This is my first missive to this terrific forum. I'm an old dog, as in I remember President Eisenhower, and last month I purchased my first Jeep, a 2002 TJ 4.0 in great shape with 70k. I began having problems with "will crank but won't start when hot but will start fine after cooling down for a half hour". i joined the forum and spent hours gleaning the numerous fine threads here, with so many contributors generous of their time. I didn't do proper diagnostics, and started throwing logical parts at the issue. Bosch fuel pump, Mopar fuel pressure regulator, Mopar crank position sensor. I also installed a heat shroud kit for the fuel rail and injectors. No fixee. So I revisited the threads, and followed some simple advice. Is it fuel or fire? I checked the plugs after a no-start episode, and they were dry. No fuel. I checked the fuel rail pressure while running and it was 49 psi. Perfect. After getting the engine hot, I shut it off and watched the pressure slowly rise to 85 PSI , on a cool day with the hood open. Wait, how can this be with a new Mopar fuel pressure regulator which will release excess fuel pressure back into the tank at just a few PSI above specs? Well, maybe the new regulator is bad. It happens. I had to think about it, I didn't want to drop the tank again. And then the light bulb turned on and i became un-stupid. The previous owner had trouble starting the jeep because the check valve in the tank had failed. Being clever and not wanting to drop the tank, he installed a new in-line check valve just in front of the gas tank. It solved the problem of the gas line draining back into the tank when the engine is stopped, but the new check valve stopped the line from venting excess pressure back into the tank. IE, the in-line check valve isolated the fuel pressure regulator from doing its job in this returnless system. So, the lesson is, do basic diagnostics before buying parts. Thanks to the forum.
My solution for Won't run when hot
- Thread starter Wheel
- Start date
Very lucky the PO told you about the check valve. Without that little snippet of knowledge you could have spent a lot more time and money. One of the advantages of getting old is it hurts too much to get angry anymore.
Making you (me) stop and slowly rewind everything that is going sideways.
Glad you got it. 
These are super machines. Welcome to the forum. 
Making you (me) stop and slowly rewind everything that is going sideways. Glad you got it. These are super machines. Welcome to the forum. Similar threads
