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so my check fuel cap light came on last week, i reset it then,and it came on again today the fuel cap looks new and the seals not broke or anything. jw y does the light keep coming on? and i always tighten my cap to 2 full turns in case anyones wondering.
 

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whats that and how do u fix it
This off a Mustang Site. But it explains what an EVAP leak is. Same thing on the Focus..


Had the same issue with mine !! I tighten the cap disconnected the battery to clear the code If that doesn't work Time for a test to find the leak


The EVAP small leak test applies vacuum to the evaporative emission (EVAP) system and monitors vacuum decay. The powertrain control module (PCM) monitors the fuel tank pressure (FTP) sensor signal to determine the vacuum decay rate. At an appropriate time, the PCM turns the EVAP canister purge valve ON (open) and the EVAP vent valve ON (closed). This allows the engine to draw a vacuum on the EVAP system. At a calibrated time, or vacuum level, the PCM turns the purge valve OFF (closed), sealing the system, and monitors the FTP sensor input in order to determine EVAP system vacuum decay. If the system detects a leak larger than a calibrated amount DTC P0442 will set.
The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
inspect for leaks in the following locations:
The EVAP system purge pipe.
The EVAP vapor pipe.
The EVAP vent hose/pipe.

The fuel fill pipe/hose and fuel fill cap.

The EVAP canister.
The EVAP canister vent valve.
The EVAP canister purge valve.
The fuel sender assembly and/or seal.
The FTP sensor seal.
The fill limiter vent valve, pressure relief valve, rollover valves, and fuel tank.
 

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^ basically what he said. Usually I use a smoke machiene to locate the leak. the evap purge valve opens and the vent valve closes this put the system under vac pressure. then the ecu watches the pressure sensor on the tank for a pressure drop. If the pressure sensor is faulty it'll think you have a leak when you don't.

Most common causes are:
faulty pressure sensor
Rotted/leaking fill tube
fuel pump/top of tank leaking (rot/bad gasket on sending unit)
vent valve not closing properly

You can remove the line up at the purge valve and pinch off the line off the charcoal canister and put the system under air pressure only like 5-10psi then use a sthetscope(I know I horribly misspelled that, the doctors thingy) and listen following the entire evap system listening for a hiss of air leak. when you hear it you found the issue. it'll be faint beacuse it's not throwing a mid/major leak.
The sthetscope is much cheaper than a $1000-$2000 smoke machiene or probally a $100-$200 diag.
 
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