1999 Jeep Grand Cherokee Vacuum / Idle / Stalling Problem

Hello all,

I have a 1999 Grand Cherokee Laredo 4.0L which was running perfectly until tonight. I went to start it but it wouldn't turn over. I thought maybe I hit the gas pedal when I tried turning it over and flooded it or something. I then tried starting it holding the gas pedal to the floor and then quickly letting it off if it turned over, that worked, it started right up but would just stall when it hit low rpm idle speed. If I held my foot lightly on the pedal it ran smooth at 1000 rpm but the second I let it off it would stall.

I then pulled out my trusty Haynes manual and looked under the troubleshooting section. There I saw the word "vacuum" which raised a red flag in my mind. When I bought the truck about 8 months ago the only HVAC option that worked was defrost and my cruise control didn't work. After searching the internet I found some postings about vacuum lines controling the HVAC venting. After a few minutes looking under the hood I found the culprit, the vacuum line that runs to the intake manifold was just hanging there. I hooked it back up and sure enough, I had cruise and other options besides defrost. At the time I thought it was a freak thing like back pressure that blew it off or something.

This all leads me back to tonight. After a lot of head scratching I thought "why not disconnect the vacuum line that was hanging there unhooked 8 months ago, after all, it would only take a few seconds and if it didn't work than that was just one more thing I could scratch off my list of possible solutions" SURE ENOUGH! just by disconnecting the vacuum line that hooks to the intake manifold the truck started right up and idled fine!!!

I also notice the steel cable running from the throtle body to somewhere under the battery had a lot of slack in it (that's a mystery zone for me since I haven't looked under there yet and have no idea what it runs to). Does that have some thing to do with the idle speed? Maybe a vacuum actuated choke?

I would just leave the vacuum line unhooked but there's a reason it's hooked up to the manifold at the factory (I think?).

Could I have a bad resevoir or valve that's not opening?

Could a vacuum problem lead to poor gas mileage? I average about 17-18 MPG.

Sorry for the long post, I just wanted to get all the facts and some of my theories out there. Also, I will post the results when I get to the bottom of the problem.

Thanks, Mike

It's hard to flood a fuel injected vehicle, very hard. Flooding is a thing for carbs. In fuel injected vehicles, pressing the gas pedal all the way down signals the computer to start in safe-mode so to speak. It will ignore a lot of sensor input and try to start.

Cruise control motor. Fuel injected vehicles don't have chokes, they don't need them. Chokes are for helping the engine suck fuel from a mostly empty carb. :)

Your idle speed is controlled by the IAC (idle air controller) like on this page.

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You need a factory service manual and vacuum diagram. A vac pump/gauge is also handy for finding leaks. I'm guessing you have a cracked line or one of the soft rubber connectors has gone bye-bye.

A bad reservoir (under the battery) is a very common problem. You can check the hoses with one of these.

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It sure can. So can a O2 sensor that's starting to go. They don't always throw a code. Then again 17-18 isn't unreasonable.