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

jeeperguy21

NAXJA Forum User
Location
Syracuse, UT
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!
 
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!
 
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?
 
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.
 
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:

DSC00946.jpg


DSC00948.jpg
 
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.
 
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.
 
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.
 
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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.
 
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:wierd:
 
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.
 
Hey igotanxj, you could be right. I'm not positive, but the old PCV valves always fed the intake manifold directly. If it just goes to the air cleaner box, it doesn't matter a whole lot, except for putting oil fumes under the hood.
 
OK, checked the 2000 FSM and the rear one (broken in this case) goes to the intake manifold and the front one goes to (or in this case comes from) the air box. So clearly, the rear one sucks the blow-by into the intake manifold and the front one is to supply filtered air to the valve cover to be sucked out by the back one.
 
OK, checked the 2000 FSM and the rear one (broken in this case) goes to the intake manifold and the front one goes to (or in this case comes from) the air box. So clearly, the rear one sucks the blow-by into the intake manifold and the front one is to supply filtered air to the valve cover to be sucked out by the back one.
You are correct. My manual shows the same thing. Looks like that's what is causing the high idle problem.
 
Okay, so to give everybody an update, I tore everything apart again last night and it seems like the intake manifold was hitting the new exhaust manifold in some places, making it so that it didn't sit just right. I got out the grinder and made some adjustments to it, and reseated it and everything seemed like it was nice and tight.

Can I just mention that the bottom bolts on the manifold SUCK? I felt like I was in a game of Twister and I couldn't win.

To answer some of the questions, the problem was NOT caused by the broken crank case breather tube, I DID replace the exhaust manifold gasket, and YES- that job was in the top 5 list of worst jobs to have to do on an XJ.

To make a long story short, the intake manifold must not have been seated properly when I assembled it the first time, which would have caused HUGE amounts of air to get into the engine, making it act like someone had the pedal to the floor. After I got it all put back together the 2nd time, I was nervous to start it up again for fear that I was going to have the same problem of the engine wanting to blow itself up from such high RPMs. But I am pleased to report that it started up normally and idled GREAT, even without the crank case vent thingy broken.

The only thing that I noticed with the crank case thingy broken was the wind noise coming from the tube. After rigging it together with a spare rubber fuel line, all is well.

I didn't realize how sad of shape my old exhaust manifold was in until I started it up again and I DIDN'T go deaf from the engine. The noise comes from behind me from the tailpipe once again. It feels and sounds like a new Jeep again. I love it.

Haven't driven it long enough to notice any difference in the APN vs the stock exhaust. All I can say is that I am so glad that job is behind me. I'm just gonna tighten down all the bolts after driving it for a day or so and hopefully never have to deal with that again.

Total work time 1st day: 12 hours
Total work time 2nd day: 2 hours

I guess one good thing is I took the time to replace my old serpentine belt and clean the throttle body while I had everything torn apart.
 
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