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Super Moderator
1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
23,878 Posts
Many times at this age it is a cracked (dry rotted) or swollen rubber hose, especially at the hose ends where they attach to a line or component. Using your underhood vacuum map as a diagram, carefully look for any breach to air in the evaporative system plumbing or components.
The evap small leak code (P0442 or P0456) can be set with a pinhole leak. It needs to be a closed, airtight system. The larger leaks are more obvious and usually easier to find.
A shop smoke machine blows cool vegetable oil smoke into the lines. It is often laced with a UV dye and the leak will be apparent by looking for the smoke. Some can be difficult to find and service, i.e.- a leak on top of the fuel tank. They can also be intermittent.
I have found leaks at the purge valve hoses where they attach to the valve under the hood. The engine heat and hydrocarbons swell and soften the ends where they fit over the purge valve fittings just enough to set the code. Wire tie wraps (zip ties) can be snugged around the hose ends to clamp it tightly onto the valve fittings.
If a battery disconnect would make a vehicle pass an emissions test, they would all do it. The PCM has to run tests (OBD II monitors) before the monitors will pass. This is what the state emissions test looks for. It is easier than an actual tailpipe emissions test. It can take 2 or 3 days to run all the OBD II emission monitors in various driving conditions called drive cycles or 'good trips'. The fuel level has to be over 1/4 tank, but not full in order for the monitors to run.
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