In an effort to help the four lethargic squirrels at the heart of Muffin's power plant breath a little better, I attempted an "upgrade." Could be read as, "turd polish."
It's widely known that short of forced induction, or major machine work there's not a whole lot you're going to do to the 2.5l to make any real gains in power. Muffin's MPFI version of the mill is already at its pinnacle in terms of OEM power, which still ain't much. I ditched the stock mechanical fan long ago, in favor of an electric fan, alleged to provide noticeable gains in performance, it may have added half a squirrel, but made no difference I could tell, placebo or otherwise. Muffin is hobbled by the auto transmission too, so pretty much worst case scenario.
That's basically been the extent of Muffin's engine performance upgrades, I've mostly relegated to the fact that it is what it is, live with it. Then a while back I joined the forum 4bangerJP, it's dedicated to the 4 cylinder Jeeps, mostly YJ, MJ, and TJ, there's a few 2.5l Dakota folks there too. Doing some reading there convinced me to try something else.
The 4.0l throttle body has exactly the same external dimensions, and bolt pattern as the 2.5l throttle body, but has a larger bore size (same size as the bore in the manifold). Allegedly the 4.0l throttle body on the 2.5l is supposed to improve throttle response, Muffin needs all the help she can get. So I found a $30 4.0l throttle body on eBay, included everything but the IAC guts, which was fine I didn't need them anyway.
While the 4.0l throttle body's external dimensions are the same, the sensors are not, you've got to swap the IAC, TPS, and MAP from the 2.5l throttle body to the 4.0l throttle body, otherwise silly things like high idle, and other erratic behavior can occur.
2.5l on the left, 4.0l on the right, notice the smaller butterfly and taper to the bore of the 2.5l.
Jeep was even nice enough to label them for me.
While I knew that I had to swap all the sensors over, I didn't really put a lot of thought into physical differences. I pulled the MAP and TPS off of my 2.5l throttle body, and swapped them over to the 4.0l throttle body. Since my "new" 4.0l throttle body was only missing the guts (solenoid), I simply pulled the guts from the 2.5l IAC housing and put them into the 4.0l's IAC housing. Simple enough right?
Reading along and looking at the pictures you probably already noticed a blatant difference, that bone head me over looked, and why they say to swap the IAC, not just swap the solenoid. There's a little tab that protrudes out on the bottom of the IAC housing, the tabs are in different spots on the 2.5l and 4.0l throttle bodies. You can't physically put a throttle body with a 4.0l IAC housing on a 2.5l intake manifold because of that little tab, and I guess it's safe to assume the opposite is true as well.
So what did I do when I noticed my "new" 4.0l throttle body wasn't seating on the throttle body gasket? When I felt interference, did I stop and investigate what the issue was? Nope, you see, I do dumb things. Instead of seeing what the problem was, I assumed, the bolt holes might not be exactly right, and continued to turn the ratchet a little harder until I heard a "pop." I thought, "Hmm, that didn't sound right." I then removed the throttle body to investigate.
Well there's your problem, broke a bolt.
But it was much worse than just a broken bolt.
Like I said, I do dumb things. Since I seem to so enjoy doing the same thing twice, I now have another 4.0l throttle body currently on its way to my house, actually cheaper than the first one I destroyed. "I'll get you next time Gadget!"