Rick's Free Auto Repair Advice

Evaporative Emissions System Primer

How does an evaporative emissions system work?

The Evaporative Emissions System is responsible for preventing fuel vapors from polluting our air.

If you came to this post because your vehicle won’t fill with gas, skip to the bottom of the post

In the old days gas tank caps were vented to the atmosphere. That allowed outside air to enter the tank as you used up the gas inside the tank. On warm days the gas expanded and gas fumes vented backwards through the vented cap to the atmosphere. In other words, it was an open system. Not anymore. Emissions laws require the entire fuel system to be closed to the atmosphere. Not only that, but the emissions laws also require the vehicle to test the fuel system to ensure that the tubing and fuel lines are intact. The whole point of testing is to make sure no fuel evaporates into the atmosphere to cause pollution. This system is now called the Evaporative Emissions System (EVAP). Each car maker is free to design their own system. But some components are basic to all. I’ll describe those components here:

Evaporative emissions system charcoal canister

Charcoal canister, purge solenoid, vent solenoid, fuel tank pressure sensor

Charcoal canister

Manufacturers load a plastic container with activated charcoal. As you fill the gas tank, fuel vapors are forced into the charcoal canister where they’re absorbed by the charcoal. The canister also absorbs gasoline vapors generated by ambient heating. On a hot summer day heat from the roadway can cause temperatures to rise in the gas tank. That heat expands the gasoline and it vaporizes. As the vapor pressures rise, the excess vapor is pushed into the canister where it too is absorbed by the charcoal. At some point the charcoal becomes saturated with gas vapors and has to be emptied. That’s where the next components come into play.

Evaporative emissions system purge and vent solenoids

purge valve, purge solenoid, vent solenoid

Purge Solenoid

Car makers connect a vacuum hose from the charcoal canister to the intake manifold and a second vacuum hose to the atmosphere. The vacuum line going to the intake manifold contains a PURGE valve and the hose going to the atmosphere contains a VENT valve. Each valve is opened and closed by an electric solenoid.


Evaporative emissions system Fuel tank pressure sensor

This sensor is connected to the

fuel tank pressure sensor

Fuel tank pressure sensor

fuel tank and monitors the pressure/vacuum conditions inside the tank. The computer uses values from this sensor to determine if there’s a leak.

vent solenoid, purge solenoid, fuel tank pressure sensor

Vent solenoid

Evaporative emissions systemLeak Detection Pump/NVLD

For a complete explanation of this Chrysler system, click here.

How it works.

The computer monitors fuel level from the sending unit in the gas tank. It know when you fill up with gas and it knows when the tank is empty. At certain benchmarks, the computer commands a purge/vent/test cycle. The computer supplies power to the PURGE solenoid and it opens the line to the intake manifold. Seconds later it sends power to the VENT solenoid. Vacuum from the intake manifold sucks air through the charcoal canister to remove gasoline vapors.

At the same time, fresh air enters the canister from through the VENT solenoid. Since fuel vapors are now flowing into the engine, the computer can detect an enriched fuel mixture and cuts down on the flow of liquid gasoline from the injectors. The computer monitors the exhaust to detect proper combustion. As soon as it detects the exhaust mixture moving towards lean, it figures that the charcoal canister has been fully purged. It then closes the VENT solenoid. With the PURGE solenoid still open to vacuum from the intake manifold, a vacuum is applied to the entire system. Once the computer is comfortable that enough time has elapsed, it closes the PURGE solenoid.

If the system is indeed intact, the vacuum condition will remain constant, as measured by the fuel tank pressure sensor. If the vacuum holds, the system passes the test. If the vacuum doesn’t hold, the computer measures the rate at which the vacuum deteriorated. That’s how it determines whether the leak is large or small.

What goes wrong with the PURGE solenoid and VENT solenoid

Both the PURGE and VENT solenoids are prone to failure. Since the VENT solenoid is open to the atmosphere, it’s also open to road dust and dirt and SPIDERS. Yup, spiders love to build nests in the vent line. Some car makers install filters to keep them out. But filters also trap road dirt and a clogged filter can interrupt the flow of fresh vent air. To test the condition of the solenoids, the computer monitors the circuitry to them. If the wires to the solenoid or the coil inside the solenoid fails, the computer reports these trouble codes:

P0443 – Evaporative Emission Control System Purge Control Valve Circuit Malfunction
P0444 – Evaporative Emission Control System Purge Control Valve Circuit Open
P0445 – Evaporative Emission Control System Purge Control Valve Circuit Shorted
P0446 – Evaporative Emission Control System Vent Control Circuit Malfunction
P0447 – Evaporative Emission Control System Vent Control Circuit Open
P0448 – Evaporative Emission Control System Vent Control Circuit Shorted
P0449 – Evaporative Emission Control System Vent Valve/Solenoid Circuit Malfunction

You can diagnose these codes by checking the wiring harness for proper ground. Unfortunately, you need a fairly high end scan tool to force the computer to actuate the solenoids. In other words, in many cases, it’s easier and cheaper to just replace the solenoid rather than test it. In most cases, these solenoids cost around $45.

What goes wrong with the Fuel Tank Pressure Sensor

The computer also tests the circuit to the fuel tank pressure sensor. If it detects a circuit failure it reports these trouble codes:

P0450 – Evaporative Emission Control System Pressure Sensor Malfunction

If you get a P0450 code, you’ve got a dilemma. These sensors are fairly expensive and once again, if you don’t have a high end scan tool, you cannot test the sensor. You can test the continuity of the wiring harness from the computer to the sensor connector. However, from what I hear from other techs, if you get this code it means the sensor has died.

Now let’s talk about the evaporative emissions system leak codes

If the EVAP system detects a leak, it will report one of the following codes

P0441 – Evaporative Emission Control System Incorrect Purge Flow
P0442 – Evaporative Emission Control System Leak Detected (small leak)
P0450 – Evaporative Emission Control System Pressure Sensor Malfunction
P0451 – Evaporative Emission Control System Pressure Sensor Range/Performance
P0452 – Evaporative Emission Control System Pressure Sensor Low Input
P0453 – Evaporative Emission Control System Pressure Sensor High Input
P0454 – Evaporative Emission Control System Pressure Sensor Intermittent
P0455 – Evaporative Emission Control System Leak Detected (gross leak)
P0456 – EVAP Leak Monitor Small Leak Detected

Here’s the bottom line—you’ve got a leak. How do you find it? Well, as a do-it-yourselfer, there’s no easy answer. A pro just hooks up a smoke machine and fills the entire fuel system with smoke. Then, when they see smoke, they know they’ve found the culprit. Want to buy a smoke machine? Go ahead, they’re $1,000. Don’t want to spend a grand or even the $100 for a shop to find the leak for you? Here’s the easy way out.

First, replace the gas cap with ONE FROM THE DEALER. Yes, you read that right. Buy a genuine dealer cap. I’ve heard plenty of techs complain about faulty aftermarket gas caps. Reset the trouble codes with your scan tool (you did buy a scan tool, right?—if you didn’t, click here).

Next, check all the hoses coming into and out of the charcoal canister. Bend and flex each one with your fingers. Look for cracks. If you find a cracked hose, take a digital and show it to the parts guys at the dealership. Then know which hoses fail most often. If you can’t find the culprit, replace all of them. Here’s why. Replacing all of the hoses will cost you less than the shop might charge just to find the leak.

There you have it. The full story on evaporative emissions leaks.

Well, not the whole story: If you’re having trouble filling your gas tank (nozzle clicks off), that’s a sign that the charcoal canister is filled with gas vapors and can’t accept any more. And THAT is an indication of an EVAP system failure. Go up to the top and read this post to understand how the system works. Replace the failed parts and you should be able to fill your tank after that.

© 2011 Rick Muscoplat



Posted on by Rick Muscoplat

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