Common check engine light problem

A common check engine light problem that I run into often would be a set code for the evaporative emission control system. Sometimes this code is caused by a loose gas cap. I have also seen defective rubber seals on the cap itself trigger the code.

Picture of gas cap

Please be aware that a gas cap is not the only problem that can set a check engine light code for this system. In fact vehicle manufacturers have actually made it much harder for a loose fuel cap to set a trouble code. But in the end it will still turn the check engine lamp on if it continues to leak fuel vapor.

This page will be about other causes of vapor leak codes. But if you are interested in more information on the loose or leaking fuel cap subject I have put together a gas cap code video that discusses this common check engine light problem. The video outlines steps you can take to correct the issue before spending time and money at the car repair center.


Emission control systems codes

First a little background on what the system is all about, why it is deployed on the automobile and what other kinds of problems can cause a code to set. The fuel evaporative emission control system reduces the amount of raw fuel vapors that are emitted into the air from a vehicle’s fuel system.

According to the government these vapors must not be allowed to escape from the fuel tank and into the atmosphere. Since the first systems were used nationwide in the late 70s several advances have been added. Some of the modern systems include a special gas tank designed to limit the amount of fuel that can be put in the tank.

This unfilled space in the tank allows for in tank storage of vapor as well as expansion and contraction of the gas itself. It is hard to believe but in the old days they use to have a vented fuel cap to avoid pressure buildup in the system. Nowadays a completely sealed gas cap is used.

Picture of charcoal canister

The on-board evaporative emission control system is responsible for venting fuel pressure to the intake manifold when it is required. One of the main parts of the fuel vapor emissions system is the charcoal canister. The canister is connected to the fuel tanks vapor line and is usually located in the engine compartment.

Fuel vapors from the gas tank are routed to the canister and stored. When the vehicle is started vapors are drawn from the canister to the intake manifold by engine vacuum. Keep in mind that different vehicle manufacturers will deploy different methods of handling this function. On many new vehicles the computer will operate relays, solenoids and valves to burn this unwanted vapor at the right time.


Canister purge valve problems

canister purge valve picture

Another common check engine light problem is the canister purge valve. This is usually an electronically controlled and vacuum operated device that has several fittings on it. One of the more common canister purge valves would be one with three fittings.

Most often these consist of a vacuum input signal that controls the device slaved to the car’s main computer, a vapor inlet port and a purge port that will run a vacuum line to the intake manifold. This component can turn on and off and burn fuel vapor at the most opportune times.

When common check engine light problems occur different codes can set for the different components that are mentioned above. If you have a code in the P0400 range this refers to an evaporative emission system malfunction. In my opinion your best bet would be to follow a diagnostic chart for that specific code this is often the shortest distance to a repaired automobile.

For more of the latest posts to this blog about auto repair this next link takes you back to the homepage from this page about the common check engine light problem.


One comment

  1. how perge valve operates?how the vapours are sucked inside the air intake? if perge is not working then what will hapen?the one connection to the canister is open for what?

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