hesitation problems driving me nuts!!!

Hey, i own a 99 GMC Sierra SLE 4x4 ext cab with a 5.3L engine i've had into the shop 3 times and still can't get the hesitation thing fixed,the injectors have been cleaned,as well as the throttle body,the tranny's been flushed,new plugs,new fuel filter,air filter, as soon as you start to drive and give some acceleration it bogs,you can hold it to the floor and it just sits there and slowly picks up speed.Any suggestions???Some of them GM mechanics are puzzled because no codes are showing up and it also had the update done as well.I've searched some forums and some say maybe the mass airflow sensor? wouldn't that show up in the codes??Please help very frustrated!!

I had a car do that exact same thing and I had a blockage in my muffler. Start your truck and see if there is much air from the tailpipe and then have someone rev it a little.

As sonny said it could be a back pressure problem. Most shops have the equipment to check for back pressure. You didn't say if you changed the wires when you did the tune up. As check the fuel pressure.

I was probably the post you saw. Thats what mine was doing. I took it to the local dealership and they told me the fuel pump so I replaced that and it didnt fix it and then they had no Idea. So I thought I would take it to another dealership for another opinion about 30 miles away and they had it all day but that is what they found was the mass air flow sensor was dirty and I think one of the Oxygen sensors on the muffler connection was loose. But that was probably me, that was my last thought before I gave up and took it to them.

In fact I still have the fuel pump sitting in the garage for a future problem if it shows up on the 06.

Thanks for all input so far, i'm going to get the mass flow checked and will keep you all updated