EVAP System

EVAP System Leak: What Is It and How to Detect One?

An EVAP (Evaporative Emission Control System) leak is an issue that might not seem like a big deal the first time it happens. Even with a leak, you will still be able to drive your car without any significant consequences. Well, at least that’s how it looks on the outside. The truth is, every vehicle can work with an EVAP system leak. However, it’s also true that if not taken care of, the leak can cause many problems leading to more expensive repairs. To help you do that, here are the most common symptoms and causes of EVAP system leaks as well as what you can do to prevent the issue from arising in the first place.

Back to the Basics

But what is the EVAP system, and why is it so important? We already covered that in a different article, but just in case you haven’t read it, here’s a quick explanation. If you want more details, we strongly suggest you take a look at our in-depth post about “How EVAP Systems Work” here.

To put it simply, the EVAP system in your vehicle takes care of the gasoline (or diesel) fumes evaporating inside the fuel tank. Typically, fuel evaporates quickly, which can generate very high pressures inside the tank, especially in hot weather.

The EVAP system takes the fumes and stores them in a charcoal canister. Then, when the need arises, the EVAP system sends the fuel back into the engine. When you think about it, the EVAP is an ingenious system. Thanks to it, every drop of fuel gets to be used in the engine, which significantly improves the fuel-efficiency.

The EVAP system also prevents fumes from escaping the fuel tank. That way, it protects the environment from toxic gasoline or diesel fumes. Obviously, in case of a leak in the system, the environment will be the first to suffer. That alone should be enough to convince you to take action as soon as an EVAP system problem is detected. Unfortunately, the environment is not the only one that will suffer if the problem is not taken care of quickly; your budget might feel it too.

Suffice to say; we strongly recommend repairing the issue ASAP.Evap System Leak

Can You Drive with an EVAP Leak?

First of all, from an environmental point of view, it’s pretty bad to drive with an EVAP leak. Gasoline and diesel fuel contain many harmful substances, which can hurt the environment. For example, unburned fuel contains aldehydes, which are one of the most significant reasons for smog. Moreover, these fuels are full of olefins and higher paraffin, which are poisonous when inhaled.

The harmful vapors might end up in your vehicle’s cabin as well. Usually, the gas tank sits below the cabin with other EVAP system components. When a leak happens, there is a high chance that some of the fumes end up inside the vehicles. That’s also why it’s frequent to smell fuel inside the car when the system is leaking.

Surely, you will be able to tell that – gasoline and diesel smell is hard to miss. However, inhaling the fumes for prolonged periods can cause several health issues. These include difficulty breathing, a burning sensation in the throat, abdominal pain, vomiting, dizziness, fatigue, headaches, etc. And it can get worse – gasoline fumes can knock you unconscious while driving, which might result in an accident.

Related Problems

Other issues may arise from an EVAP leak as well. The most obvious one is an increase in fuel consumption. As we previously said, the system stores the vapors for later use. In case of a leak, the fumes will dissipate in the atmosphere and won’t reach the combustion chambers. If the leak is on a vacuum line, it might also change the air-fuel ratio which may cause stutters and misfires.

Furthermore, your engine might have trouble starting in the morning. Usually, the EVAP system uses the stored vapors in the charcoal canister when you start the car. If there is not enough gas inside the canister, the engine will have trouble starting correctly or might run rough at idle at times.

Also, in some severe cases, you might even experience a drop in performance. When the ECU decides to use the stored vapors, there might be a moment when the engine loses power. The ECU will quickly respond by correcting the air-fuel ratio by injecting more fuel, though, so it’ll be hard to tell from the driver’s perspective.

Finally, the “Check Engine Light” will almost certainly light up when an evaporative emission system leak is detected. Be aware, though – the EVAP system will not always be the culprit. That’s why it is still best to confirm all other symptoms before jumping to a conclusion.

If you want more info about EVAP system DTC codes, read this article here.

Most Common EVAP Leak Locations

The EVAP system consists of many parts, and leaks might happen in either one of them. However, in our experience so far, some parts fail more often than others. Here is the list of possible faulty parts and how to fix them:

Gas Cap
Make sure the gas cap seal is in good condition when refilling your car.

Gas Cap

A defective or not adequately secured cap is the most common issue referring to an EVAP leak. Most often than not, if the gas cap is not correctly tightened and vapors escape, the “Check Engine Light” might illuminate. Fortunately, it takes only a few seconds to check the gas cap. Make sure that it’s closed correctly – a clicking sound will show you when to stop. If the gas cap still doesn’t close properly, you should replace it with a new one. They are pretty cheap nowadays.

On older vehicles, inspect around the filler neck for rust. It’s not uncommon to see bits of rust preventing the gas cap seals to sit correctly and cause the check engine light to come up.

Charcoal Canister

A leak in the charcoal canister is another common issue. Since the charcoal canister is often located under the car near the gas tank, rock and debris flying from the pavement sometimes hit the canister and crack it. For the same reason hoses that connect to it might get disconnected or severed. This is especially frequent in cars driving in winter conditions.

Charcoal canisters are also prone to clogging but that is a totally different subject.

Canister Vent Valve

A leak in the canister vent valve is another common issue. The CVV takes care of pushing the vapors in the charcoal canister, often closing and opening. Usually, older vent valves won’t seal properly, and some of the fumes will escape in the environment. On multiple car models, the canister vent valve is also position right beside the charcoal canister so it can also be hit by rocks and debris and crack. If that’s the case, replacing the canister vent valve should remedy the issue.

Seals and Hoses

Faulty O-ring seals or cracked hoses are the most common cause of EVAP leaks and that’s why EVAP system problems are sometimes are to figure out. Rubber hoses and seals tend to dry up and crack and allow the fumes to leak. Even worse, there are also tons of them in an EVAP system. To detect this kind of leak, the first thing to do is to visually inspect every hose and line connections.

If no obvious problem is found, the use of an EVAP smoke machine will be required.

How to Detect a Leak in the System?

In case of a more extensive leak, drops of gasoline will always end up on the floor. Check under your vehicle to locate the leak. That said, large leaks are pretty rare, and this test won’t help you if only vapors escape from the system.

Inspecting a canister vent valve leak using a vacuum pump
Inspecting a canister vent valve leak using a vacuum pump

You can check for smaller leaks in the valve with a hand vacuum pump. For example, you can check the canister vent valve with this method. Use the pump to create a vacuum in the valve. If the vacuum drops, it means that the valve doesn’t seal properly.

Just like with a punctured tire, you can also use soap and water to locate the leak. This test includes pressurizing the system with air and a tiny bit of soapy solution. Bubbles should form on the leak location.

Of all the tests available, the smoke test is by far the most reliable. However, this test requires expensive equipment, such as a smoke machine, which can easily cost more than $1000. There are some DIY techniques involving smoke of some kind, but they are a bit complicated and don’t seem very safe or very reliable. It’s still possible, though. If you are interested in trying that, Google it and you’ll find some pretty neat tricks.

In Brief

EVAP system leaks aren’t that widespread in recent vehicles but are a bummer in older ones. Most lines and Evap system components are made out of plastic and seals and hoses tend to crack with time. It’s quite frequent to find cracked purge valve hoses causing the check engine light to come up.

If you intend to find and fix an EVAP leak on your own vehicle, we strongly suggest you get your hands on your car’s repair manual first. Having access to your car’s vacuum distribution and EVAP system diagram will significantly speed up the process. Furthermore, you’ll find everything you need to know concerning the manufacturer’s recommended procedure to test ever EVAP system component as well as troubleshooting charts. Trust me, you won’t regret it!

About Ivo G

Hey guys and girls, I’m Ivo, a passionate car enthusiast with a particular weakness for rear-wheel-drive and stick-shift gearbox cars. Meanwhile, I don’t mind getting my hands dirty – I have experience as a Certified Head of Service at a Toyota Dealership, and knowledge that I wish to share with you.

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