It’s been a while since the first time this article was published back in 6/01 on the site Jeepwire.com (also known as 4x4wire.com). After the article was published I received many emails which asked if they needed a 4.0L Throttle body spacer for this to work. Other people emailed wondering how to fix their idle which shot up after the install. By now most people know about the conversion but not so many people have read the original article. More then likely they heard about it from a friend or someone else mentioning it on a forum. Then they go off and try to do the conversion without doing any research. This is probably the biggest problem. As a result, I’m here to address all those problems and hopefully hit on some new stuff.
We first got the idea from John Dorflinger. He worked at a Jeep Dealership dismantling engines that needed to be rebuilt. He suggested using Throttle body (TB) from a 4.0L, for better air flow. John, already installed one on his Jeep and tested it at work. His results showed a decent boost in the amount of air able to enter the engine.
John was kind enough to send a 4.0L core throttle body to test. The core was stripped down to the bare and had no sensors. John stated that they weren’t necessary since I’d be reusing all of the sensors from the old throttle body. This is where a lot of people get into trouble. They reuse the 4.0L sensors and run into idle problems. The AIS housing is different for the two engines. The 4.0L uses a larger plunger so you will not only have to replace the sensor but also the housing. This requires some small Tamper-proof torx. They are small and can break off easily. From the factory, they use a thread compound so some heat might loosen then up. The 2.5L AIS housing will have 2.5L cast into it so there should be no confusion as to which one is from the old TB. The AIS controls the idle speed by letting air bypass the throttle plate. The AIS plunger opens allowing air to bypass the throttle plate and enter the combustion chamber.
The throttle body was from a late model 4.0L. For YJ's that would be from 1991- present and for XJ's that's from 1987 to present. Earlier 4.0L's used a 3 bolt design unlike the later throttle bodies. The three bolt throttle bodies will not work. Also in 1996, the 2.5L and 4.0L had some changes done for the new model year. ODB-II was being implemented which meant more sensors. The old MAP was mounted to the firewall and had a tube that connected to the intake manifold, just below the TB. The new system moved the MAP sensor onto the throttle body. So make sure you have the right TB. How will you know? Well, if you know what year your Jeep is then that will tell you. If all else fails, go out and look. It will save you headaches later on. Also, some people seem to think that 2.5L TJ’s come with 4.0L TB’s already on them. They are wrong. Other then having the MAP on the housing, there is no other differences between TJ TB’s and YJ TB’s.
So now you know what TB your Jeep has, how do you know what will fit? That’s an easy one. If you have the MAP on the firewall then you want one from a YJ, XJ, MJ or ZJ up to 1995. For those with the MAP on the TB, you’ll want to look for one from a TJ, XJ, ZJ or WJ from 1997 on. Now there might be some cross over in late 1995-1996 so you will have to look and make sure which one it had.
What makes the 4.0L TB so different from the 2.5L TB is the bore. The 2.5L TB has a bore that’s about 47mm at the base. Looking down the bore, it is easy to see the differences between them. The walls of the 2.5L are much thicker. Now some people might ask why they can’t just bore out the 2.5 L TB. Well, it can be done but a new throttle plate will need to be made. Since not everyone has the means to bore out the center and fabricate new valves, a off the shelf TB is a better option. So, how much larger is a 4.0L TB’s bore? Well the 4.0L TB measures in at 54mm’s at the base. That’s almost a half an inch difference. Now there are a few people who bore out their TB’s to gain a bit more. A few companies sell 62mm TB’s. Some people have bored out their TB under the throttle plate. The TB originally had a slight taper from the mouth to the base. So, they essentially remove the taper. As long as they do not touch the surface where the throttle plate and wall meet, the TB should be fine. Another trick that’s popular is port matching. If you compare the bore of a stock 4.0L TB to the hole in the intake manifold, you will notice it is much smaller. To remove any bottle necks, some have been grinding the port opening so it matches the opening on the TB. If you plan on doing this, you will want to either remove the intake or else stuff the intake with rags to prevent the shavings from entering the engine. Is boring and port matching necessary? No, you can take a stock 4.0L TB and install it without changing anything else and still see a difference.
Now I had previously added a Throttle Body Spacer (TBS) from Poweraid. This was before Poweraid made one for the 2.5L. It is not necessary to have the spacer to use the 4.0L TB. It did make the install a bit easier since it replaced the Torx bolts with standard bolts. There is no difference between installing the TB with the TBS or on a stock intake.
To remove the throttle body unclip the throttle cables. The throttle cable pops off with a screwdriver. The Jeep also had a hand throttle so that also had to be removed. The two sensors have plugs to the firewall wire harness, unclip those and disconnect the plugs. Remove the four bolts at the base of the TB and lift it up. Make sure the gasket is removed and nothing falls into the engine. Most things that do fall in can be easily 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 to ensure no leaks. It is a good idea to buy a new gasket rather then reusing the original one. The Jeep had a Poweraid TBS added a few months earlier so the gaskets were still new. Once the sensors are removed, it a good time to clean the AIS sensor and housing. If they are dirty, the plunger could leak and a idle problem.
Once the AIS housing, AIS Sensor and TPS (MAP if you have a post-96’ TB) is back together the new TB can be reinstalled. Make sure all the gaskets are back on and the sensors are tight. You may want to use some thread compound on the bolts to keep them from walking out. Don't forget to reconnect the throttle cable and the plugs to the wire harness.
With a turn of the key, the engine came to life then proceeded to rev to 2000 Rpm. Something wasn’t right. The problem was quickly found. The AIS plunger leaked from gunk that has collected on the tip. With that fixed I tried again. With a pump of the throttle the engine roared and then settled back down to its regular idle speed. No strange sounds that would have to alarm us that we sucked a rag or bolt into the engine. Which was a good sign.
Since it is a computer controlled engine, it will take a few miles for the ECM to readjust to the new TB. 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 at higher speeds. The throttle responded much faster then before. Also, the engine is able to lug at below idle speeds without dying. This is great when idling up an obstacle.
Other then that I 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. Because it is an OEM part, it will pass any visual inspection. It will also pass the sniffer test as long as everything else is running fine. So while you're in a salvage yard's looking for that Dana 44 or 8.8” be sure to grab a 4.0L throttle. It only takes a few minutes to swap even if you've got only hand-tools. I think you'll be happy you did. Special thanks goes out to John Dorflinger.
