Vapor Cannister Vacuum Leak

[bnoggle]

Has anyone ever experienced a vapor cannister having a vacuum leak on one of our trucks? What were the symptoms? I have replaced every vacuum line in the truck, the thermostatically controlled switches, the vacuum connectors, and pulled and plugged lines. The leak occurs when the engine is warmed up, before that the engine runs very smoothly. I have narrowed it down to a hissing sound around the vapor cannister. What else could it be? Any ideas are appreciated.

[bd]

What happens if you pinch off the vacuum lines one-at-a-time?

[Dr_Snooz]

The evap system allows pressure in the fuel tank to vent to the atmosphere through the evap canister. The fuel vapors are trapped by the canister and stored until the engine is running. At certain times, the evap system will purge the canister by allowing engine vacuum to be drawn through the canister. Thus the fuel vapors are burned in normal combustion and don't contribute to dreaded smog.

Sooooo......

All that to say that finding a vacuum leak at the canister is not necessarily abnormal, but if the system is malfunctioning, it could be pulling vacuum at the wrong times. The evap system doesn't often go haywire, but it isn't unheard of.

[bnoggle]

If I pinch off the vacuum lines at the cannister the engine does run a little bit better. I have gone around the rest of the truck, and found nowhere else that it runs better with a blocked vacuum line.

[bd]

Do the canister vacuum lines have the configuration illustrated above?

[bnoggle]

Do the canister vacuum lines have the configuration illustrated above?

Yes


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[bd]

Then the canister has one 3/8" "bowl vent" hose running to the carburetor fuel bowl, one 5/16" "tank vent" hose connecting to the steel line from the fuel tank, one 1/4" "purge" hose that connects to engine manifold vacuum below the carburetor, and one 3/32" "purge signal" hose that connects to one of the TVS trees.  There is also a short 3/32" hose that tees into the 1/4" 'purge' hose and a covered, but open, "vent" port.  Pinching which hose makes a difference in the engine's smoothness?

[bnoggle]

Then the canister has one 3/8" "bowl vent" hose running to the carburetor fuel bowl, one 5/16" "tank vent" hose connecting to the steel line from the fuel tank, one 1/4" "purge" hose that connects to engine manifold vacuum below the carburetor, and one 3/32" "purge signal" hose that connects to one of the TVS trees.  There is also a short 3/32" hose that tees into the 1/4" 'purge' hose and a covered, but open, "vent" port.  Pinching which hose makes a difference in the engine's smoothness?

Blocking the 1/4 in purge hose seems to make it run better.


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[bd]

If you remove the 3/32" vacuum hoses connecting to the canister purge valve and canister bowl vent valve and apply vacuum to the respective nipples, will they hold vacuum or do they bleed off?  Likewise, if you remove the 1/4" vacuum hose from the canister purge valve and apply vacuum to the 1/4" valve nipple, will it hold vacuum or bleed off?

[bnoggle]

If you remove the 3/32" vacuum hoses connecting to the canister purge valve and canister bowl vent valve and apply vacuum to the respective nipples, will they hold vacuum or do they bleed off?  Likewise, if you remove the 1/4" vacuum hose from the canister purge valve and apply vacuum to the 1/4" valve nipple, will it hold vacuum or bleed off?

They will both bleed off quickly.


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[bd]

There are leaks in the valve diaphragms.  Either procure OEM valve service kits and replace them or source a new canister.

[bnoggle]

Ok, thank you


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[bnoggle]

I replaced the canister this morning, and the truck is idling much better than before, but there is definitely still a roughness to the engine at idle, and around 40 mph. I'm wondering if there is not a problem with the EGR valve as well.