EVAP SYS MED. LEAK.... ok... that's all well and good, but what the heck is an EVAP SYSTEM??

Help plz.

Jimbo

 

EVAP System:

The Evaporative Emission (EVAP) Control system used on all vehicles is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon (charcoal) storage device (canister) to hold the vapors when the vehicle is not operating. When the engine is operating, the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.

The evaporative system includes the following components:

*Fuel tank
*Evaporative emission canister vent solenoid
*Fuel tank pressure sensor
*Fuel pipes and hoses
*Vapor lines
*Fuel cap
*Evaporative emission canister
*Purge lines
*Purge valve solenoid

EVAP System Operation:

The EVAP purge solenoid valve allows manifold vacuum to purge the canister. The Powertrain Control Module (PCM) supplies a ground to energize the EVAP purge solenoid valve (purge on). The EVAP purge solenoid control is Pulse Width Modulated (PWM) or turned on and off several times a second.

The EVAP canister purge PWM duty cycle varies according to the operating conditions determined by the mass air flow, the fuel trim, the engine coolant temperature, and the intake air temperature. For certain EVAP tests, the diagnostic will be disabled if the TP angle increases to above 75% .

The evaporative leak detection diagnostic strategy is based on applying vacuum to the EVAP system and monitoring for vacuum decay.

The fuel level sensor input to the PCM is used to determine if the fuel level in the tank is correct to run the EVAP diagnostic tests. To ensure sufficient volume in the tank to begin the various diagnostic tests, the fuel level must be between 15% and 85% .

The PCM monitors the fuel tank pressure/vacuum level via the fuel tank pressure sensor input.

Results of Incorrect Operation:

Poor idle, stalling and poor driveability can be caused by the following:

*Malfunctioning purge solenoid.
*Damaged canister.
*Hoses/lines split, cracked and/or not connected properly.

Evidence of fuel loss or fuel vapor odor can be caused by the following:

*Liquid fuel leaking from the fuel lines
*Cracked or damaged canister
*Inoperative canister control valve

Vacuum hoses that are:

*Disconnected
*Mis-routed
*Kinked
*Deteriorated
*Damaged

The evaporative leak detection diagnostic strategy is based on applying vacuum to the EVAP system and monitoring vacuum decay.

The PCM monitors vacuum level via the fuel tank pressure sensor input. At an appropriate time, the EVAP purge solenoid and the EVAP vent solenoid are turned on, allowing engine vacuum to draw a small vacuum on the entire evaporative emission system. After the desired vacuum level has been achieved, the EVAP purge solenoid is turned off, sealing the system. A leak is detected by monitoring for a decrease in vacuum level over a given time period, all other variables remaining constant. A small leak in the system causes DTC P0442 to be set.

If the desired vacuum level cannot be achieved in the test described above, a large leak or a faulty EVAP purge solenoid is indicated. This can be caused by the following conditions:

*Disconnected or faulty fuel tank pressure sensor.
*Missing or faulty fuel cap.
*Disconnected, damaged, pinched, or blocked EVAP purge line.
*Disconnected or damaged EVAP vent hose.
*Disconnected, damaged, pinched, or blocked fuel tank vapor line.
*Disconnected or faulty EVAP canister solenoid.
*Disconnected or faulty EVAP vent solenoid.
*Open ignition feed circuit to the EVAP vent or purge solenoid.
*Damaged EVAP canister.

Any of the above conditions sets DTC P0440. A restricted or blocked EVAP canister vent path is detected by drawing vacuum into the EVAP system. The PCM turns off the EVAP vent solenoid and the EVAP purge solenoid (EVAP vent solenoid Open, EVAP purge PWM 0% ). The PCM monitors the fuel tank pressure sensor input. With the EVAP vent solenoid open, any vacuum in the system should decrease quickly unless the vent is blocked. A blockage is caused by the following conditions:

*Faulty EVAP vent solenoid (stuck closed).
*Plugged, kinked, or pinched vent hose.
*Shorted EVAP vent solenoid driver circuit.
*Plugged evaporative canister.

If any of the above conditions are present, DTC P0446 sets.

The PCM checks for conditions that cause the EVAP system to purge continuously by commanding the EVAP vent solenoid on and the EVAP purge solenoid off (EVAP vent solenoid CLOSED, EVAP purge PWM 0% ). If fuel tank pressure level increases during the test, a continuous purge flow condition is indicated. This can be caused by the following conditions:

*EVAP purge solenoid leaking.
*EVAP purge and engine vacuum source lines switched at the EVAP purge solenoid.
*EVAP purge solenoid driver circuit grounded.

If any of the above conditions are present, DTC P1441 sets.

Refer to the appropriate DTCs for further diagnostic procedures regarding the EVAP system.

Visual Check of Evaporative Emission Canister

*Cracked or damaged, replace the canister.
*Fuel leaking from the canister, replace canister and check lines and line routing.



Check your hoses....could be something came undone...

 

I have seen both bad gas caps & loose gas caps cause the P0442 code before. Most of the time the cap is just a bit loose. I have never seen a 'good' gas cap go bad, they failed from day 1. Sounds like a 'b' movie...

Damn DevilsReject, what a post! Very informative, thanks!

 

Yeah, gas caps can typically give you that code rather easily. Most people just dont turn it all the way or the seal gets messed up. Pretty common occurance....

 

We actually see many gas caps fail, causing evap codes. Take the cap off and look at the little valve in the center, if there is rust in there, it is most likely a leaky cap. That valve is a one way valve (in only). They get rusty and leak.

 

what else could it be, bought a new gas cap, and disconected battery cable annd turned code off. about 25 miles later, it came back on.
does that mean something else, or do i need to get it professionally cut off? i am single mom, and seem to always get screwed unless i seem to have some info and at least appear to know a little of what i am talking about?

 

I'd figure just going in and telling them that you have a problem with your evap emissions system since it was throwing a code would set you apart from joe/jane average, terri.

And damn - Devils Reject - that WAS one hell of a write up.

 

Don't forget to mention to the service manager that you already tried a new gas cap. That would be their first guess also, I'm sure.