OMG what next

I have a vacuum leak issue since putting a new engine in that i've been looking for and as a result, my four wheel drive won't kick in. The light comes on but it won't engage. Anyway, this morning i got stuck in about a foot of snow in a circle, while i was trying to get out, the 4 wheel kicked in momentarily and i got out. After I got out, i went and ate when i came back out and tried to start my 91 cherokee 4.0l auto, it wouldn't run, it turned over fine just wouldn't stay running, like it was trying. I waited a bit, then tried it and it started limped it home and it died twice while I was getting there. When i got home i thought it may be some condensation or something so i tried to let it idle for a while but it wouldn't stay running for very long. It starts and idles steady then every 30 seconds or so the tachometer drops all the way down like the engines quit then the engine kicks back in and the idle revs it back up and then it repeats, not steadily but it repeats. I have a freind that said it may be the alternator, but i have no clue about the vacuum issue or this. Help anyone, please!

i'm new to jeeps but i will share knowledge from other cars i've had.

usually when the tach drops to zero but the engine is still turning, that indicates that whatever gives the car the tach signal is farking up. usually the hall sensor/distributer.

Any check engine lights? i want to guess its the crank position sensor. apparently a known thing to fark up in these cars, and as far as i know... would do what you are talking about.

Well banditt is partially right, the cps could have been causing the funky starting issue you were having, as when they start to go out sometimes they'll start and sometimes they won't. As for it idling and then the rpms dropping off and acting like that it definately sounds like a vacuum leak. I'd start by checking all the connections on the intake manifold to see if any of the fittings are cracked or very loose. Also check the vacuum canister behind the front bumper on passenger side, and check the vacuum leading to the MAP sensor on the driver side. Others will chime in with other possibilities to check.

i believe it is the cps as i jiggled the wires on it and it runs fine now, i dont know if i need to replace it or not as it was due to being stuck in very deep snow. Any ideas? or could it just have been wet?

So, i haven't replaced anything yet as i'm looking for the most likely culprit. Here's what i gather could be the problem part but i'm not positive on some of the things:

Ballast resistor - from what i gather this just relays the power to the fuel pump, mine's in tact and looks good, but i cleaned the contacts anyway.

Fuel Pump Relay - this is kind of like a fuse for the power sent to the ballast resistor, i think if this was not functioning the fuel pump would not be working at all

Map Sensor - Again this seems to be a part that the jeep would just not start

IAC - this part from what i understand causes uncontrollable idle, i dont have that issue, it runs fine for 30-40 minutes at a time then craps out

TPS - this part from what i understand causes uncontrollable idle, i dont have that issue, it runs fine for 30-40 minutes at a time then craps out

CPS - this could be the issue, but from what i've read it causes you not to shift, mine shifts fine

Any help on this would be great, once again, i start the jeep, it starts fine, runs good, nice and steady. Then after a while for no apparent reason, it starts getting real choppy and sputtering, and will finally die or keep sputtering until it stabilizes again (usually dies). this just started yesterday, as i got stuck and put alot of revving and stress on the engine (the most since i put it in 2 months ago) i checked all the vacuum connections and can't find a leak unless theres one i'm missing, is there one that attached to the front axle?

I usually don't like just throwing parts at things, but my wife's 98 ZJ would die for no apparent reason. There was a code stored, no check engine light though. I replaced both the CPS and the hall effect sensor (AKA distributor ignition pickup). Have had no stalling problems since. Sounds like you may have 2 problems though, have to solve one first, then see what happens with the other one....

OK, i replaced the CPS to no avail and bypassed the ballast resistor, so its neither of those. I'm planning on trying the fuel pump relay, IAC and TPS tomorrow, does anyone have any other ideas to help?

Own an early 90's GM vehicle? Pull the MAP and swap it for yours just to test. The jeep will in fact start without one, but once your system gets idle to a steady point the MAP may not be feeding correct information so it leans everything out and dies. Cheap part, might as well replace if it's looking old.

i replaced the tps and it worked! thank god. Thanks for help