Failed smog test ; gross air leak at vapor recovery system. Replaced gas cap from parts house with a dealer supplied gas cap and the gross leak is gone but a minor air leak is still being reported by the On Board Computer. Have checked the hoses I can see and they appear fine no cracking etc. Last night I pumped smoke into the tank through a " Jim Cap " ( 2" rubber cap w/hose clamp ) and got nothing from the the collection of hoses at the carbon canister, fuel pump assembly at the fuel tank. Any suggestions now?

The evaporative (EVAP) emission system is all about controlling fuel vapors. When a leak develops, it allows the vapors to enter the atmosphere which is harmful to the environment. Modern vehicles therefore have been designed with a system to detect when a leak is present. There are several different Diagnostic Trouble Codes (DTCs) than can be set for EVAP leaks including those for very small, small, or gross (large) leaks. The most common gross/large leak is from a loose or damaged fuel cap. Because it's the most common, people often assume that the gas cap is the culprit even though it could be elsewhere in the system. Some vehicles even have a fuel cap indicator light that makes this same assumption. If you have a DTC for an EVAP leak and/or the fuel cap indicator light is on and you've already replaced the gas cap, then an EVAP leak test should be performed to locate the true source of the leak. You can read more about large EVAP leaks and the associated warning indicators here: https://www.yourmechanic.com/article/what-does-the-fuel-cap-warning-light-mean-by-spencer-cates

EGR valve ? Which pollutant is worse on gas analysis ?

"Tighten gas cap" means

1. That there is an air in-leak somewhere in the vapor recovery system of the car. Hoses and hose connections is a suspect place. So is the electric carbon canister vent valve. OR
2. It also can mean out of spec gasoline from the discount gas station.

If this was my car I would:

1. Top up with some decent gasoline.
2. Have the local shop clear the code and see if it comes back. Maybe do this several times until I am sure all the suspect gasoline is gone.
3. If it does, and as a last (expensive) resort, have the shop or dealer "smoke test" the emission system for leaks.

Good luck.

I researched this and found the following information. Hope it helps: P0455 means a gross leak detected in the Evaporative Emissions System. Some call this a "gas cap" code, because the most common cause a loose or failed gas cap. But it could be caused by any vapor leak in the fuel tank, filler neck, or anywhere else in the vapor management system. If it isn't a gas cap, you will need tol take it someplace where they have a smoke machine that can find the leak using smoke.

P0455 means a gross leak detected in the Evaporative Emissions System. Some call this a "gas cap" code, because the most common cause a loose or failed gas cap. But it could be caused by any vapor leak in the fuel tank, filler neck, or anywhere else in the vapor management system. If it isn't a gas cap, you will need tol take it someplace where they have a smoke machine that can find the leak using smoke.

It's an emission control related problem, there is a leak in the fuel vapor recovery system and could be caused by a number of things, the most common are a vapor hose off at the air filter so check the air filter hose connections, it's common for the evap system purge hose to be disconnected from the air filter, next is a gas cap that is not sealeing, try replacing the gas cap with an OEM grade gas cap, then disconnect the negative battery cable for ten seconds, this clears the codes, wait and see if the check engine light comes back on and which codes came back.

Fuel tankes have only one large acess on the top i sugest checkin the rubber gasket for cracks or perishing also check along the fuel lines for damp spots use your nose to locat the strongest area of gas then go from there hope this helps:)

EVAP is evaporative emissions which include the raw fuel vapors from the tank. They are known as hydrocarbons. The evap system contains these vapors and disposes of them through the burning process in the engine. You have valves and solenoids controlled by the computer to purge them from the system. Some of the parts that may cause this test to fail are the vapor lines to and from the tank, the fuel cap (loose or leaking), the carbon canister, and the purge valve or solenoid. It is a somewhat difficult system to test because it is under very low pressure. With all the components in the system, it is hard to say which is at fault. There is a pressure tester provided by Rotunda (Ford service tool) that is made to diagnose this particular problem. You local dealer should have one. About all you can do yourself is visually check the lines and the fuel cap for obvious problems. The MIL light may be on for this reason also. I hope this helps.

means small evaporative emmisions leak. there is a vapor leak in the fuel system, either a bad or loose gas cap, cracked filler neck etc. id try the cap first, if not that you can fix yourself. a shop will pressurize the system with nitrogen and smoke and locate the leak in this manner. good luck

I have the same failure that occurred when smogging my Ford Ranger. The EVAP test was just added a couple of years ago and it tests for leaks in the gas tank vapor recovery system. What this is a system of vapor recovery lines that connect from the gas tank and goes to a canister then returns to the intake and on the next engine start and gets used, instead of leaking into the air.

The only issue is these lines can develop a leak at any of the connection points or along the rubber portions and can be a pain to replace depending on your application. So until you fix the leak you will fail this test since they pressurize the system and if there is a leak it won't hold the pressure.