I recently started smelling gas in my cab while driving. I have a 3.9 87 and i was getting 17mpg but then i started smelling gas and now im down to 13. How can I check for error codes, and where can I find what the codes mean? I know the codes wont tell me why. Im just curious to know how to do that. I think i may have a leaky needle

 

I would start looking for a gasoline leak, then worry about the codes later. Check all connections around the carb first. That is the most likely location if you are smelling gas in the cab. Is that a throttle body injected carb? Check the injectors. A gasoline leak could result in an engine fire as well as bad milage. After an engine fire, your milage doesn't matter.
I get the error codes off my 1988 by turning the ignition key on/off/on/off/on and counting the flashing engine light. The number of flashes gives you the code. Codes are easy to find online. I got mine here some place.

 

no theres no throttle body. Its a straight carb setup. I checked all the fuel lines and filter and it still smells. I can smell alot of unburned gas when im un hooking my trailer.

 

has the truck been off road lately? smelling gas in the back sounds like a hole in the tank or possibly the lines right off the pump... oh and the codes are in the 1st gen FAQ on this site, theres a link to a list of the PCM trouble codes close to the top of the post.

 

nope no off roading... running on some old stock wrangler tires so i dont wanna chance the off road. I need to know how large of a tire i can use tho.

 

Check the fuel lines that go between the inside of the driver's side frame rail and the carb. The lines will sometimes rub against the lip on the bottom of the firewall and cause a leak.

My1991was leaking there when I bought it and I ended up using JBWeld to fix it.