I have a 98 Ranger, 4.0L automatic. I have had the truck serviced several times but seem to keep having the same recurring problem. When I first start the truck up in the morning (it's parked outside) the idle fluctuates and the engine stumbles for about the first couple of minutes. If you put it in gear it acts like it might die although this has never happened. After the couple of minutes pass the engine just seems to snap out of it, the idle smooths out and it runs fine. I don't know if I can remember everything that has been replaced but I know for a fact we're talking the EGR valve, the MAF sensor, the idle contol valve, the coil pack and I think the plugs. The plug wires were checked and appeared to be good. I use a very reputable mechanic and he has always done good by me. My check engine light came on months ago and I have had my mechanic check the codes several times. The odd part is 2 out of 3 times there was no code registered. The only time there was one indicated both left and right banks lean. My mechanic said this was a strange code that coulkd indicate both my O2 sensors were bad. He said this seldom ever happens and he did not understand it. He cleared the CEL and told me to drive the truck and see if the CEL came back on again. He said if the CEL came back on within a few days he felt sure there might be a potential O2 sensors problem. I drove the truck for almost a month before the CEL came back on again. My gas milage has not changed from what I was getting prior to the CEL coming on and outside of the morning idle problem the truck runs fine. Anyone got any ideas??

 

You may have the start of a leaky intake manifold gasket, getting unmetered air and so running lean

 

I have also had those codes come up unexplained. After checking everythiing you mentioned I checked FUEL pressure. I was lucky only the filter needed changed -- no codes or issues since.

Your start up problems could be related because with a clogged filter it would take awhile to build good fuel pressure.