Hmmm. Sounds like they may have reset the computer before you bought it
A code reader is your next best thing now. I had one for my 86 E-150, and it was easy enough. It was a cheap one, only had one led on it, so it was a little difficult. I would not be surprised to find out it's a bad 02 sensor, from the slight rough idle. But a code reader is the only sure bet. Good luck,,,,98

 

Your cap STILL may not be making a good seal. A slight, occasional rough idle is standard. How is your fuel economy? If it is holding around 15 HWY, it can't be too much wrong. You could try some good fuel system cleaner that cleans all the way through the tail pipe. If there were some sort of deposit on your 2, O2 sensors, it might clean them off.

 

what is a good fuel cleaner? I know you aren't talking about cheap crap like STP injector cleaner

 

I can't remember the brand name but I got it at NAPA and it cost about $10 a bottle. It even came with a free Hot Wheels car. It did wonders with my idle. Was the best money I ever spent. Just go there and look at the good stuff, and remember, you get what you pay for.

 

My check engine light had come on at 75,000 and I scanned it, came up as an EGR valve problem. I cleaned it and re-installed, cleared the code but it came back on a day later. Before going to the dealer on Mon. for a new EGR Valve I decided to replace the fuel filter, it needed it anyway but the CEL light went off after a couple of cycles and has not re-appeared in the past 29,000.
Couldn't hurt.
Good luck!

 

The only fuel additive I have ever used, that made any difference at all was Redline fuel system cleaner. This stuff stopped a rough idle in one half tank! I have only seen it in the one place I bought it from, but I believe alot of speedshops and Pep Boys would have it, too.