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Old 09-15-2012, 05:21 PM   #1
Join Date: Sep 2012
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Cool 4.0L Intake swap

Jeep YJ intake upgrade
After researching various documents regarding the intake manifold upgrade for the older 4.0L Jeep inline 6 cylinders I decided to give it a go. The following is what I found to be true on my 1989 YJ.
The manifold from a 1999 – 2004 is designed to increase horsepower as due to new emission requirements the 1999 -2004 engines lacked the horsepower of the earlier model inline Jeep 6 cylinders and the engineers redisigned the manifold for greater flow. As you look at the difference it is apparent that the newer manifold allows for more even and smoother air flow. According to articles this manifold could increase horsepower as much as 15. I was speculative.
Searching the internet I found several outlets to purchase the manifold and did so on Ebay for a reasonalble amount. After cleaning and purchasing new intake gaskets I tackled the challenge.
I found that this is NOT a bolt on application for my YJ. I did have to cut the power steering/AC bracket to allow clearance. There is mounting holes on the new intake that I suppose could be used for AC however mine did not have it.
The clearence for the exhaust manifold was my biggest issue. After trimming the manifold between the runners I finally had a good fit. The next issue was dowel pins on the head for aligning the manifold. The newer manifold does not have the holes drilled for the dowels. After matching up the manifolds and drilling the new mainfold to accept the dowels the manifold bolted up with no problems.
After installation I used my throttle body and injectors with rail. They were installed with no problems. The vacumn lines were rerouted quickly with more than enough ports on the manifold.
After starting the engine the computer took a few minutes to settle the engine to normal operating conditions. The operation was normal as with the old intake manifold.
Conclusion: This is NOT a bolt on application but could be accomplished with normal mechanic tools. Horsepower gains???? I really doubt that a 15 horsepower gain could be seen. In my case from normal driving I would guess maybe 5 horsepower.
I really think that the cost and frustration of this swap was NOT worth the effort.
If someone else has done this and noticed a high increase in horsepower I would like to compare notes.
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Old 09-16-2012, 01:11 AM   #2
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Your 1989 YJ came with a 4.0 with fuel injection?

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Old 09-16-2012, 02:53 AM   #3
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Sounds like someone did a head swap to get MPI in his 89.

I don't believe the 15hp gains also. 4 wheeler also said it was only 5hp.
For you Vets and Active Jeepers
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Old 09-18-2012, 06:41 PM   #4
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Originally Posted by Nubby55 View Post
Your 1989 YJ came with a 4.0 with fuel injection?
Upgraded with noticeable horsepower gain. However it is a swap for an experienced mechanic.
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