EVAP System Vapor Pressure Negative – A Detailed Explanation

Car evaporative emissions systems are a great evolution to control fuel consumption and protect the atmosphere.

It’s vital to understand the system design, how it works, and what the average EVAP system vapor pressure negative is to determine if your system is suffering from a leak, thus, applying a proper solution.

I will help you out with a detailed explanation about normal EVAP system vapor pressure and more! Follow along! 

About The EVAP System 

evap system vapor pressure negative

What Is It? 

The system is made to redirect the fuel vapor in the fuel tank into the combustion chamber at the correct time and not to let it escape into the atmosphere.

Thus, it will not pollute the environment. Plus, it helps increase fuel efficiency and avoid some performance issues.

Why Do You Need It? 

The mixture of air and fuel vapor in the fuel tank should be released somewhere. Yet, we can’t emit it into the air; thus, the canister with “activated” charcoal is needed.

The part will absorb the vapors, store them until the special conditions are satisfied (such as constant driving and operating at normal temperature), and send them back to the intake system.

Once the system can’t keep the gas vapors properly or there’s a leak, toxic smoke will escape and harm the environment even when you’re not driving your vehicle.

Thus, the EVAP system vapor pressure at idle should be maintained in an acceptable range.

How Does It Work?

Gas Cap

A fuel cap plays a more important role than most people thought. It not only works as a cover of the filler neck but also guarantees the proper performance of the emission system.

The gas cap’s seal retains the vapors tightly inside the tank. This tight seal closes the system and assists the positive pressure building up.

Incorrectly closing the lid can lead to the check engine light flashing and the DTC code being detected.

Fuel Tank

Most drivers know that a fuel tank is a place to store gas or diesel, yet it also keeps a large amount of fuel vapor. Thus, I recommend not to overfill your tank to optimize the operation.

If you insist, excessive fuel will occur in the EVAP system, causing excess fuel vapor emission to the atmosphere.

Fuel Tank Pressure Sensors

FTPS is multi-functional. It tells the car owners the volume of fuel available, sends information about the fuel tank’s pressure to the ECU and signals the drivers if a detected leak happens in the system.

Canister Vent Valve

Some automakers also call this detail the canister purge valve. The component is in charge of sealing the fuel vapors within the charcoal canister.

Besides, it also transfers vapors to the air intake if needed.

When there’s electrical flow, the canister purge valve works as a solenoid to open the vapor lines.

The detail’s closing and opening process is controlled by the PCM. PCM will decide when it’s the right time to open the valve.

I notice that the canister purge valve is subjected to replacement the most among others.

Charcoal Canister

The charcoal canister is where the fuel vapors are stored. You will find it in a small plastic box near the gas tank. In older models, it’s located inside the engine bay.

The component consists of a lot of activated charcoal that works as a sponge. It captures and stores the gas to use later when needed. You can bypass the EVAP canister in case of malfunction.

Liquid-vapor Separator

Liquid-vapor separator sits on the gas tank’s top. You can easily conclude from its name that the component separates the fuel vapor from the liquid form.

If this part didn’t exist, your charcoal canister would be over full with fuel, leading to malfunction.

Vent Control Valve

The detail is also known as the canister close valve. As its name indicates, it’s responsible for closing the charcoal canister.

The PCM usually uses this function to detect leakage from the EVAP system.

EVAP System Error Codes

All parts of the EVAP system should function properly to ensure performance, yet malfunction sometimes is unavoidable.

However, you can use an OBD-II scan tool to detect the problem. Here are some of the common issues with their fault codes:

  • P0442, P0441, P0440: EVAP system codes
  • P0445, P0444, P0443: Purge valve codes
  • P0449, P0448, P0447, P0446: Canister vent valve
  • P0454, P0453, P0452, P0451, P0450: Pressure sensor codes
  • P0457, P0456, P0455: Leak codes

What Is EVAP System Vapor Pressure Negative?

what should the evap system vapor pressure be

The EVAP system includes three main parts: the canister vent solenoid (CVS), canister purge solenoid (CPS), and fuel tank pressure sensor (FTPS).

All components work together to measure very small pressure changes with the measurement of inches of water (in.-H2O).

EVAP vp h20 normal range should be around −10.0 in.-H2O negative pressure.

What Is Normal EVAP System Vapor Pressure? 

EVAP system vapor pressure normal range (positive pressure) is from 3.0 to 4.0 in.-H2O. Meanwhile, EVAP vp h20 negative is approximately −10.0 in.-H2O.

The positive pressure buildup of about 42.0 in.-H2O (or 1.5 psi) and the negative pressure buildup of−21.0 in.-H2O (or at −1.5 in.-Hg) are the amount of EVAP vp released from the gas cap.

Understanding Of Different Phases

“Workhorse” Phase

It’s the phase when the sensor will detect a slight negative EVAP system vapor pressure. It’s caused by the charcoal canister and different filter restrictions.

In this phase, the ECU will be informed of a vacuum ranging from -3.73mmHg to -4.47mmHg.

If the negative pressure exceeds 14.95mmHg, the code P1450 (Excessive Vacuum Buildup) will likely appear.

Gas Vapor Integrity Phase

The test conditions in this phase are: 15% to 85% fuel tank full and other states are satisfied. During this phase, the sensor should sense a negative pressure of -14.95mmHg.

If not, the PCM will proceed with the test several times. If the expected result is not achieved after that, the system will be identified with the code P0455 (Gross Leak).

If the value is more than supposed to be, the ECU will show error code P1450 (Excessive Vacuum Buildup).

Vacuum Stabilization Phase

Suppose all the previous tests are done successfully with the target value achieved; this phase will maintain that pressure with the close of purge solenoids and canister vents.

If the result fluctuates during this test, a P1450 code will appear.

Vacuum Hold And Decay Phase

This phase will mostly test the time that the negative pressure decreases.

For example, the gross leak will be assumed if the starting result of 13.08mmHg fails to 0mmHg in a few seconds, or a large leak will be concluded if the value drops to 3.74mmHg for more than 30 seconds.

The ECU will confirm no leaks when there’s a constant pressure of around 30 seconds.

Vacuum Release Phase

In this phase, the ECU will open the canister vent solenoid, making the pressure in the tank reach that of the atmosphere.

This process will take around 30 to 60 seconds. The phase will tell you if the vent solenoid opens fully or not.

Vapor Generation Phase

After the previous phase, the canister vent solenoid will be closed for the pressure to increase inside the tank.

In general, positive pressure rises quickly. Yet, if it is not over 4.7mmHg, there will be the conclusion of a 1-mm leak, leading to the code P0442.

Smoke Test For EVAP Leaks

Step 1. Prepare 

Mechanics usually conduct the smoke test to know whether the EVAP_vp normal range is achieved.

If you want to do it yourself, I suggest you’d better not perform the test outdoors but inside their garage, where there’s no breeze.

Apply good light and prepare a good, strong flashlight. Use a smoke machine to limit the applied pressure. Choose a gas cap adapter or EVAP service port adapter to access and test the system.

Remember to make the test at a very low pressure of about 12 inches of water (½ PSI). Plus, always remain the value lower than 7 PSI to avoid damaging the hoses and sensors.

Step 2. Connect the smock source to the EVAP port and identify leaks

Open your car’s hood and find the green plastic cap covering the port. You will see a valve core under the lid, but your smoke machine should be able to push the smoke through it.

Now, check for leaks.

If there’s no smoke port, use a proper adaptor to attach the smoke machine to the gas tank through the filter. Then, perform the test by pushing the smoke in and looking for any leaks.

Step 3. Replace the part causing leakage

Once you identify where the leak comes from, change that part immediately.


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Are EVAP Systems Available In Diesel Engines?

No, diesel engines don’t need the systems as the systems aim to collect, stabilize and return the evaporated fuel to the engine. Yet, diesel is heavier than gas and does not evaporate that easily.

Plus, diesel has a higher boiling point than water, leading to no evaporation and the unnecessary of having EVAP systems.

Since these two types of fuel are different, it’s also possible that diesel will ruin a gas engine if you’re confused and mis-fill them.

Will A Bad EVAP System Decrease The Car Performance?

In general, it will not significantly degrade the performance, given that the purge valve is closed.

Even in this situation, you may not feel anything while driving, yet the check engine light will light up.

If the problem lies in any other components in the system, it’s almost impossible to notice power loss unless you drive your car for a very long period.

The Bottom Lines

I hope now you understand the EVAP system vapor pressure negative as well as the system’s operation.

EVAP vp normal range should be around −10.0 in.-H2O for negative pressure and from 3.0 to 4.0 in.-H2O for positive pressure.

Otherwise, the system will detect a small (gross) or large leak, depending on the specific situation.

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