Somewhere in the annals of hot rod history someone decided to toss a bigger carb-uretor on an engine for more fuel and air, and thus more power. Sounds like a plan to us. The modern equivalent is to put a larger throttle body on your multi-port injected Jeep engine. If you have a 4.0L, then that’s not so cheap or easy because it requires expensive aftermarket parts and/or custom machining. But if you own an MPI 2.5L in a YJ, XJ, or TJ, then it’s pretty cheap and easy. The 58mm throttle bodies on 4.0Ls are just larger versions of the 52mm throttle bodies on 2.5Ls. Does this swap make more horsepower? That’s debatable, but the 2.5L in our little TJ seems more responsive and peppy off the line. This is probably the result of the much cleaner throttle body as opposed to the dirty gunked-up one it replaced, but hell, we feel it. At $35 for a used throttle body and an hour’s worth of work, we’ll take the butt-dyno numbers as proof enough. The mechanical part of this is pretty easy. First pull the air tube off the old throttle body, loosening the band clamps with a flat-blade screwdriver. Then pop the throttle cable off, unplug the sensors from the wiring harness, and use a 10mm socket to remove the throttle body from the intake and swap your old sensors to the new throttle body. Reinstallation is reverse of removal. Easy. Follow along as we show you our swap and give some tips along the way.
Our 4.0L throttle body came from AMC 4x4 Salvage in Glendale, Arizona, which sells them used for $35 without sensors or $65 with sensors. It’s best to swap your existing sensors if they’re good including: the throttle position sensor (TPS); the idle air control (IAC) valve motor; the IAC valve housing; the manifold absolute pressure (MAP) sensor. On some earlier YJs and XJs the MAP is mounted on the firewall, not the throttle body. If you get a throttle body without the provision for the MAP, you can tie it into a source of manifold vacuum.
You’ll need small Torx bits including one T20 security bit for the screws on the IAC valve housing. The 2.5 IAC won’t work with the 4.0L IAC housing and the 2.5L engine won’t idle correctly with the 4.0L IAC. Also the 4.0L throttle body has a tab on the base that interferes with the four-cylinder intake. Nick Cardenas from AMC 4x4 also recommends running the engine for a while before removal of the stock throttle body. The heat will help with loosening the thread lock on the easy-to-strip bolts that hold the sensors on the throttle body. Shown are the two IAC housings. You need the 2.5L IAC and IAC housing on the 4.0L throttle body.
We cleaned the new throttle body and the old sensors with some throttle body cleaner before installation. If you want to run the sensors already on the 4.0L throttle body, you also need to know if your wiring harness has round or flat pins before buying the new part. This changed around 1997. Just for reference our ’97 TJ has round pins (shown), but according to Cardenas, some ’97 TJs have flat pins. Avoid overly rusty or worn throttle bodies.