Red88

Just bought an 88 cherokee with a 4.0. Worked on it all weekend replaced some leaky gaskets, front U-joints etc. Tonight I replaced the brake booster which was leaking badly. Previous to this the jeep ran fine although you could hear a very loud hissing in the cab coming from behind the brake pedal. Anyway I replaced it and went to go for the test drive and now it wont hold an idle. It will run with your foot on the gas and hold a fairly low rpm (no tach for an actual number). But as soon as you let off the gas completely it dies. I am guessing I knocked a wire loose when I did the brake booster but cant seem to find anything no connected. I removed the air cleaner box, and windshield washer reservoir as well as obviously the brake booster. So I think that whatever it would be would be on the drives side of the engine bay but I just cant seem to find anything. There is a small (coke can size) cylinder shaped thing with a bunch of wires coming out of/ going into it on the edge of the engine bay near the air cleaner. It appears to have a cable coming out of it going to part of the throttle linkage. What is this it seems like it would have to do with cold idle speeds? How do I test if its working? What else should I look for?

LilyBayXJ

This may seem like a stupid answer, I'm not sure if it's what you're looking for, but I had a similar problem with my girlfriend's '98 ZJ with the 318 (5.2). Would run if my foot was on the gas, but stall out at idle. It had been sitting for a while. I drained the gas, put 6 gallons of fresh 91 octane in, added a little fuel stabilizer, and it ran like a champ. If your Cherokee had been sitting for a while, it would start, but when it was driven it most likely stirred up some sediment in the gas tank, causing the Jeep to run like crap.

Red88

The jeep did sit for a while but as it started and held a fine idle previous to the brake booster job I think there is more to it than this. I highly suspect a pulled loose wire or something similar. Does anyone know what the piece of electronics next to the air cleaner ( fender side) that has a cable going to the throttle linkage does?