SouthernWASP

99 xj se 4.0 5 spd. Long history with this. No prior problems for 200K. Short story is when hot randomly looses power stumbles backfires dies. Won't start until cold. Repeat. Went through this couple years @ 200K. Went thru sensor checks. All read good. Due to problem & mileage replaced cks, cps, iac, battery temp sensor, o2 sensors, fuel pump, relays, battery, alternator, plugs, wires, dist cap, rotor button, air in spare tire and bath water. Was definitely a thermal breakdown problem. Something worked but not sure what. Same problem back again @250K. Went thru all same checks of sensors, etc. All checks ok. Thermal breakdown again I believe. Don't want to go thru all this again. Let idle for hour without problem. Go drive eventually dies.Convincing myself its the cks. Any thoughts, suggestions? Daily driver. Getting desperate. Don't wanna sell but...

Turbo X_J

Wiring.

tjwalker

You could try testing the crank sensor immediately upon the engine dying. That might yield a clue.

Also keep in mind that your "ignition coil" can fail thermally. It is much more rare, but it absolutely can happen. You can test the coil for both primary and secondary resistances with a meter and a manual. Again, test only when the engine won't start. I didn't see any mention of the coil in your initial post.

SouthernWASP

​​​​​​​Previously changed coil. Checked previous cks when hot. Was ok.

Idunno

I had this with a car. I'd be driving along at a steady speed and then the engine would just quit. Turned out the gas cap was bad - not venting - and the vacuum inside the tank eventually reached a value that the pump could not over-power. Five minutes on the side of the road and I was good to go again. I mentioned it to an older fellow at a pit stop. He took off my gas cap and threw it into a corn field. Then he charged me a couple bucks for a new one and the problem was solved. That was on a carb'd 4-banger.

I also read that a failed fuel injector (won't close) that hangs up depletes the fuel pressure on the rail into one cylinder, causing the backfire you mention, and then the engine shuts down for lack of fuel pressure to any of the other injectors. It might be one injector that is taking too much heat, causes the internal hang-up, and then re-seats when it cools.

boxburn

I've also seen vehicles with this issue. It'd be a really quick and easy one to test next time it dies. Just pull the cap off and if you hear the air rush in, there's a venting issue.