Starting Problem 1996 Yukon

[lmroberts]

I am experiencing warm start problems. By that I mean it starts good when it is cold,
but after I let it set for 5 or ten minutes it is hard to start and it seems I have to press the pedal down and it starts. It runs fine and doesn't miss, has good power for an engine with 250,000 Mi. There are no codes showing faults or pending codes. The heat sensor for the computer seems to be functioning. It at some times will back fire into the manifold not the exhaust. My mech pulled the plugs suspecting a leakey injector and did not find any excess gas in any cylinders, but found the last cylinders plug badly fouled. Have replaced plugs and the problem is still there although it has not backfired since.Could this be a faulty O2 sensor? I need to continue driving this car for at least 6 more months. As I am retired and on social security and will need to pay some bills off before investing in a new vehicle. I would appreciate any help. Thanks