How to fix EVAP Canister Problems
To fix an EVAP canister problem, read the trouble code first, then work from the cheap parts to the expensive one: tighten or replace the gas cap, look for cracked hoses, and test the purge valve with a hand vacuum pump. If the charcoal canister is only clogged, you can blow it clear with 50 psi of compressed air. If the canister is cracked or the purge valve leaks vacuum, replace the canister and purge valve assembly.
The evaporative emission control (EVAP) system traps gasoline vapour from the fuel tank so it does not escape into the air. If you have ever left the gas cap loose on a modern car and seen the check engine light come on, you have met it. The engine computer (powertrain control module) turns on the light and stores an EVAP diagnostic trouble code (DTC) when it finds a leak or a fault anywhere in the system.
An EVAP fault rarely changes how the car drives, so it is easy to ignore. Don't: it almost always means the car fails a state emissions test, and the car releases more fuel vapour until it is fixed.
TL;DR
- Cause: a loose or bad gas cap, a cracked hose, a leaking purge valve, or a clogged or cracked charcoal canister.
- Symptoms: check engine light, fuel smell, rough idle, hard starting, poor mileage, hard refuelling, failed emissions test.
- First step: scan the code, then check the gas cap and hoses before buying parts.
- Test: a good purge valve holds vacuum from a hand pump; if the vacuum leaks, replace the valve and canister.
- Clean: blow 50 psi of air through the vent pipe for 1 to 2 minutes.
- Warranty: US federal law covers the purge valve, fuel cap and vapour canister for 2 years or 24,000 miles.
If this is not your case, read Mercedes Benz Vapor Canister Purge Solenoid Troubleshooting Guide.
Table Of Content
- What is the purpose of the EVAP canister?
- What happens when the EVAP canister goes bad?
- Is driving with a faulty EVAP canister dangerous?
- How do you test an EVAP canister?
- How do you clean an EVAP charcoal canister?
- Is a bad EVAP canister covered by warranty?
1. What Is The Purpose Of The EVAP Canister?
The EVAP canister is connected to your fuel tank by the tank vent line. It holds around 2 pounds of activated charcoal, which traps gasoline vapour until the purge valve opens. Engine vacuum then pulls the vapour out of the charcoal and into the intake manifold, where it is burned. This keeps the vapour out of the air.
Pulling the stored vapour into the engine is called purging, and it needs the purge valve. The powertrain control module opens the purge valve, outside air is drawn in over the charcoal, and the vapour is pushed into the engine and burned with the normal fuel charge.
A faulty canister can also hurt engine performance and slow down acceleration, as the symptoms below show.
2. What Happens When The EVAP Canister Goes Bad?
A failed EVAP canister upsets the whole evaporative emission control system. These are the symptoms that usually point to it:
| Symptom | Likely cause | What to do |
|---|---|---|
| Check engine light with an EVAP code | Leak in the cap, hoses, purge valve or canister | Scan the code with an OBD2 scanner, then smoke test for the leak |
| Strong fuel smell | Cracked or saturated canister | Inspect the canister and hoses, replace if cracked |
| Rough idle, rpm hunting | Vacuum leak at the canister, purge valve or hoses | Vacuum test the purge valve and check the hoses |
| Hard starting | Vacuum leak upsetting the air-fuel ratio | Find and fix the vacuum leak |
| Poor gas mileage | Fuel vapour lost instead of burned | Fix the leak, clean or replace the canister |
| Fuel pump nozzle clicks off, hard to refuel | Clogged canister cannot take tank vapour | Blow out or replace the canister |
| Failed emissions test | Any EVAP fault | Repair, clear the code and retest |
Gas Mileage Becomes Poor
Low gas mileage means the car is not running at its best, or that the fuel system is leaking. A faulty EVAP canister is one of several possible causes.
The fuel vapour that should be burned in the engine escapes before it reaches the combustion chamber. That lost fuel lowers your mileage and raises your fuel bill. The canister may also become saturated or clogged as a result.
Engine Performance Issues
A damaged EVAP canister often shows up as poor engine performance. The engine runs inefficiently and gives little power for quick acceleration, so the car feels sluggish even when you press the pedal hard.
The sluggish acceleration comes from incomplete combustion. Replace the defective canister promptly.
Starting The Engine Is Difficult
Hard starting is another sign of a failing EVAP canister. A problem with the charcoal canister can cause a vacuum leak, and that can make the car hard to start.
A vacuum leak lets unmetered air into the engine. The air-fuel ratio becomes too lean and combustion suffers. If the leak is not fixed, the car may eventually not start at all.
Check Engine Light Illuminates
When the engine computer detects a problem in the EVAP system, it turns on the check engine light. The light can come on for many other reasons too, and fueleconomy.gov notes that it can be something as minor as a loose gas cap.
So scan the car for trouble codes with an OBD2 scanner first. If the code points to an EVAP leak, use a smoke machine (an EVAP leak tester such as the ones OTC makes) to find exactly where it leaks.
Vehicle Experiences Harsh Idling
A rough idle is a common sign of a damaged EVAP canister. The rpm becomes unstable and you may feel the car shake. A healthy idle is usually around 1000 rpm.
If the idle rpm keeps moving up and down, you have an idle problem. Worn spark plugs, dirty fuel injectors and vacuum leaks can all cause it.
A damaged EVAP canister, or the hoses attached to it, can cause that vacuum leak. Fix it quickly, because a lean running engine can be damaged over time.
The Smell Of Gas
A fuel smell is one of the classic EVAP leak symptoms. It has many possible causes, but a broken EVAP canister is one of the most common. A damaged canister can give off a strong fuel odour.
Emissions Test Fails
A defective EVAP canister will make the car fail an emissions test, and the check engine light will be on as well.
It Becomes Difficult To Fill The Gas Tank
When you refuel, the vapour pushed out of the tank goes to the EVAP canister. If the canister is clogged, the vapour cannot get in and backs up into the filler neck, which makes refuelling difficult.
3. Is Driving With A Faulty EVAP Canister Dangerous?
Driving with a bad EVAP canister is technically safe. On its own it may not make the car drive badly, but it will stop it passing an emissions test.
A faulty canister lets fuel vapour escape into the air and keeps the check engine light on, which can hide a new fault. Fix it as soon as you can.
4. How Do You Test An EVAP Canister?
The charcoal canister holds fuel vapour until the engine can burn it. When the EVAP system reports a leak, test the canister and purge valve to find out where it is.
1st step
Find the charcoal canister. It is a black cylinder, often mounted in one corner of the engine compartment.
2nd Step
Look the canister over. Make sure there are no visible cracks or gaps in the housing.
3rd step
Connect a handheld vacuum pump to the purge valve on the top of the canister.
4th step
Work the hand pump and watch the valve. On a working canister and purge valve assembly, the valve opens when the pump is used.
5th step
While pumping, look and listen at the purge valve. The valve should stay open, but no vacuum should leak from the canister. If you find a vacuum leak, replace the purge valve and canister assembly.
Remove the hand pump from the purge valve. Start the car and let it idle in park. Watch the engine compartment and make sure there is no fuel smell coming from the canister.
5. How Do You Clean An EVAP Charcoal Canister?
The charcoal filter inside the canister absorbs excess fuel vapour from the tank and fuel system while the engine is off. It works like a sponge. When the engine runs again, the vapour is drawn back into the combustion chamber. Over time the buildup can clog the canister so vapour cannot get in or out.
1st step
Use a flat-head screwdriver to remove the lines from the top and bottom of the charcoal canister. Three lines run to the top and one to the bottom. If needed, remove the canister from its bracket with a ratchet and socket.
2nd Step
Place the canister on a flat surface. Run the air compressor until it reaches at least 50 psi, then switch it off.
3rd step
Fit the rubber tip of the air nozzle onto the canister's outer vent control valve pipe. This pipe sits at the very end of the top row and is slightly larger than the other two pipes.
4th step
With one hand, cover the openings of the other two top pipes with your fingertips. With the other hand, pull the trigger and blow 50 psi of air into the canister for 1 to 2 minutes. Then take one finger off the top pipes and hold it over the opening of the pipe on the bottom of the canister.
Blow air into the control valve pipe again and feel for air coming out of the bottom pipe. If it flows freely from the top to the bottom, the canister is clean. Refit the canister and reconnect the lines.
6. Is A Bad EVAP Canister Covered By Warranty?
In the US it may be. The EPA's emission warranty fact sheet lists the purge valve, fuel filler cap, purge solenoid and vapour storage canister as emission parts. On 1995 and newer light-duty cars and trucks under 8,500 pounds, these are covered for the first 2 years or 24,000 miles, whichever comes first. Check your Mercedes warranty booklet, because manufacturers can offer longer cover.
Get to know the symptoms above. Catching an EVAP fault early keeps your car passing its emissions test and running as it should. If the code points at the purge valve rather than the canister, the purge solenoid guide covers how to test it. For how Mercedes fault codes are read and decoded in general, see What's The P13DF Mercedes Fault Code?.