Once finished with the heater core replacement the jeep only starts if I hold the throttle then once it starts it will only stay idling with the throttle applied then after a little while it will run like normal on its own.. this only happens once a while. Any idea what it could be? Like I said it started to happen right after the heater core replacement. Also once it gets running it’s good for the day and it happens randomly about the next day after sitting for a while Might do it might not

 

I'm going to assume you have a 99 xj based on your tag but it's always useful to provide that information..

Anyway, if the current problem resulted from the core replacement work,, based on my own post replacement issue, I suggest you go over every connector under the hood and under the dash, cleaning and reseating each connector.

​​​​​What you did during the core work can vary; some folks take the dash completely out- I didn't, some take the steering column out, I didn't, but in any case a lot of connections have been stressed. Also check the HVAC vacuum lines

.... when I did my 2000 got everything back together, no left over parts, nice new copper core, everything worked except when you hit the driver door lock the rear wiper wiped one stroke. I reseated c100, which I never disconnected, and problem solved.

Also keep in mind that the problem could be a coincidence re the core replacement.

Sounds like an intermittent electrical issue with iac/TPS/ temperature sensor.