the 4.0L engine (that is comon to the 91 build) is rated at 350,000 miles under "normal" operating conditions. Mine has over 214,000 on it and prolly has more than that due to larger tires and the cable was unhooked for several months a couple years back. I've replaced the water pump and valve cover gasket. Other than normal maintenance like plugs and such, these are original miles, and there is plenty of life left on her. Don't assume that high miles is any indicator of a "loosing engine battle".... I've ragged mine out over the years and still do when I need to get in front of the local yuppies in their $75,000 "SUV"'s on the highway and such. She's got plenty of spur and although I don't expect to get 350,000 miles out of her, I do expect to tick past 300,000. If you haven't properly maintained your '91 YJ, it could be possible that you are in fact due for a replacement/overhaul/rebuild.... Just remember, you can't get your 15 year old MOPAR Jeep to run like a Corvette, even if you took the body and frame off a 'vet, and put your Jeep's frame and body on it, it still wouldn't run like a 'vet!