Evaporative Emissions System (EVAP) Description

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Read Some Popular EVAP System Questions And Answers

The evaporative emission (EVAP) control system limits the fuel vapors from escaping into the atmosphere. The EVAP transfers the fuel vapors from the sealed fuel tank to an activated carbon storage device, the EVAP canister. The EVAP canister stores the vapors until the engine is able to use the extra fuel vapor. When the engine is able to use the extra fuel vapor, the intake air flow purges the fuel vapor from the carbon element and then the normal combustion process consumes the fuel vapor. The system is required in order to detect the evaporative fuel system leaks as small as 0.040 inch between the fuel filler cap and the EVAP canister purge valve. The system can test the evaporative system integrity by applying a vacuum signal, ported or manifold, to the fuel tank in order to create a small vacuum.

The powertrain control module (PCM) then monitors the ability of the system to maintain the vacuum. If the vacuum remains for a specified period of time, then there are no evaporative leaks, and a PASS is reported by the PCM. If there is a leak, the system either will not achieve a vacuum, or a vacuum cannot be maintained. Usually a fault can only be detected after a cold start with a trip of sufficient length and driving conditions to run the needed tests. The enhanced evaporative system diagnostic conducts sub-tests in order to detect the fault conditions. If the diagnostic fails a sub-test, the PCM stores a diagnostic trouble code (DTC) in order to indicate the type of fault detected. The EVAP diagnostic tests include the following tests:

Weak Vacuum Test:

This tests for large leaks and blockages in the EVAP system. The fuel tank pressure (FTP) sensor detects a low fuel tank pressure resulting from a large vacuum leak or a restriction in the vacuum supply to the fuel tank. The FTP sensor value should increase over a period of time. If the PCM does not detect an increase, a malfunction is indicated by setting DTC P0440.

Weak Vacuum Follow-up Test:

This tests the EVAP system after the Weak Vacuum Test has failed. The FTP sensor looks for high tank vacuum over a period of time. If this occurs the Weak Vacuum Test will pass. If Weak Vacuum does not pass, the test continues through this ignition cycle.

Vacuum Decay Test:

This tests for small leaks in the EVAP system. The decay rate is determined by measuring the change in the fuel tank pressure over a period of time. If the decay rate is more than a calibrated value, another Vacuum Decay Test will be initiated. If vacuum decay is still more than a calibrated value, a malfunction is indicated by setting DTC P0442.

Fuel Vapor Build Pressure Test:

This test is used to determine the vacuum leak size. When the vacuum has decayed to near zero, the PCM subtracts the measured slope from the vacuum decay to calculate the leak size. If the calculation indicates that a leak is present, the Small Leak Test will fail.

Canister Vent Restriction Test:

This tests for a restriction in the EVAP vent system. The FTP sensor looks for sufficient vacuum from the engine intake manifold. After a period of time and conditions have been met, this test will pass. If the vacuum value is too high, a malfunction is indicated by setting DTC P0446.

Purge Valve Leak Test:

This tests for small leaks to the intake manifold. This is accomplished by sealing the EVAP system purge valve and the EVAP vent valve and allowing the PCM to monitor the FTP sensor. The FTP sensor value should not increase. If the PCM detects an increase, a malfunction is indicated by setting DTC P1441.

Diagnostic Test Failure. DTC Possible Causes

Weak Vacuum Test and Large Leak Test: P0440 A large leak, above .060 in. or blockage somewhere in the system.

Vacuum Decay Test Small Leak Test: P0442 A small leak somewhere in the system, .020-.060 in.

Canister Vent Restriction Test: P0446 A restriction in the EVAP vent system Purge Valve Leak Test P1441 A stuck open or partially open purge valve.

Fuel Tank Pressure Sensor: The fuel tank pressure (FTP) sensor measures the difference between the air pressure, or vacuum, in the fuel tank and the outside air pressure. The sensor mounts at the top of the fuel tank sending unit. The PCM supplies a 5 volt reference voltage and ground to the sensor. The sensor provides a signal voltage between 0.1-4.9 volts to the PCM. When the air pressure in the fuel tank is equal to the outside air pressure, such as when the fuel fill cap is removed, the output voltage of the sensor will measure 1.3-1.7 volts. When the air pressure in the tank is 1.25 kPa (4.5 inches Hg), the sensor output voltage should measure 1.25 kPa (0.5 volts). The sensor voltage increases to approximately 4.5 volts at -3.75 kPa (14 inches Hg).

EVAP Canister Purge Valve: When energized, the evaporative canister purge valve allows the fuel vapor to flow from the EVAP canister to the engine. The normally closed valve is pulse width modulated by the PCM in order to precisely control the vapor flow. The valve opens during the Enhanced Evaporative Diagnostic Test in order to create a vacuum in the fuel tank and then closed in order to seal the system.

EVAP Vent Valve: The evaporative canister vent valve not only allows the fresh outside air to the EVAP canister during the purge modes, but also allows the diagnostic to pull a vacuum on the fuel tank by closing the vent valve. This valve is normally open.

EVAP Canister: The EVAP canister, filled with charcoal pellets, stores the fuel vapors from the fuel tank. Vacuum is pulled from the fuel tank through the vapor pipe. The system goes into open loop. Fresh air is mixed with the vapors and pulled to the intake manifold from the canister through the purge line (1). The engine vacuum purges the EVAP canister during normal driving.

What is code to turn off light? What can happen if you don't fix it? Oldsmobile. -That depends on what the stored computer code number is. Most EVAP system codes will have no ill effect of the vehicles operation, other than a constant check engine light and unable to pass emissions testing.

I have P0446 and I changed the purged solenoid, vent solenoid, and gas cap and still same. Chevy Malibu. -That depends on what the stored computer code number is. Most EVAP system codes will have no ill effect of the vehicles operation, other than a constant check engine light and unable to pass emissions testing.

If you're using a smoke machine and smoke is coming out to the breather is that normal? Pontiac Grand Am. -It depends if you've got everything closed off. You should only use the smoke machine after the vent valve and purge solenoids are commanded closed, and the gas cap is on tightly, or on an adapter tightly. If you've introduced smoke into the system without doing the steps listed above, you can have smoke coming out everywhere and it will not help you find your leak.

When does the EVAP test occur -- At cold engine start, on highway driving? After resetting the DTC P1441, P1442 and P1446, it takes 3 to 4 days for the MIL light to turn ON and for the INC on status to disappear. DTC small leak, but found a open CCV (large vacuum leak), so the CCV got replaced. Chevrolet Monte Carlo. -Several parameters must be met to run this test. Highway driving, then sitting at a stop, engine warm, A/C off.

what is the evaporation control valve? That's what my code is for. Is that in the canister? 2006 GMC truck. -If you mean the vent valve, it is in the rear under the spare tire, or on the rear of the gas tank. If you mean the purge valve solenoid, it is on the engine.

2001 Pontiac Grand Am GT ABS/TRAC off warning light on but shows no codes. Is there a reset button for these warning lights to go off? -No rest button. You need to have the computer scanned for codes, and the problem repaired, then the computer scanner can clear the codes.

2002 Ford E-250. Vacuum forms in gas tank after driving a few miles. The bottom of the fuel tank gets sucked upwards. Turn off engine the gas tank pops back into place. No check engine codes. Replaced fuel pump, fuel vapor separators, and fuel tank pressure sensor. After two days time problem started again. -You do not have a sensor problem, you have a vent valve problem. It is not venting when it should be, thus causing an extreme vacuum.

Any idea of what the cost may be to have the EVAP Leak addressed for a 2000 GMC Jimmy? -That depends on what the problem is. If it is just a vent valve, then expect to pay about $150 parts and labor. If it is a fuel tank seal, than maybe $300 pats and labor. Could just be a bad gas cap. If the leak is hard to find, then labor hours will be most of the cost. And this depends on what the shop charges. Dealerships range from $90- $130 per hour. Local shops are $60 - $100.

I have a 99 Hyundai accent and recently found out the car I recently purchased is missing the evap canister. Of course check engine light is on. I have been having problems after driving for a while I blow a noticable amount of white smoke out the tailpipe. I am hoping buying and installing one will remove that issue. Also is not a blown head gasket. Any ideas. -A missing canister should not cause the engine to blow smoke. You have other problems.

Since you are very smart let me throw this at you. I have a 2009 Dodge Ram Hemi came with 26 Gallon tank no codes. I decided I wanted a 32 gallon tank so I bought one from salvage yard and removed 26 and moved over the fuel pump assembly which is same part number for both. Hooked all back up and everything looks fine but now I am getting a gas cap light and check engine where engine is lit solid. Check with the scanner I have it says 0440 evap emissions code. What do you think I did wrong to cause this error. I have rechecked the connections which is about 6 things if you include both ends of quick connects and hoses. I did mess up wire harness with one purple wire and one black wire that connects to rear of tank by the vent hose and had to make a wire harness with spade terminals. I doubt it is really gas cap since the gas cap worked before on 26. What do you think it could be? Maybe some junk that got in the lines? -You may have blown a fuse, have a leak in an EVAP line or the pump unit seal, or maybe the pressure sensor and computer is not calibrated for a larger tank.

P2421 Evaporative Emission System Vent Valve Stuck Open on a 2007 Volvo XC90. How do I fix? -Replace the valve.

I have a 2002 Chevy Avalanche with the warning light showing. Code P0446 comes up which is associated with EVAP Vent System Performance. Any idea what would be causing this and how to fix? -Probably the EVAP vent valve is bad. Very common problem. It is located above the spare tire. There is a redesigned kit at the Dealer to repair this problem.

I hope you can help me i have a 2004 dodge stratus and i have been through so much with car my engine light is on and has been for ever i need a smog and the code i am getting says that my car is running lean i have changed everything possible what are my options please help. -Check for vacuum leaks,check fuel pressure and check all the oxygen sensor for proper operation.

2002 Honda Passport (same as Isuzu Rodeo). Codes P0446 and P1441. Using a Mity-pump, vacuum holds at the purge valve and in the line from the valve to the canister (line pulled off of canister and vacuum pulled from that end). Purge valve opens when a voltage is applied from a 9V battery. Vent valve at canister has open flow and valve closes when a voltage is applied from a 9V battery. I did not test vacuum between gas cap (seal looks fine) and canister (lines are too large for pump). Canister itself will hold pressure, but not vacuum. Can this be the problem? -Yes, the canister not holding vacuum could mean that it is cracked. Vacuum pulls it one way, pressure the other. But, just because the vent and purge valve seem to be working when voltage applied, does not mean the computer is applying voltage properly. You need to use a scanner that can turn these items on and off. That will rule out the computer, valves and wiring.

I have a 2001 Hyundai Accent 1.6L and I have had the EVAP CONTROL CANISTER replaced twice now. Is there anything I can pick up at like Auto Zone to help keep it clean? -If the canister is getting full of dirt, there may be a filter for it. Check with your dealer for a redesigned unit. If it is filling up with gas, then you have a problem in the system

I have a 2001 Rodeo that keeps having the same error code saying bad gas cap. I replaced it with one but still got the error. I was told it has to have a factory cap. I am also getting horrible mileage 8.6. I was told the bad gas cap would cause this also. any ideas? -If the gas cap is not fixing it, then you have an EVAP system leak. Possibly a bad purge valve or vent valve. The check gas cap light comes on because the computer senses a leak,but cannot determine what it is. A large leak acts like a gas cap loose or missing, and is a common thing to happen, so thats why the system has this warning.

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