No Idle, 1998 Grand Cherokee
 

today I tried to crank my grand cherokee because it has been sitting for quite some time (6+ months) and it would not start (surprise!). I then attempted to jump it, and while it was hooked up, the car started but absolutely would not idle. It sounded normal other than it would shut off as soon as I took my foot off of the gas. I let it sit and charge for a while, then tried to crank it by itself. Nothing. No lights, instruments, or noises. I'm assuming the battery needs replacing as it seems it cannot hold a charge (Correct?), but why would it not idle at all?

I'm betting it's your battery cable clamps. When you jump-started it, you clamped on the cable ends and when it started it was running on the alternator. as soon as it slowed down, the power wasn't there enough to keep it running. JMHO.

When the battery goes completely dead and the vehicle sits for a long time the PCM has to go through a relearn process. Replace the battery and start the engine, you will have to keep your foot on the gas for a few minutes until the PCM wakes up and pulls its head out of its arse.

Battery
 

Ok thanks. Update: I installed the new battery and it sounds great! may need to replace the pan head gasket though.