Good afternoon,

Today my Service Engine Soon light popped on. When I pulled the code it came up as P0455 which is Evaporative System Leak Detected (gross leak/no flow). I just purchased a new gas cap a few weeks ago. I took it off to make sure it was OK and there was a pretty good seal on the tank. There was a loud hissing sound when I pulled the cap off. I also have not observed any fuel leaks around the tank or the fill tube. Has anyone had this issue come up and how did you fix it? All of the parts associated with this system cost over $100 so I'd like to make an informed decision about what to buy first. The two most likely culprits appear to be the Canister Purge Valve or the EVAP Fuel Tank Pressure Sensor. I checked out the lines running from the purge canister (there are three) and they look OK. I'm leaning towards replacing the Purge Valve first. Who can help?

 

You need someone with a (low pressure) smoke machine to find the leak.
Watch this video before you start throwing parts at the problem.
This guy in the video (Eric O.) is my favorite automotive diagnostician on the internet.
Some EVAP parts are still available under the Chevy part numbers.

 

the code sets when the ftp sensor does not detect a large enough pressure drop when the evap system has the purge valve open.
the vent valve must close to seal the system during the test. these commonly get stuck. the valve is powered at all times with the key in run...provide it grd and you should hear it click when it closes.
if you hear nothing...replace it and the evap filter....just make sure the vent valve has battery voltage.

-if the vent valve is closing...you should check the ftp sensor reading with a scan tool. it should be near 0"h20 engine off and the fuel cap off. install the cap and run the engine and command the purge valve open. the pressure should drop. if it doesn't...you have either:
-a very large evap leak
-no purge air flow...blocked/ open- purge line/bad purge valve
-a bad ftp sensor

 

Here's a diagram to go along with tech2's explanation of the EVAP testing process.

 

An update on my Chevy Tracker. My mechanic found the problem and I did not believe it until I saw it. Someone disconnected the vacuum line from the Tank Pressure Control Solenoid Valve and stuck a old broken drill bit in the vacuum line. Then they plugged the connection on the valve. My mechanic threw away the drill bit, drilled out the obstruction, plugged in the vacuum line, and the code went away. I owned the car for over six years before this code popped up, so that ghetto fix worked for quite a few years....

 

1. Congratulations on getting this problem repaired.
2. Thanks for reporting back with the fix.
3. Was the "Tank Pressure Control Solenoid Valve" bolted to the engine and did it look something like the valve in the picture below?

 

I'm revisiting an older thread because my P0455 code is back.

I have a new fuel cap (the old one failed a pressure test when I was doing an emissions test), and when I remove the cap there is always a big hissing noise so I'm sure the new fuel cap is good.

I removed the evap canister vent solenoid and tested it. It was open with no voltage applied. I applied 12 volts, the solenoid engaged, and no air passed through.

I removed the evap canister purge valve and tested it. It was closed with no voltage applied and no air passed through. I applied 12 volts and it opened. This is also the picture that L84sky posted above.

I have not found any damaged vacuum lines.

I need to test these two valves in the car to see if they are getting the proper voltage and my fuel pressure sensor. The fuel pressure sensor is hard to find so I hope it is good.

 

According to the Chevy FSM, DTC P0455 can be intermittent. A Tracker specific scanner
may be required to troubleshoot.

 

I knew the vent valve was normally open and the purge valve was normally closed. I had assumed the control signal from the PCM was simply and ON/OFF command.
While that is true for the vent walve, it turns out the purge valve is Pulse Width Modulated (PWM) by the PCM.


This a screenshot of my Tracker idling in the garage. Neither EVAP valve is being commanded
to do anything. The fuel pressure sensor is reading zero inches of water.


Driving down the road I noticed the purge valve was reading 7% open. I didn't expect a partially
open valve.



Plotting the purge valve command and RPM showed a definite correlation.



The purge valve closed when I came to a stop sign.



However, he vent valve stayed open (uncommanded) the whole time.



The Chevy FSM theory of operation clearly states the PWM control of the purge valve.
I either did not read this or did not understand it until today.



The vent valve only closes during the EVAP leak test (which never ran during my drive
cycle). I'm still puzzled by the construction of the EVAP canister. The drawing above
suggests air can pass between the left and right sides. If that's true, it would seem
to me pressure cannot build up in the fuel tank unless the vent valve is closed.
The fuel tank pressure reading stayed at or very close to zero the whole drive cycle.
Also I don't ever remember a hissing sound when I remove my gas cap.

 

you don't want vac or pressure to build up in the tank; leak test is the exception.

air can pass on both side of the cannister but it wants to draw purge from the tank and charcoal pellets...when it does t his fresh air is drawn in to stop the tank and cannister from collapsing. If the tank couldn't breathe when the engine was pulling fuel out...the tank would collapse.

the evap vent filter should really be a maintenance item especially if you drive on dirt roads. They will plug up regardless of there location.
if you have a hissing sound when the cap is removed...something is wrong