I have a 2000 GMC Sierra with the 5.3. The engine will start and idle with no problems. I can rev the engine in park just fine but when I put it in gear it falls on its face and backfires a little. Feels like no fuel when in gear for a block or two, then runs fine. Does it hot or cold, but runs just fine after the first block. Also doesn't show any check engine lights.

I checked my fuel pressure regulator but no gas in the vacuum line. I thought maybe fuel pump but don't think it would run fine after the first block. Letting the fuel pump run a little before starting doesn't seem to make a difference.

Any ideas?

Thanks,

Dave