Misfires and Their Causes
Generally, the term "misfire" refers to an incomplete combustion process inside the cylinder. When this becomes severe enough, the driver will feel a jerking action from the engine and/or powertrain. Often the owner will bring the vehicle into a shop complaining that the timing is "off." This is partially correct because a misfire does involve a mis-timed combustion event. However, the base ignition timing being out of adjustment is only one reason for a misfire to occur—and not the most likely.
Common Causes for an Engine Misfire
An ignition system problem is one of the most common reasons for an engine to misfire. As the spark plugs, ignition cables, distributor cap and rotor, and ignition coil wear over time, their ability to transfer the needed spark to ignite the air/fuel mixture inside the combustion chambers becomes compromised. In the early stages, the spark will only be weaker and the actual misfire will be subtle. As the ignition components continue to wear, the misfire will intensify and the combustion process can be interrupted completely. This will cause a severe jerk or shock in the operation of the engine (the engine may even backfire through the air intake system, producing a loud "pop").
The lean misfire is another common reason for an engine "miss"—this is due to an imbalanced air/fuel ratio (too much air/too little fuel). Since an engine needs a richer (more fuel) mixture for a smooth idle, this problem may be more noticeable when the vehicle is idling. The lean misfire may decrease or disappear as the engine speed increases because the efficiency of the volumetric flow into the combustion chambers increases dramatically. This is one reason why a vehicle gets better mileage on the freeway than in the city. An EGR valve that is stuck open, a leaking Intake Manifold Gasket, a defective Mass Air Flow Sensor, a weak or failing fuel pump, or a plugged fuel filter are some of the many causes for a lean misfire.
Mechanical problems can also cause an engine to misfire. Common causes of a mechanical misfire are worn piston rings, valves, cylinder walls, or lobes on a camshaft; a leaking head gasket or intake manifold gasket; damaged or broken rocker arms; defective fuel injectors (and/or the electronics that control them); and a slipped or incorrectly-installed timing belt or timing chain. Generally, this type of misfire has more of a "thumping" feel to it. It is usually noticeable regardless of engine speed; in fact, it may even intensify as the engine speed increases.
Sometimes, the engine has nothing to do with a misfire. One common cause for "jerky" performance that feels like a misfire is a problem in the transmission and its ability to properly up- or down-shift. If the misfire occurs during higher speeds, it could be a problem with the operation of the overdrive gear or a chattering clutch in the Lockup Torque Converter. If the vehicle jerks or feels like it is "missing" during deceleration, it could be due to harsh transmission downshifts, badly warped rotors, out of round brake drums, and/or sticking brake pads or brake shoes.
Make sure that you have the vehicle properly inspected in order to determine the root cause of the misfire. Entire engines have been replaced to solve a wrongly perceived mechanical misfire problem that was actually rooted in the transfer case, transmission, driveshaft, or front/rear differential.
What are oxides of nitrogen and how are they created? (NOx)
A group of highly reactive gases that contain nitrogen and oxygen in varying amounts. Many of the nitrogen oxides are colorless and odorless. The common pollutant nitrogen dioxide (NO2) can often be seen combined with particles in the air as a reddish-brown layer over many urban areas. Nitrogen oxides are formed when the oxygen and nitrogen in the air react with each other during combustion. The formation of nitrogen oxides is favored by high temperatures and excess oxygen (more than is needed to burn the fuel). The primary sources of nitrogen oxides are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels. NOx is generally created in your combustion chamber with when temperatures reach above 2500 degrees faranheit. The cause of it mabe because of a lean misfire, engine cooling system malfunction, advanced timing, faulty air injection system, or faulty EGR valve.
What is Carbon Monoxide? (CO)
A colorless, odorless gas that forms when carbon in fuel is not burned completely. Carbon monoxide is a component of exhaust from motor vehicles and engines. Carbon monoxide emissions increase when conditions are poor for combustion; thus, the highest carbon monoxide levels tend to occur when the weather is very cold or at high elevations where there is less oxygen in the air to burn the fuel.
It usually means too much fuel, not enough air
What are Hydrocarbons? (HC)
Gasoline and diesel fuels are mixtures of hydrocarbons (made of hydrogen, oxygen and carbon atoms.) Hydrocarbons are burned by combining with oxygen. Nitrogen and sulphur atoms are also present and combine with oxygen when burned to produce gases. Automotive engines emit several types of pollutants.
Chemical compounds that contain hydrogen and carbon. Most motor vehicles and engines are powered by hydrocarbon-based fuels such as gasoline and diesel. Hydrocarbon pollution results when unburned or partially burned fuel is emitted from the engine as exhaust, and also when fuel evaporates directly into the atmosphere. Hydrocarbons include many toxic compounds that cause cancer and other adverse health effects. Hydrocarbons also react with nitrogen oxides in the presence of sunlight to form ozone. Hydrocarbons, which may take the form of gases, tiny particles, or droplets, come from a great variety of industrial and natural processes.
It has to do with spark/ignition. Usually too early or too late. Too much air not enough fuel.
The oxygen sensor serves a very simple purpose--it measures the level of oxygen, nitrogen and fuel leaving the engine cylinders. If the sensor detects a high level of fuel, the ECU reduces the amount of fuel entering the engine. If the oxygen sensor detects too much air, the ECU delivers more fuel to the engine.
The O2 sensor measures the oxygen content of the exhaust. The O2 sensor’s sensing ability comes about by producing a small voltage proportionate to the exhaust oxygen content. In other words, if the oxygen content is low it produces a high voltage (0.90 Volts - Rich mixture) and if the oxygen content is high it produces a low voltage (0.10 Volts - Lean mixture). Although theoretically the O2 sensor should cycle between 0.00 volts and 1.00 volts, in reality it cycles between 0.10 volts and 0.90 volts.
What is an EGR valve and how does it operate?
Heat is produced as a by-product of the combustion of air and fuel in the cylinders of an internal combustion engine. If the temperature in the combustion chamber gets too high, oxides of Nitrogen (NOx) are created. The cylinder combustion chamber temperature can be controlled by introducing an inert gas into the cylinder. This inert gas will dilute the intake mixture and in the end lower combustion temperatures. This lower combustion temperature is achieved by reduction of the oxygen content in the combustion mixture. Exhaust Gas Recirculation (EGR) systems were deigned to do that.
During light acceleration or cruise conditions, the vacuum signal increases and pulls the diaphragm and valve open. This allows exhaust gas to flow into the intake system.
How does air injection work?
If the engines used to run our cars burned fuel with 100% efficiency, hydrocarbon emissions would not be a problem. However, engines are not perfect, and under various conditions of operation there are times when a significant amount of fuel passes through the engine unburned. To control these emissions, we need to complete the burning process before the unburned fuel reaches the atmosphere.
Air injection systems use the exhaust system to do this.
What is a three way catalytic converter?
A catalytic converter is an exhaust emission control device which converts toxic chemicals in the exhaust of an internal combustion engine into less noxious substances. The catalytic converter is installed in the exhaust system between the exhaust manifold and the muffler so exhaust gases must pass through it.
A Three way catalytic converter – This type of converter is nearly identical to a conventional converter with the exception of the catalyst used. A conventional converter, using platinum and palladium, as a catalyst, reduces emissions of HC and CO only. The TWC uses rhodium, with or without platinum, as its catalyst. Rhodium helps reduce NOx emission, as well as HC and CO.
What is an emission Control Systems?
Advances in engine and vehicle technology continually reduce the toxicity of exhaust leaving the engine, but these alone have generally been proved insufficient to meet emissions goals. Therefore, technologies to detoxify the exhaust are an essential part of emissions control. Hydrocarbons, carbon monoxide and oxides of nitrogen are created during the combustion process and are emitted into the atmosphere from the tail pipe. This has given rise to the creation of emission control systems. Systems that help reduces exhaust emissions.
What is the California Smog Check Program?
California requires vehicles built in 1976 and later to participate in the biennial (every two years) Smog Check Program. Some gas-powered vehicles that are six yrs old or newer may not have to participate but will have to pay a smog abatement fee for the first six years instead of being required to pass a smog test. The six year exception doesn’t apply to vehicles that were registered out of the state of California. Other vehicles that are reguired to the smog check are Diesel powered vehicles that were built 1998 and later and vehicles weighing 14,000 lbs or less.
Anyone wishing to sell a vehicle that is over four years old must first have a smog check performed. It is the seller’s responsibility to get the smog certificate prior to the sale. If the vehicle is registered in California and was acquired from a spouse, domestic partner, sibling, child, parent, grandparent, or grandchild it is exempt.