My 1998 4.0L has had a problem with a hard start/rough idle when hot for about three years now. I have thrown parts at it, have taken it to two shops, and not been able to pin down the problem. Hoping this forum’s knowledge can help:

Symptoms

- When hot, it will start hard and idle rough for about five minutes before the idle smooths out. The idle is nice and smooth after that.
- Once in the three years of this problem, it has thrown a P0303 code after really bad start on a 100 degree day.
- The problem is much worse at high altitudes. At 10k’ it will missfire multiple cylinders and die (no codes though). The second time you crank it starts right up, no problems, but the idle is very rough for a few minutes
- It smells like gas after the hard start (I am thinking this ate up my cat because I recently had to have it replaced)
- Occasionally, something will make a screeching sound when you start it up (may be unrelated?)

Work so far

- I have taken it to two shops. One recommended replacing the fuel pump due to a failed check valve and one thought that a bad exhaust leak was cooking the injectors. I now have a new fuel pump and exhaust manifold and the problem hasn’t gone away.
- Over the past couple of years, I have replaced the fuel injectors, fuel pump, IAC, TPS, MAP sensor, cleaned the throttle body, replaced the CPS, replaced the PCV valves and tubes (all with mopar sensors and a bosch fuel pump). I also have a fairly new distributor, ignition coil, plugs, wires, and distributor cap.
- Finally bought a fuel pressure tester and checked for a fuel pressure leak using the procedure in the FSM. It was on the nose at 49 psi when running, 45 psi five minutes after shutoff, and 36 psi after thirty minutes. This checks good and would seem to rule out a leaky injector or the new fuel pump’s check valve being bad.

Next steps:

Given the amount of $ and time I have thrown at this without results - I would like to attack in a more methodical way by testing and replacing only known bad parts. Generally sounds like it could be a heat soak problem - maybe the altitude part is really just because the underhood temperature stays higher for longer up there. I want to apply the heat soak band-aids as a last resort (injector wrap and e-fan timer) because it didn’t do this for the first 15 years and 180,000 miles, and the 98s are supposedly a lot less susceptible to heat soak than the 00-01.

The question for the forum: given we know the fuel side is good, what would you test next?