EVAP (Evaporative Emission Control System) Misery Of The EPA

I do appreciate the fact that the government has worked hard to clean up the environment. However, I don’t appreciate the fact that the systems that do so invoke a lot of cost and complication to what was a simple clean engine compartment. EVAP systems are just that, a nightmare to debug, and can take on many DTC codes at once. There are a thousands of suggestions and even videos on the internet, some are helpful and others just make matters worse. Buying diagnostic equipment can cost thousands of dollars, and taking your ride to a dealer can cost you around $300.00 or more to replace a simple $15.00 valve. To make matters worse, the manufactures have now evolved into a bigger nightmare by adding a third valve and even worse a pump. This especially bugs me since any devices added to the drive train causes a reduction in horsepower and as far as my understanding takes me, increases fuel consumption, thus adding more emissions! No offence but I would much rather have the manufacture add another safety device to protect my family or some upgrade to my ride then to waste it on such devices. In any event you can see that I am totally enthused about the intervention of such systems, and by virtue of doing my own work, have spent a lot of time working on the same only to discover that they all have similar but with a few different quirks. Hence I will start with the first rule of thumb.

 

First: No two OBD II codes in two different cars are always the same. For instance, many folks will swear that an P0446 is a leak in a GM and some claim it’s a “Gas Cap” Nothing is further from the truth it’s a “GM DTC P0446 (Restricted/Blocked EVAP Vent Path)” which is explicit to the test resulting in a higher or “excessive” vacuum state in the system, which points to certain devices, vent valve, blockage, blocked canister etc. and has nothing to do with a leak. In Fords it sets a P1450 code which states “Excessive Vacuum in System” The moral of the story is pull the DTC OBD codes directly for the Manufacture, model & year from a manufactures web site or trustworthy database.

Secondly: Fixing one problem does often lead to another, don’t be discouraged, push on!

Third: Learn how to interpret the codes a P0455 is a gross leak in most units which is mainly interpreted as a cap problem. In newer models (some 2006 and up) they now have a P0457 which explicitly says it’s a gas cap. I have seen both codes set at once and find out it’s a major leak in the system, such as in a rotted filler neck. Too many folks have rolled into a handy dandy auto shop to spend $20.00 bucks or more for a cap they never needed.

Forth: Find a buddy with a full blown scan tool if possible. These units can set special test and allow you to read the values of codes, sensors, etc. Or buy one used on EBAY I got a real nice Mac Task Master retails around 2K new I picked it up for $300.00 a few years old updated to 2006. It’s worth every penny especially with the right models you can also test ABS and Air Bag problems. You can easily update older versions within two model years for about $200.00 on EBay. For the most part the software and even the device is from one manufacture despite the name on the unit. SPX is the parent company to OTC which builds diagnostic instruments not only under its own lines “OTC & Genesis” but for people like Snap On Mac etc.

Fifth: Build a simple smoke machine. I have my own $30.00 contraption of which I will share with you all. This simple device has saved me and others thousands of dollars in repairs. Keep in mind some shops charge upwards of $100.00 to run this test. Problems related to open leaks in EVAP and vacuum systems can be found with this simple device. For instance earlier corvettes were loaded with vacuum systems from headlight door controls, dampers, climate switches, check valves, vacuum switches etc. and sometimes finding a leak was close to impossible and time consuming. A system smoker makes this job easy.

 

Basically despite all the long verbiage that you will find; The Simplified Explanation of The EVAP System “Recycles Gas Vapors In Your Tank & Burns Them In The Engine, Thus Keeping Them From Reaching The Atmosphere”.

In Detail: The Evaporative Emission (EVAP) Control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the charcoal canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid to the atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the PCM commands the EVAP purge solenoid “on”, allowing engine vacuum to be applied to the canister. With the EVAP vent solenoid “off”, fresh air is drawn through the vent solenoid and the vent line to the canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid into the intake manifold where it is consumed during combustion. The PCM uses several tests to determine if the EVAP system is leaking.

 

Large Leak Test
The Large Leak diagnostic tests for large leaks and blockages in the EVAP system. With the engine running, the PCM commands the EVAP vent solenoid “on” and commands the EVAP purge solenoid “on”, allowing engine vacuum into the EVAP system. The PCM monitors the fuel tank pressure (FTP) sensor signal to verify that the system is able to reach a predetermined level of vacuum within a set amount of time. Next, the PCM commands the EVAP purge solenoid “off”, to seal the system, and monitors the vacuum level for decay. If the PCM does not detect that the predetermined vacuum level is achieved, or the vacuum decay rate is more than a calibrated level on two consecutive tests, DTC P0455 will be set.

Small Leak Test
If the large leak test passes, the PCM will test for small leaks by continuing to monitor the FTP sensor for a change in FTP sensor signal over a period of time. If the decay rate is more than a calibrated value, the PCM will rerun the test. If the test fails again, a DTC P0442 will be set.

Canister Vent Restriction Test
If the EVAP Vent system is restricted, fuel vapors will not be properly purged from the EVAP canister. The PCM tests this possibility as discussed next. The PCM commands the EVAP purge solenoid “on”, commands the EVAP vent solenoid “off”, and monitors the FTP sensor signal for an increase in vacuum. If the vacuum increases more than a calibrated value, DTC P0446 will be set.

Purge Solenoid Leak Test
If the EVAP purge solenoid does not seal properly, fuel vapors could enter the engine at an undesired time, causing drivability concerns. The PCM tests for this possibility as follows. The PCM commands the EVAP purge solenoid “off” and the vent solenoid “on”, to seal the EVAP system, and then monitors the FTP for an increase in vacuum. If the PCM detects that the EVAP system vacuum increases more than a calibrated value, DTC P0496 will be set.

Check Gas Cap Message
The PCM sends a class 2 message to the driver information center (DIC) to illuminate the Check Gas Cap message when any of the following occur: * The Large Leak Test failed due to a malfunction in the EVAP system * The Small Leak Test failed due to a malfunction in the EVAP system, typically DTC P0445 & P0457.

EVAP System Monitor Operation
The EVAP System Monitor is a PCM diagnostic that runs once per trip that monitors the EVAP system in order to detect a loss of system integrity or a leak in the system (leaks that range in size from 0.020" to 0.040" in diameter). It also checks the ability of the EVAP system to flow fuel vapors in the system. The PCM controls the Purge solenoid to open and close the path of vacuum to the canister at off-idle speeds. The PCM monitors changes in the fuel tank pressure (FTP) sensor signal with the EVAP vent valve closed. If the PCM detects the system cannot hold proper vacuum, a trouble code is set and the current engine conditions are stored in Freeze Frame. Refer to the EVAP System Graphic as needed.

Possible Causes of an EVAP System Failure
1 Cracks, leaks or disconnected hoses in the fuel vapor lines, components, plastic connectors or lines

2 Charcoal canister is damaged or saturated with fuel (due to EVAP vent valve problems)

3 EVAP service port is leaking

4 Fuel filler cap (gas cap) is loose or missing

5 PCM has failed

EVAP Purge Conditions
The PCM purges the canister under the following conditions:

1 After the engine has reached operating temperature

2 At wide open or part throttle (as long as the engine is not overheated)

3 The engine can be in open or closed loop Fuel Control in a purge event.

Simplified Circuit: Obviously not to scale! However one can easily follow the lines. The system is depictive of most systems without the addition of the third thermo valve or pump. The Remote EVAP Vent Valve is sometimes part of the Charcoal Canister Assembly, it is shown in the vented cap style and also can be found with an additional hose attached that directs any fumes away from its mounted position. Generally the Green line is connected to the tank via. The fuel pump assembly. The Service Valve & EVAP Purge Valve are found mounted on the engine. Keep in mind what appears to be simple! as it is does in this drawing, not turn out to be when issues arise.