I worked as an auto mechanic for 5 years. It is OBDII and at one point I know that I did hook it up to a scanner but I don't think it showed much...the scanner I was using though could only acess it via "global OBDII". I remember looking it over pretty good and everything seemed to be in order other than the cyl 4 misfire code. I'm SURE I would have noticed if it wasn't going into closed loop.
I also know the mechanic who my dad uses on a regular basis and I have complete confidence in him. He has spent much more time with the truck than I have and I know that he has a Snap-On MODIS scanner so he undoubtedly got a better data readout than I did and after several weeks and several trips back to him (trying several different things) the problem still exists. It does seem to get pretty poor gas mileage.
Unfortunately, I don't know what all he tested and did to the truck. It's been over a year since he has messed with it and probably 8 months since I tinkered with it. Having left the Mechanic business for the time being I no longer have access to a scanner nor have the time to really spend with the truck. My dad has simply accepted the problem as an inherent flaw and has given up even trying to get it fixed. The truck starts and runs so he just drives it AS IS.
Given that it is a Cylinder 4 misfire code....there are some pretty basic checks that I am sure the mechanic went through. Spark, fuel, compression, etc....the basic things that a cylinder needs for combustion. The main reason I posted onto this forum was in hopes that there might possibly be a common (yet elusive) problem that throws that code on these engines....such as maybe inherently weak valve springs or something of that nature. I have often found that regardless of how strange the problem seems SOMEONE on these forums usually has had it before.