Jeep Cherokee revving uncontrolably high

A couple of weeks ago my 2000 inline 6 cylinder Jeep Cherokee got very loud. Meinekee told me it was a cracked rear exhaust manifold. I purchased a manifold and had a roving mechanic put it on. However, when he started the vehicle after the repair, we found that it immediately revs to redline and stays there. It’s obviously undrivable, and we can’t even leave it running to diagnose. The throttle body appears to be closed. Has anyone out there ever seen this before? Thanks!

A racing engine is sometimes caused by a vacuum leak. Check all hoses that attach to the intake tube, throttle body, and intake manifold. If you can’t find one which is deteriorated, or loose, and leaking, disconnect and plug everyone of them (towards the intake). If the idle is slower, unplug and reconnect each hose, one at a time. When the engine again races, you’ve found the cause.

On my Jeep the crankcase vent tube went into the throttle body with just a rubber plug. Nothing really held it in. This would fall out sometimes. Don’t remember it racing that high. This isn’t one of those combo gaskets that can be put in upside down is it? Anyway, you saved a little money and got a goofball for a mechanic.

Thank you for your responses. I talked to a couple garages as well, and most agree that it is probably a vacuum leak. “Opera House” you’re right about the goofball mechanic! I was hoping to start a good relationship with a local guy - so much for that!