I just did the 2.5 to 4.0 throttle body swap in my 91 YJ. Now the Jeep idels at 2000 RPMs!! is there something that I can do to fix this? I have tried reseting the computer (turning the key and disconnecting the NEG battery cable-- no luck!) Please help I enjoy the extra horses but I am afraid am am ruining my engine!

When you resest your computer, did you leave it unplugged for a bit, or just pull the neg cable and then put it back on? I've heard it's good to let it sit there for a minute or so before hooking back up. Also, trying checking for codes. To do this, cycle your key all the way on(don't start) and all the way off three times, and the third leave it on and count the flashes from the engine light. Make sense?

I understand the code trick (cheap way to find out what is wrong!) Anything else I could Check?

I don't really know what else could be wrong. I'd try checking codes if it were me, then go from there. I'm sure someone else has some good insite or has had this problem before. Make sure you have a good seal between the TB and the intake.

Did you use the 2.5l's IAC motor and houising? That is a must. The housing for the 2.5 is marked. It is held onto the throttle plate with 4 tamper proof torx screws.

http://www2.hawaii.edu/~jyokomur/tec...y01/index.html

Roger switched the AIS housing but I am still at 1500- 1700 RPMs. I am nearing wits end with this one =--- Help! anybody!!

And you also used the original 2.5l's A.I.S. motor? The A.I.S. motors have a different part number between the 4.0 and 2.5. They are matched with the housings. I think the difference is the angle of the pintle. You should have used all the old parts from your 2.5 but the throttle plates themselves.

If that is correct, check closely for vacuum leaks. Carefully inspect the throttle plate base gasket. Also make sure you didn't leave any hoses off. Remember, air is also fuel. So any vacuum leaks act the same way as opening the throttle.

There is also the slight possibility that your old A.I.S. motor is bad.
If you have access to a scanner, there is a function that will allow you to set minimum air flow (the screw on the throttle plates). It also has a function that will allow you to set the rpm to any desired speed. i.e. can dial engine rpm to 1700 or what ever.

Also make sure that you swapped over the T.P.S. correctly. It is possible to stab the sensor on the wrong side of the shaft. This will give high t.p.s. readings, and make the pcm pull the a.i.s. open. This is part of a de-cell function programmed into the pcm.

Do you have a vaccum leak? the computer might sense more air and it is richening it up to compensate witch might be making it idle higher.

Fixed the problem! There is a small allen screw near the the front of the throttle boddy that allows you to adjust the angle of the butterfly valve in the throttle body. I started up the Jeep and made the adjustments and what do you know it works now!! Thanks to everyone who helped out with your responses.
Rob

This has been an interesting post. As I read it I found that there wasn't really enough information to give a good diagnisis.

An Idle Control Circuit will vary with load, temp, volume of air being sucked in, etc.

Here we've got some variables:
1. The Idle appears constant.
2. There's a swap of parts afoot.
3. There's no specs for the idle of either of the parts.

My rule of thumb is that if the Idle Control circuit is steady... it's not a vacuum/air leak screwing up the computer.

Most Idle circuits will have some sort of "rest" position for the throttle plate in the throttle body housing. It may be capped, hidden or just plain seald off with goop, but usually it's there.

Also in EFI circuits the idle is "pre-set" per the IAC/IAS sensor and the throttle plate rest position.

You may have just lucked out by closing off the throttle plate.

My guess is that there is a specific sequence of events that must be made to accuractley set the idle, rather than just backing off the throttle plate.

But, I could be wrong... I have been before. Anyone want to chime in on this? [img]images/graemlins/confused.gif[/img]