Replaced exhaust manifold, start engine and the RPM's go out of control!

[jeeperguy21]

I was so happy to have finally gotten the new exhaust manifold put on my '99 XJ. After I finally got done hooking up everything the way it was before, I started it up and the engine will rev out of control.

It does the same thing after multiple starts. I have to kill the engine immediately because the engine wants to blow itself up. What is wrong?!

Let me know if you know what causes this or have had the same problem. I gotta drive this thing to work in the morning!

[ECKSJAY]

You've got a leak somewhere in the intake. Take a few mins off, go with a flashlight and a deep breath, and look at the seams. Happened to a buddy of mine after an engine swap and his intake was just a slight bit crooked at one end. Loosened up a few bolts, tapped it in place, tightened down, and it worked fine. Check out the throttle body and all the vac hoses on the intake manifold, too. Good luck!

[jeeperguy21]

I'm gonna shoot myself. Already been working on this thing for 8+ hours. It looked and felt like it was seated properly when i installed it. So with the intake being a little crooked, like your buddy's was, why would that cause the RPMs to be so high?

[ECKSJAY]

Because it's letting too much air in. When you step on the gas, it opens the butterfly in the throttle body, right? That lets more air in, but it's under a controlled manner. If you've got half of the intake away from the head, you've got a MASSIVE intake leak...which is letting a ridiculous amount of air in...which also makes the engine race.

[jeeperguy21]

After looking it over, I found something broken, but I don't know what purpose it serves. What is this and could it be causing the problem or not? Here are 2 pics of it:

[bigfrd8]

That is part of your crankcase vent system. it will not cause RPM problems. like the others said you either don't have the manifold seated properly or you left some vacuum lines loose.

[hubs97xj]

CCV. Big freakin vacuum leak. Could be your culprit.

[ERratIC]

The other end of the hose goes to the airbox or intake pipe...I think. Which would not cause your problem since the air leak is before the throttle body. Should still be fixed though.

[hubs97xj]

That tubing runs to the intake manifold. <edit> come to think of it, when I had the booster hose off the manifold, the idle jumped, but it didn't runaway completely. I'm thinking you've got a bigger leak, and a guess would be that the manifold isn't seated fully on the alignment pins. It can appear to be fully seated looking down on it, and still be open along the bottom.

[StaleAle]

Fix that because yes it will cause the RPMs to jump to about 2000. Its the EGR system and the front hose to the intake is the crank case breather. Make sure your brake assist hose is connected as well to the manifold.

With the EGR disconnected you should not hit the rev limiter so it could be a gasket (did you put it on backwards) and the EGR being unplugged causing a big vac leak.

[dgrigorenko]

You definatly have a leak. i have done mine twice and the first time had no problems, but on the second one my intake manifold was hitting the exhaust manifold ever so slightly causing it to not get a good seal, which caused the exact same issues. i took a ball pein hammer to the exhast manifold and ever so slightly bent the last tube (closest to firewall) and i am talking SLIGHT just a few taps made enough clearance. i was good to go from there. oh while you are in there you might want to think about replacing your motor mounts.... just a thought. i hope this helps


G

[igotanxj]

Originally Posted by jeeperguy21

After looking it over, I found something broken, but I don't know what purpose it serves. What is this and could it be causing the problem or not? Here are 2 pics of it:

show us where the other end of that hose goes first. im too lazy to go look at my jeep

[JeepFreak21]

Originally Posted by igotanxj

show us where the other end of that hose goes first. im too lazy to go look at my jeep

It goes to the air box.
Billy

[winterbeater]

That line sucks the blow-by that collects in the cylinder head into the intake manifold. You could slip a tight piece of hose right over that to fix it. You want to fix it for sure, but I think your intake is probably leaking somewhere else too. But fix it first to find out. Or plug your finger over the part not attached to the valve cover and have someone start it to find out.

[igotanxj]

Originally Posted by JeepFreak21

It goes to the air box.
Billy

I thought there were two, one that goes to the airbox and one to the manifold?