lead connector for any arc damage.

Poor earth to the engine. Electricity will only flow if it has somewhere to flow to. If the engine isn’t properly earthed, the spark will not jump from the positive to the negative electrode. If the earth lead is poor, it can reduce the power of the spark.

Check the engine earth lead from the engine to the car body. Also check the lead from the battery negative to the car body.

Replace as necessary or upgrade to a better earth lead kit.

Damaged HT leads. HT leads are easily damaged if cheap HT lead removal tools are used. They slip off and slice the shroud which can let water in. Check the leads for damage.

Poor connection from the HT lead to the coil pack The previously mentioned HT leads run from the spark plugs to the coil pack. The coil pack creates the high amperage power that is sent to the spark plugs. If the connection to the coil pack is poor, the power could be affected.

Check and clean the connections to the HT leads. Visually check the HT lead connector for any arc damage.

Poor connection from the ECU to the coil pack. The coil pack is controlled by the ECU so if there is a poor connection, the coil pack may not be receiving the signal to tell it to send the power to the spark plugs. Check and clean the main connectors.

Damaged wires between the ECU and the coil pack. Almost all of the wiring is wrapped up in bundles so if you have a damaged wire, you are up shit creek without a paddle. It’ll take ages to trace the wires to find the damage. The easier option id to check the wire continuity with a multimeter and also check that a voltage is being received at the coil pack using the same multimeter.

If you find a wire that isn’t connected somewhere along its length, the easiest option is to run a new wire.

Poor connection at the ECU connector. Disconnect the connector, clean and inspect the terminals. Check for continuity between the terminals and the coil pack connectors.

Faulty coil pack. The coil pack creates high amperage charges up to 40 times a second. It comes as no surprise that these can fail. Remove and replace as necessary.

Faulty ECU. This one is very unlikely however, as a matter of completion I thought I’d better add it. If the ECU isn’t sending out the “fire” signal to the coil pack, the coil pack will sit there doing nothing. There really would be no repair for this one. It’d be a new ECU.

No Or Low Compression

Piston ring. This is a fairly common problem on the 450 fortwo and Roadster. The piston rings seal the piston against the cylinder wall to help create compression. If the piston rings are damaged, the compression will be reduced to the point where a misfire occurs. The engine will require reconditioning.

Performing a compression test will tell you if you have compression issues.

Damaged valve. The head of the engine contains mechanical valves that open and close to allow the air and fuel into the cylinder and to allow the exhaust gas out of the engine. It’s fairly common for the exhaust valves to overheat and burn. This causes damage to the head of the valve so it fails to seat properly.

The compression will be reduced to the point where a misfire occurs. The engine will require reconditioning. Performing a compression test will tell you if you have compression issues.

Damaged cylinder wall. If you run low on oil, the tip falls off a spark plug ro the piston ring becomes damages, it can score the cylinder wall. When the damage becomes too deep for the piston ring to seal, compression will escape from the cylinder.

The compression will be reduced to the point where a misfire occurs. The engine will require reconditioning.

Performing a compression test will tell you if you have compression issues.

Damaged head gasket. There is a gasket between the main engine block and the cylinder head. If this gasket fails, compression will be lost out the side of the engine. A new gasket will need to be fitted.

Performing a compression test will tell you if you have compression issues.

Loose spark plugs. Often over-looked but, if the spark plugs aren’t seated properly, the compression can escape from around the spark plug thread. The fire from the cylinder can also pass the spark plug and damage other things.

You can just reseat the spark plugs if you wish but I would recommend fitting new plugs as the fire seal on the old plugs will have already been compressed and may also now be damaged.

Remove the spark plugs and check for any fire damage.

Failing Knock Sensor

The knock sensor listens to the engine and determines if the detonation is happening at the correct time. If the sensor is failing or isn’t bolted down correctly, it will misread and believe there’s a misfire. I have even seen a car with bad engine mounts bring up a misfire due to excessive engine movement.

If you have damaged con rod bearings, this will create a knocking which will sound really bad but can also bring up a misfire code.