At 01:44 PM 8/21/03 -0400, you wrote:
>Any thoughts on this one? Michael 01 4.7L 4x4 QC
Let's hope that a loose or faulty gas cap is the culprit,
as others have posted. Otherwise, nailing it down can
be difficult and requires a special detection device.
Apparently, some diy can be rigged up but it's complicated
and clumsy to work with :-(
Anyhoo, some general info.:
The EPA has an official list of components that defines
the evaporative emission control system:
* Fuel tank
* Canister vent solenoid valve
* Fuel tank vacuum or pressure sensor
* Fuel lines
* Vapor lines
* Fuel tank cap
* Charcoal (EVAP) canister
* Purge lines
* Purge solenoid valve.(Vapor Management Valve)
Failure of any of these components will compromise the overall system,
causing hydrocarbon emissions to increase beyond the limit, sometimes
even if the car is parked. Most likely, certain conditions were detected
when your start-up on-board diagnostic test and a dtc was stored and the
MIL triggered. must be able to verify
Two parameters define proper EVAP function: system integrity, as in leaks
or blocked lines, and proper purge airflow from the canister to the engine.
In laboratory settings, it has been determined that an EVAP hose with a
leak only 0.020 in. in diameter is capable of increasing HC emissions
enough to fail the FTP certification. Those tests also show it is possible
to detect a leak that small using on-board diagnostic equipment. However,
real-world testing at that level showed extremely high failure rates, so the
EPA set a 0.040 in. leak as the standard for illuminating the MIL for leak
detection. For Dakotas, a leak between .40" to .90" was probably detected
when the pcm performed the EVAP leak detention test on start-up.
At this time there are two basic strategies for checking system integrity.
Some manufacturers use a pressure sensor; either built into the fuel tank
unit or mounted somewhere else in the fuel vapor control system .
At the appropriate point in a drive cycle, the PCM will close the canister vent
solenoid valve, use the engine to pull a vacuum of about 14 inches of water
on the system , close the purge valve and measure how long it takes for the
vacuum to decay to a specified level. If it decays too fast, there is a leak.
If it doesn't decay fast enough, there is a blocked line. The same test can be
performed in the service bay, basically a leak-down test using a hand operated
device like the Waekon Model 46568 EVAP System Master Diagnostic Kit
(read, an expensive diagnostic device).
I have no idea what a service shop would charge for this test. I have also
read (on another forum) that a 4.7L (cannot remember the year) had the
same problem and they could not find the leak with this test :-(
Let's hope that the culprit is a loose gas cap :-) Tighten the cap (2 clicks).
Clear code (either via scanner or disconnect -ive battery cable as others
have suggested. Sounds like the worst that'll happen is that you'll drop
excessive HCs into the air wherever you go :-P Good luck!
Bob
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