I have a 1997 wrangler that appears to be bipolar.. When it sits all night, it will start like a champ on the first try. However, throughout the day, it will take 4 or 5 tries before it starts, and sometimes I have to have my foot on the gas a little bit. It seems the longer it sits, the easier it is to start.. I recently replaced a bad o2 sensor (and had to replace the spark plugs as a result of the bad o2 sensor, it was a year before the bad sensor was diagnosed and fixed).
My jeep had a similar issue earlier this year, except it was cold starts that it had a problem with.. My fuel filter went bad and wasn't keeping pressure in the fuel system, so I replaced the filter and it started like a champ afterwards. (But didn't run well because of the o2 sensor) Well my jeep started having cold start issues again.. If it sits for more than 5 seconds now, it will be nearly impossible to start.
Little confused (normal) first you say its a hot start problem, then later it's a cold start?
Umm, basics first. Is it a 4.0? Do you have the basics when it won't start? Spark and fuel?
If it's a hot restart and a 4.0, make sure after checking you have adequate fuel pressure, that all the insulators are in place around the injectors. The 4.0 had a problem with heat from the intake causing the fuel to boil in the injectors. Made it a bugger to restart hot. Had to crank and crank till enough cool fuel got to the injectors to let them work properly.
Also on hot restart problems, is the fuel pump running? You could have a problem with the fuel pump relay sticking when hot, and/or even the pump hanging up when it's hot.
Swap the fuel pump relay with one of the other ones and see if it makes a difference when it acts up.
If it's a cold start, basics again, before you even try and start it, see if you have fuel pressure. No pressure, sounds like your fuel pressure regulator is bad.
And the fuel pressure regulator is NOT the fuel filter. Regulator sits on top of the tank, filter is in the tank on the bottom of the fuel pump.
Takes care of checking for fuel.
As for spark, does it have it when you are trying to start it?
Could be something as simple as the CPS starting to go on you, or maybe even the ADS relay hanging up.
97 Sahara, 74 Renegade, 72 CJ5, 55 Willy's (the Beast) Boss 302, 6:34 gearing, Bose 901 speakers.
God I miss that Jeep. Forgot the kinda Jeep, 80 SX4.
Ha sorry bout the confusion. It is indeed the 4.0 engine. I replaced the fuel pressure regulator to fix a cold start problem I had, and now it has the issues with hot starts.
I swapped the relays and it didn't help, and my fuel pump is running.
However I'll check the injector issue like you mentioned, because after I turn the engine off, I can put my ear by the injectors and I hear a supressed hissing and a small boiling noise. After I start hearing this noise, it takes forever to start. If I let it cool (or turn my key a zillion times to the on position to run the fuel pump) it'll start quicker. Thanks pal!! (: