Gasoline odor in Nissan Quest

I have a 1994 Nissan Quest with 192K miles on it. Recently I've been getting a strong gasoline odor that lasts for several minutes after I start it up. I noticed the odor after I took it into the dealer for a pinging problem (I suspected timing for the pinging but the dealer says that the computer would take care of timing so they didn't do anything for that). Still get the pinging sound even after I use higher octane gas.

I don't think the odor is a gas cap problem, as I have been making certain the gas cap is tightly in place. If this is a fuel injector problem what is this likely to cost me? I rarely drive this car now until I can get it fixed.

Thanks

GLeidner