Have you checked your PCV and Vacuum lines?

 

I really hope its not a vacuum leak. They can be a pain to track down.

What really gets me is that it only does this after it warms up, but I guess a lot of things could act differently when heated. The next thing I'm going to try is a new fuel filter. I've not checked the fuel pressure yet, but its probably due a new filter anyway.

If I figure this thing out, I'll post my solution. It looks like several other people are having similar problems.

 

I believe I found the problem.

Today I received my OBDII to USB cable, and hooked my laptop up to the Blazer. After clearing all the codes, I drove around town for a while. The Blazer threw another "Lean Fuel Trim" code, but no more O2 codes. I cleared it again, started it back up, and started the hunt for vacuum leaks. As I was looking I happened to notice that the hose running from the valve cover to the intake air piping was disconnected (I must have knocked it off when I took it apart to install the MAF sensor). After reconnecting it I drove it around town again, and it never seemed to hesitate again. When I got home I checked the codes, and it hasn't yet thrown a code for lean fuel.

I'm hoping this fixed the problem. If it does, then my protip to those with similar problems is to check your airflow piping from the air box to the intake carefully; this engine seems to be very picky about its airflow. If you have a leak in the hose coming off of the intake to the valve cover, that could be a problem as well.

 

That will definitely do it. The reason isn't because the truck is picky per-say, it is because you have introduced a source of outside air that is not being detected by the MAF sensor. When you think about it, it makes complete sense. The MAF sensor is one of the most important sensors in your engine when it comes to responsiveness.

 

It was just strange that it had no driveability issues with the MAF sensor removed when the previous owner replaced the air box with one of those large, conical filters. Of course, the person who did the mod also cut the hose from the air piping to the valve cover and placed tiny air filters on the exposed ends, so it wasn't exactly sucking straight air with that setup at the time.

Either way, I'm happy to say I have yet to have another problem with it. Those mods may have been trying to circumvent another problem, and in the process of restoring it to the factory setup I have may solved the problem.

 

mmhhmmm, i disconnected my MAF, it runs better, my problem is when coming to a stop, the idle dips low, then can bog before acceleration, none of that happens with the MAF disconnected, I allready cleaned the MAF correctly, no change.
Vacuum is moderate about 18 at idle, also with the MAF connected the idle seems to fluctuate after warm up.
so tomorrow i will clean the egr valve and see what happens. any other suggestions would be great!!