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Old 11-10-2015, 05:52 AM
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Manifold install issues

I just finished an exhaust manifold replacement...took my time and followed a detailed set of instructions that I found on WF. Now when I start the Jeep the engine revs so high that I have to immediately turn it off...also a fairly strong exhaust smell considering the motor is running for 1-2 seconds.

I've triple checked all connections, torqued the bolts, changed the o rings on the injectors...I can't locate where so much air is getting into motor.

Would love to hear your thoughts. 1999 Sport 4.0. Thanks!

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Old 11-10-2015, 06:17 AM   #2
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double check the seal on the intake to cylinder head, using carb spray on the mating surface of the intake runners going to the cylinder head. if rpm drops on a particular runner, you have your vacuum leak. most likely will be front or last runner if the manifold isn't seated properly.

also, double check that IAC connector again. make sure it's plugged in all the way and not damaged. i once plugged mine back in and bent the prongs in the sensor, causing an absence of power/signal to the sensor and allowing more air to meter past the butterfly because the IAC probe is retracted.

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Old 11-10-2015, 06:21 AM   #3
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btw, how high does the RPM go, what did you torque the bolts down to, and in what sequence?

flange mounting bolts 1-5 and 8-13 should be 23 ft-lbs, and 6-7 should be 17 ft-lbs
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Old 11-12-2015, 08:37 PM
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btw, how high does the RPM go, what did you torque the bolts down to, and in what sequence? flange mounting bolts 1-5 and 8-13 should be 23 ft-lbs, and 6-7 should be 17 ft-lbs
RPM would go redline if I didn't turn the engine off right away. I torqued the bolts to spec but didn't follow the sequence.
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Old 11-12-2015, 10:55 PM   #5
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ok so you're smelling strong exhaust when it runs for 2 seconds, as mentioned in your first post. leads me to believe you're not seated all the way on the exhaust and intake manifold. if you have a piece of thin cardboard or a feeler gauge set handy, check in between the head and manifolds for gaps top and bottom, or you can try to wiggle the intake manifold to see if it gives some.

give some detail about your year model tj, engine, and what parts you used for the gasket and exhaust manifold, i.e. brand and part# manifold, and if you used the supplied gasket or one sourced separately.
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Old 11-12-2015, 11:04 PM   #6
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if you ever disassemble the manifolds again, check the imprint on the gasket to see if you were in fact seated.

there are dowel pins on the head, between intake runners 1-2 and 5-6, that the intake manifold has to be located on. they'll be in line with the bottom row bolts. if you can somehow crawl under and find the location of these dowels through the intake manifold tabs, there should be no gaps on either end. i know it will be difficult with the motor mount bracket and front axle in the way, so goodluck.
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Old 11-12-2015, 11:12 PM   #7
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RPM would go redline if I didn't turn the engine off right away. I torqued the bolts to spec but didn't follow the sequence.
You didn’t get the intake manifold seated back in place correctly which is causing a big vacuum leak. That's the cause of your high rpms. Nothing else. You have to really take your time insuring the intake manifold is properly positioned over the positioning studs on the head, otherwise you will end up with the vacuum leak you have now.

Essentially, you have to start the job over, sorry. Take your time next time. And don't bother trying to tighten the bolts more which could end up causing a major problem.
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Old 11-16-2015, 10:33 AM
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You didn’t get the intake manifold seated back in place correctly which is causing a big vacuum leak. That's the cause of your high rpms. Nothing else. You have to really take your time insuring the intake manifold is properly positioned over the positioning studs on the head, otherwise you will end up with the vacuum leak you have now. Essentially, you have to start the job over, sorry. Take your time next time. And don't bother trying to tighten the bolts more which could end up causing a major problem.
I did the entire job over and am still having the same issue. The intake is definitely seated properly. Any other ideas? Thanks for the help!
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Old 11-16-2015, 03:43 PM   #9
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I just did this 2 weeks ago. I followed advice on this forum, but here is one I took and used. Use the copper gasket spray, and assure all surfaces are clean. Spray both sides of the gasket, and the manifold surfaces, and assure the intake manifold is lined up on the pins correctly, and it will seal up like a champ. The copper gasket spray is what really makes the difference though. Good luck with it
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Old 11-16-2015, 05:27 PM   #10
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FWIW, it's really easy on the 4.0L for the intake manifold to "look" like it's seated properly, but not be. Take a really good (bright light and hold it up from the bottom. I had the same issue when I did mine a couple of years ago. It looked perfect until I got some better light on it.
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Old 11-16-2015, 07:57 PM   #11
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FWIW, it's really easy on the 4.0L for the intake manifold to "look" like it's seated properly, but not be. Take a really good (bright light and hold it up from the bottom. I had the same issue when I did mine a couple of years ago. It looked perfect until I got some better light on it.
X2. The above symptom is a classic indication of a vacuum leak from the intake manifold area. Also make sure the large black rubber vacuum line is connected and leak-free between the intake manifold and power brake booster.
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Old 11-16-2015, 09:28 PM   #12
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How did the gasket look when you disassembled the manifolds?

What exhaust manifold did you install? i was having issues with vacuum leaks on mine because the header i installed had a thicker flange than the intake manifold. so, maybe compare the old and the new exhaust manifold flange for that matter.

Also, go back to your IAC. does the probe appear to be retracted, and are the pins for the connector straight and not bent? make sure the pigtail is plugged all the way in.

might also wanna check the TPS for the same reason with the pigtail.

this thing's a bugger, isn't it? i did this particular job at least 3 times within a 2 month period this year...got to the point where i didn't need the book after the 2nd time LOL
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Old 11-16-2015, 10:27 PM
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How did the gasket look when you disassembled the manifolds? What exhaust manifold did you install? i was having issues with vacuum leaks on mine because the header i installed had a thicker flange than the intake manifold. so, maybe compare the old and the new exhaust manifold flange for that matter. Also, go back to your IAC. does the probe appear to be retracted, and are the pins for the connector straight and not bent? make sure the pigtail is plugged all the way in. might also wanna check the TPS for the same reason with the pigtail. this thing's a bugger, isn't it? i did this particular job at least 3 times within a 2 month period this year...got to the point where i didn't need the book after the 2nd time LOL
Manifold looked ok but I replaced with a felpro one. Exhaust manifold is from eBay with the sections that's will expand when hot.
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Old 11-16-2015, 11:39 PM   #14
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Exhaust manifold is from eBay with the sections that's will expand when hot.
Those are flex joints to help the exhaust header avoid cracking as the engine moves around from torque.
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Old 11-20-2015, 04:16 PM
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How did the gasket look when you disassembled the manifolds? What exhaust manifold did you install? i was having issues with vacuum leaks on mine because the header i installed had a thicker flange than the intake manifold. so, maybe compare the old and the new exhaust manifold flange for that matter. Also, go back to your IAC. does the probe appear to be retracted, and are the pins for the connector straight and not bent? make sure the pigtail is plugged all the way in. might also wanna check the TPS for the same reason with the pigtail. this thing's a bugger, isn't it? i did this particular job at least 3 times within a 2 month period this year...got to the point where i didn't need the book after the 2nd time LOL
I measured the flange and they're almost the same thickness. Less than 1/8" difference. I'm going to measure with a digital caliper to get exact measurements.
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Old 11-21-2015, 06:35 PM
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How did the gasket look when you disassembled the manifolds? What exhaust manifold did you install? i was having issues with vacuum leaks on mine because the header i installed had a thicker flange than the intake manifold. so, maybe compare the old and the new exhaust manifold flange for that matter. Also, go back to your IAC. does the probe appear to be retracted, and are the pins for the connector straight and not bent? make sure the pigtail is plugged all the way in. might also wanna check the TPS for the same reason with the pigtail. this thing's a bugger, isn't it? i did this particular job at least 3 times within a 2 month period this year...got to the point where i didn't need the book after the 2nd time LOL
The new exhaust manifold flange is .533 inches, me one is .507 inches. Would that make a difference?
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Old 02-12-2016, 01:34 AM   #17
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Bumping this thread to see if you ever resolved the issue... I also have a '99 4.0, and just reinstalled my intake manifold tonight after replacing the exhaust manifold. I used a Dorman stainless steel replacement, and now I'm a little worried the flange is a different thickness than the original.

It was a huge pain to get the bolts torqued, and most of them don't actually have a straight-on approach where you can use a torque wrench correctly. The one that I did (#6) was WAY tighter than I could get the ones directly under the intake with a 9/16" open-end wrench. I'm going to go back with a cheater and put more torque on them tomorrow night. This design is for the birds...

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