Vapor Leak and Poor Gas Mileage

I have a 1998 Honda Accord, automatic with 179,000 miles. The malfunction indicator light came on so I had the car smoked and found that it needed a purge valve. I replaced the purge valve and smoked the car again which indicated the car still has a vapor leak and the malfunction light stayed on. The only place visible smoke is coming from is from the fuel filler neck (place where fuel or gas is put into the car) when the gas cap is off.
Can anyone give me any helpful advice where the leak may be? My car is also getting horrible gas mileage. I was going to replace the fuel filter but I heard it was in the gas tank, is this right and is it replaceable? Would replacing the gas cap help either the vapor leak or gas mileage?
Thank you, I appreciate your help.

Well, I can’t See it from here. I guess you will just have to do it the old fashioned way. Follow the smoke back to the source.

What is the exact code? There are a few for EVAP system leaks. Another idea, have you had the gas cap checked for leaks? A new one is a cheap test.

If you fill beyond the fist click when getting gas, you could be causing this problem. If your lucky and you stop overfilling, then it might clear up on its own.

Otherwise you may have a fuel line leaking somewhere and smoke is not going to find it. You have to sniff for unburned hydrocarbons.

The fuel system is no longer sealed when the gas cap is removed, so a leak there is hardly surprising.

An evaporative emission system leak won’t generally cause poor fuel economy so something else is going on regarding that issue. What is the actual error code?

Replacing the fuel filter isn’t going to have an effect on either problem.


When you use the smoke machine to check for leaks, you put smoke into the test port, which should be in the engine bay. Before you do that, you close the vent valve with the scan tool. But the fuel filler cap needs to be installed. Otherwise, all the smoke will come out of the filler neck.

Put the cap back on, close the vent valve and try again.

But you have to close that vent valve also

For those who care, I believe the problem with my car has been fixed. I had the Canister Purge Valve replaced, reset the code (P1456) and the malfunction light has not came back on in 2 weeks of driving. I’m also getting better gas mileage(about 26 mpg) than I originally indicated; I had initially miscalculated the mileage.

We do care. Thanks for letting us know how it turned out.

If problem returns, the canister itself may need to be cleaned or replaced. At 180K, wouldn’t be unusual for the canister to go on the fritz.