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Customized Wiring Tech » Blog Archives » Jeep 4.0L Fuel System
The 4.0 needs 45 psi. This article says the regulator for your year engine is in the tank. Older 4.0's had them in the fuel rail with a return line to the tank. The way I see it, you have two choices, get the pump/regulator/sending unit from the newer Jeep or swap your fuel rail for the older style. IMO the fuel rail swap would be easier, but I'm not sure if it is a direct bolt on.
It makes sense that it would drive fine going down the road, because it is using fuel thereby not allowing it to build up over 45psi. If you have a 75psi pump with no regulator on a 45 psi system, at idle, something is going to give. My guess is going to be a hose clamp or o-ring.