HOUSTON, July 16 (Reuters) - Los Angeles gasoline differentials showed gains on Monday as a regional refinery undergoing maintenance reported planned flaring, traders said. L.A. Jail-delivery CARBOB gasoline was seen done at 8.00 and 8.50 cents over August RBOB futures on the New York Mercantile Exchange, up 1.5 cents per gallon from Friday's levels, traders said. The increase came after Exxon Mobil Corp on Sunday reported that planned flaring would occur through Monday at its 149,500 barrels-per-day (bpd) refinery in Torrance, California. San Francisco Bay market July CARBOB was 3 cents under the L.A. price, traders said. Gasoline in the Portland, Oregon market climbed 2.5 cents per gallon to 15 cents over August NYMEX RBOB futures. July CARB diesel in L.A. held steady at 5.5 cents over August NYMEX heating oil futures, while CARB diesel in the Bay also was flat at 2.75 cents over. Portland diesel was seen done at 22 cents over August NYMEX heating oil, traders said. L.A. jet fuel differentials mirrored Friday's levels of 9 cents over August NYMEX heating oil, as did those in San Francisco.