Well, just about reworked this 96 4.0 GEMS engine except for rebuilding. In the past year have put new plugs, wires, tps, Iac, all sensors except MAF, cleaned throttle body. It idles fine now on cold starts but about half the time after 45 min or longer runs it will hessitate to idle up correctly. What could cause it to do this only when hot? Will having the values reset in the electronic system at a cost of $100 at dealer help this problem or could it be in the plugs and wires, or coil or MAF sensor I have not replaced? I have not replaced the fuel pump since I was told the truck would not run at all if the pump was bad. thanks for your comments.

 

Fuel pumps act up when they start to go bad, sometimes they do just give up without warning. When my fuel pump was starting to go, the 2 symptoms it was doing was weird idle issues when hot, and it would hit flat spots when accelerating,almost like I would let up on the gas a little.

You can check the fuel pressure pretty easily. There is a valve on the passenger side of the fuel rail, should have a screw off cap. Use a tire pressure gauge and check the pressure while idleing. Should be like 34-36psi.

 

Like Kenk said it very well could be the fuel pump, it could also be the fuel temp sensor.
First check the fuel psi next time it does it.
Use a cheap dial tire psi gauge.

 

easy check- wiggle the wires going to the MAF when it is hot and acting up. My connector was bad on mine. Same symptoms. It is right above the dang manifold and eats up a lot of heat. Just something else to check.