You will likely find it useful to purchase your own OBD-II scanner - particularly since a decent one will work with pretty much any OBD-II vehicle (unlike OBD-I, where manufacturers were allowed to set their own standards for the whole wretched system.)
The "fuel filler cap" trouble code (yep - it's helpful to know which one you're actually getting - this is why I suggest dropping $60-100 on your own reader...) is generally a catch-all for an evapourative emissions fault somewhere in the fuel tank area. It could also be the fill hose from the fill port to the fuel tank (it's a large-diameter segment of elastomer hose,) it could even be the access plate on the front of the tank is just starting to leak - not enough to let fuel out, but enough to let fuel vapour out. This would be considered an "evapourative emissions fault". And, since most XJ's seem to have the access plate on the upper front of the fuel tank, a vapour leak there would start somewhere between 1/2-2/3 full...
But, when you're trying to get help diagnosing an OBD trouble code, here's what it's helpful for all of us to know:
- The specific code being thrown (OBD-I will be a two-digit numerical code. OBD-II codes take the form of a letter, zero or one, and three numerical digits. The letter indicates the general system, "zero or one" - zero is an SAE standard code, while one is a vendor specific code - and the three digits will give the specific system and fault.)
- The model year, engine, transmission, and powertrain options (Any pertinent mods like oversize tyres and gearing changes can also be useful - but you covered that rather well. Mileage on the clock is also a useful datum in many cases - and you did provide that.)
- If you have noted any other symptoms in operation.
- If you have done any visual inspections of any systems - what did you find?
Since we can't see your rig, we have to depend on you to be our eyes (and ears!) We're perfectly willing to help you learn and repair, but it does take some information for us to do so...
