Vehicle Emissions System Problems And Repairs

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Based on the amount of repairs we do on emission components on just about every type of vehicle, I thought I would write an article talking more about the parts and workings of the emission control systems on late model vehicles.

More often than not, when we get a car or truck that comes in the shop with a check engine light on, it will usually be a problem with one or more parts of the emission controls. There are many components to the system, and they all have a certain job to do, but each one of them needs to work in order for the complete system to function properly. Basically, the main function of emission controls is a two part process.

The first is to limit the amount of fuel vapor or fumes that can escape the gas tank. This is done with a few different parts which I will get into in a bit. The second function is to re-use some of the exhaust gases by introducing them back into the intake path to get burned again during another combustion sequence. Again, some select parts do that job and all of them need to be functioning to work properly.

Fuel Vapors

When you pump gasoline into the fuel tank, you are also adding a whole lot of air as well. This fuel filled air, if just let to escape to the atmosphere, is what leads to ozone and greenhouse gases and other environmental problems. What is done to control this is the utilization of what’s called an Evaporative Emission Control System. This system takes the fumes from the gas tank, and can either send them to be burnt along with the fuel mixture, or be absorbed in a charcoal canister, or at times be vented to the air.

To burn the fumes off, they are sent through a Purge Solenoid which forces them into the intake stream. Extra or excess fumes can be absorbed in the Evap Canister, also known as a Charcoal Canister. There is an element inside the canister that can take the fuel portion out of the air and keep it from getting out to the atmosphere. Also, the Evap Vent Valve or Vent Solenoid is used to vent the remaining cleaner air from the canister to the atmosphere. As you can see, that is a pretty involved system and even just one little part being faulty will mess up the entire process. A faulty purge solenoid, or vent valve are the most common problems here due to either or both of them sticking open or closed. Too much flow, or not enough flow will result and set a code which will turn on the check engine light. A pressure sensor can tell if there is a leak in the system if it cannot hold a vacuum for a certain period of time, as requested by the computer. Lots of possibilities here for issues, as you can see!

EGR Valve

To take care of the exhaust gasses to send a little less ‘bad’ air through the catalytic converter, is the duty of the EGR system. EGR stands for Exhaust Gas Recirculation. This involves a pipe or pipes from the exhaust manifold to what is called the EGR valve. This valve regulates the flow of spent exhaust gas back into the intake system. There is a pintle valve inside which is commanded open and closed by the vehicles computer, as deemed necessary. When there is a problem with this system, you can experience a rough idle or stalling when the valve is stuck open. A commanded state that is not actually happening inside the valve is another issue that will set the check engine light on and cause emission failure. EGR valve replacement is a very common occurrence when problems arise in this system. Also, excessive carbon build up can lead to problems as well, and require extensive cleaning of the passages for the gases.

Those are just some of the things we have to deal with when checking the emission control system on your vehicle. There are many different functions to be performed, and all of which need to work together. Among all of the other engine monitoring sensors, you can get an idea of how easy it is for there to be a problem somewhere. The best bet, when you see your check engine light to come on, is to get it checked out as soon as possible. Other problems can result if you leave it go for too long. Just some friendly advice…


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