HOUSTON, July 17 (Reuters) - California gasoline differentials took a sharp dive on Tuesday as a Los Angeles-area refinery restarted a key unit, traders said. Los Angeles July-delivery CARBOB gasoline fell by 13 cents per gallon to finish at a 5-cent discount to August RBOB futures on the New York Mercantile Exchange on news that Tesoro Corp had restarted a gasoline-making fluid catalytic cracking unit at its 103,800 barrel-per-day (bpd) refinery in Wilmington, California. Tesoro had said work was ongoing at the plant, but did not specify which units. Traders said the FCCU was shut last week. San Francisco Bay market CARBOB was 4 cents under the L.A. price. Planned work also was ongoing at Exxon Mobil Corp's 149,500 bpd refinery in Torrance, California, and Phillips 66's 139,000 bpd Wilmington, California, refinery. Traders said they expected those plants to restart units in the coming days as well. L.A. CARB diesel slipped 1.25 cents to 4.00 cents over August NYMEX heating oil futures, while Bay CARB diesel climbed a quarter cent per gallon to a bid-offer spread of 2.25/3.25 cents over, traders said. L.A. jet fuel joined the slide as well, falling by a penny per gallon to 7.50 cents over. In the Portland, Oregon, market, gasoline fell 3 cents per gallon to 15 cents over NYMEX RBOB, more than paring Monday's 2.5-cent gain. Portland diesel also retreated, falling 2 cents to 20 cents over NYMEX heating oil, paring some of Monday's 6-cent gain. Both markets gained on Monday on talk of BP Plc shutting its Olympic Pipeline for maintenance, but sources familiar with the line's operations said the work would wrap up this week. The system connects four refineries in the Pacific Northwest. A BP spokesman declined comment.