4.0L Throttle body for your 2.5L, v2.0
We're willing to bet that if you have a 2.5LYJ, you're looking for some more ponies under your hood. If you've installed larger tires and haven't changed the gears yet, you probably haven't seen 5th gear in a while. Just getting the Jeep to move require feathering the clutch more then you really want to. If that's you then you will want to read on. For those with TJ's you won't need to do this. We were told that TJ's already have a larger throttle body which is shared with the 4.0L. (*Correction: see below)
Turbo City currently sells bored throttle bodies for 2.5L's and 4.0L's. They claim to be good for 4-6 horses and is a direct bolt-on. The only down size is the cost and they require a core. But if you have a TJ then this might be your only choise. With our project, we did not have to have any special milling done so we are able to return the engine to stock if there is the need.
We first got the idea from John Dorflinger. He suggested trying out a 4.0L throttle body on our 2.5L. He had already done so and saw good results and though we should share this with the rest of you. John was kind enough to send us a used 4.0L core throttle body for our test. Since we had a Poweraid Throttle body spacer meant for a 4.0L, we knew this would work but with what results? The throttle body was from a late model 4.0L. For YJ's that's from 1991- present and for XJ's that's from 1987 to present. Earlier Inline 6's used a 3 bolt design unlike the later throttle bodies. The three bolt throttle bodies won't work for this project. The easiest way to spot these is that the engine has to be a MPFI not not a TBI engine. If you've got a TBI engine then this won't work for you.
If you haven't installed a Poweraid Throttle Body Spacer yet, then you will need to get some torx bits to remove it. We also found out that we needed some special tamperproof torx to swap the some other parts. Tamperproof torx are just like regular torx but with a small post in the middle to prevent a standard torx from working. The hardest thing might be finding a place that sells these bits. Since the usual places didn't have them we went to the net and clicked our way to eBay.com. A couple clicks and a few days later we were the proud owners of a new tamperproof torx set. That place has just about everything you could ever want and some good deals on tools.
We used the couple day's of down time by cleaning up the old throttle body. This isn't necessary for the install but years of sitting around caused a lot of scale to form on the surface of the throttle body. Since we wanted to keep the cost down, we used what we had sitting around. Because the throttle body is aluminum, nothing too abrasive should be used. We ended up using an abrasive pad and then went to a screwdriver for the tougher stuff. When that didn't work we pulled out the buffing wheel and dressing compound. Remember you don't have to do any of this unless you want to.
To remove the throttle body we had to unclip the throttle cables. The throttle cable pops off with a screwdriver. The Jeep also had a hand throttle so that had to be removed too. The two sensors have plugs to the firewall wire harness, unclip those and remove the plugs. Remove the four bolts and lift the throttle body. Make sure the gasket is removed and nothing falls into the engine. Because of the design of the engine most things that do fall in can be retrieved from the intake manifold. Any part of the gasket that doesn't come off with the throttle body will have to be scraped off. It is a good idea to buy a new gasket rather then reusing the original one. The Jeep we were working on had a Poweraid Throttle Body spacer added a few months earlier so the gasket was reused.
The throttle body uses a lot of torx bolts for some reason. Since our throttle body didn't have any sensors with it we had to transfer the old sensors to the new throttle body. The only exterior difference is that there is a small housing bolted to the side of the throttle body that has the engine size cast into it. The housing holds the AIS motor to it. The AIS controls the idle speed by letting air bypass the throttle plate. All it is is a valve that opens up to let air into the throttle body by bypassing the throttle valve. The other sensor is the TPS or Throttle Position Sensor. As the name suggests it lets the computer know the position of the throttle valve.
With the two throttle bodies nest to each other and all the sensors removed it is easy to see the difference between the two. The 2.5L's throttle body's throttle valve was much smaller then the 4.0L's. The 2.5L has a lot more material around the throttle body which has been milled out of the 4.0L. The other dimensions of the throttle body are the same so everything will bolt back into place without any modifications. It is a good idea to clean off the AIS motor and housing since tar collects in there. Also, take the time to clean the channels on the new throttle body if you haven't already. There is a small paper gasket for the AIS motor that should be retained and reused.
Once everything is back together the 4.0L throttle body can be reinstalled. Make sure all the gaskets are back on and the sensors are tight. You may want to use some Locktite on the bolts to keep them from walking out. The Factory uses a similar product when building these. Don't forget to reconnect the throttle cable and the two plugs to the wire harness. Conclusions
The first thing we were wondering if if it would work with all the other engine modifications. So far the engine has a Poweraid TB spacer, Jacob's Omni-Pak, 8mm MSD spark plug wires, Open element K&N filter and Accel U-Groove spark plugs. Behind it sits the Carsound Hi-Flow Cat and Borla Cat-back. With a turn of the key, the engine came to life. No warning lights came on so that was a good sign. With a pump of the throttle the engine roared and then settled back down as the throttle closed. No strange sounds that would have to alarm us that we sucked a rag or bolt into the engine.
Because this is a computer controlled engine, it will take a few miles for it to readjust to the new throttle body. Initial though on our first test drive, There was a noticeable change in torque. There seems to be a little more at low speeds. This was more noticeable when crusing down the Interstate. Some area's where we had to down shift into 4th gear could be taken in 5th. Accelerating from a stop was a little better too. One thing we did notice was that with the new throttle body, we get a audible whistle from the intake. It's especially noticeable when opening up the throttle. Since engine noise isn't a problem with us it's not a problem. It can be heard above the howl of the 35 inch MTR's.
Other then that we think this is a inexpensive way to get a little more power out of the 2.5L and since it is a OEM part, they are easy to get a hold of. We also like the fact that the we get to keep the original throttle body in case we have any problems with state emissions. So while you're in those salvage yard's looking for those Dana 44's and Ford 8.8”'s be sure to grab a 4.0L throttle body if you can't afford the whole engine. It only takes a few minutes to swap in if you've got all the tools handy. We think you'll be happy you did. And special thanks goes out to John Dorflinger.