2.5 to 4.0 throttle body, Idle issues, Again! - JeepForum.com
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Unread 02-20-2013, 12:54 PM   #1
az_zoner
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2.5 to 4.0 throttle body, Idle issues, Again!

I ask some question like this in a thread but didít get any response so I thought Iíd post them to see if anyone has info or experienced the throttle body swap like I am. This is to a freshly rebuilt engine 2.5 in a 91 YJ

I just did the 2.5l to 4.0l throttle body swap. I started with the 4.0 boday and 2.5 housing and AIS and could not get it to idle down. It stayed just below 2k rpm. I went back to the 2.5 throttle body and sensor and it ran fine. So I tried again with the 4.0l body and 2.5 housing and sensor. The rpm was back up. I do disconnect the battery, ground cables out, and turn on the ignition, and wait several hours but it doesnít seem to help. The only way I can get it to idle down to about 800 RPM is to run the 4.0 body and the 4.0 sensor. This set up seems to not run as smooth as the old 2.5 set up. I actually took it for a drive when the 4.0 body and 2.5 sensor was on with a high idle and it seem to run much better. Just canít figure out the high idle. Last night I left the battery disconnected overnight. Still high this morning

So if I left the 4.0 housing and sensor on there, what would be the down fall? I only have about 20 miles on this rebuilt motor so I'm not rapping it out yet to really tell but if there is a downside I'd like to know.

What can you do with these sensors as far as cleaning? Will carb cleaner hurt them? I cleaned the housing up. Maybe itís time for a new sensor. But sense it runs on the 2.5 body I just donít know. Confused!

Thanks,

Craig

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Unread 02-20-2013, 01:14 PM   #2
Que89YJ
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The longest you need to dosconnect for is 10 sec. to reset the ecu. Carb cleaner is ok for the sensors and throttlebody. You used the throttle position sensor(TPS) and IAC from the 2.5 and the throttlebody from the 4.0? If the idle was still high do a quick spray arount the throttlebody with carb cleaner to make sure there are no leaks. You have the right idea in dosconnecting the battery after every swap to reset the ecu.
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Unread 02-20-2013, 01:27 PM   #3
0verkillYJ
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I know EVERYONE will tell me I am wrong, but I had a high idle problem after an alternator upgrade. I had tried EVERYTHING! I finally stuck the IAC that came with my 4.0 TB when I bought it on Ebay and it now runs perfect. Now I am assuming it is a 4.0 IAC, I guess it could be a 2.5. But I replaced the 2.5 IAC 3 times under warranty and it did not fix it. Not until I pulled out the used 4.0 one I have had for a year because I didnt throw it out and it fixed it.
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95 YJ, 2.5 AX15 conversion, 4.0 TB, SOA +1.5", 33" DC Crusher Tires, 8.8 Rear w/welded tubes + LSD, Spartan Locker HP30 Front w/solid axle swap, Rocky Road OTT kit, JKS Disconnects, AA SYE, Teraflex Shock Bar in Rear, 1/2" Boomerang Shackles, RE Add a Leafs, Homebrew Traction Bar, Monstalined in&out, LED Tail lights, 1700 Watt Stereo
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Unread 02-20-2013, 02:23 PM   #4
az_zoner
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Quote:
Originally Posted by 0verkillYJ View Post
I know EVERYONE will tell me I am wrong, but I had a high idle problem after an alternator upgrade. I had tried EVERYTHING! I finally stuck the IAC that came with my 4.0 TB when I bought it on Ebay and it now runs perfect. Now I am assuming it is a 4.0 IAC, I guess it could be a 2.5. But I replaced the 2.5 IAC 3 times under warranty and it did not fix it. Not until I pulled out the used 4.0 one I have had for a year because I didnt throw it out and it fixed it.
You know that you need to change both the housing and sensor at the same time? The 4.0 is a larger (physical size) sensor and the 2.5 sensor in a 4.0 housing will leave a gap around the sensor.

cb
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Unread 02-20-2013, 02:28 PM   #5
az_zoner
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Quote:
Originally Posted by Que89YJ View Post
The longest you need to dosconnect for is 10 sec. to reset the ecu. Carb cleaner is ok for the sensors and throttlebody. You used the throttle position sensor(TPS) and IAC from the 2.5 and the throttlebody from the 4.0? If the idle was still high do a quick spray around the throttlebody with carb cleaner to make sure there are no leaks. You have the right idea in dosconnecting the battery after every swap to reset the ecu.
Correct, I first tried the 4.0 throttle body and the 2.5 IAC and housing. that config gave it a 1800 rpm idle. Only when I use the 4.0 throttle body and 4.0 IAC and housing can I get it to come down to 800 rpm. But it just doesn't seem to have the response that it should. I will try and see if I have a leak under the throttle body tonight.

Thanks,

cb
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Unread 02-20-2013, 03:01 PM   #6
0verkillYJ
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Quote:
Originally Posted by az_zoner View Post

Correct, I first tried the 4.0 throttle body and the 2.5 IAC and housing. that config gave it a 1800 rpm idle. Only when I use the 4.0 throttle body and 4.0 IAC and housing can I get it to come down to 800 rpm. But it just doesn't seem to have the response that it should. I will try and see if I have a leak under the throttle body tonight.

Thanks,

cb
yeah, when you hit the gas the throttle opens so the iac has nothing to do with that. However you should be using the 2.5 tps
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Unread 02-20-2013, 06:42 PM   #7
az_zoner
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I went back and double checked that I had ground the tab off enough to not interfere. I restarted and it still idles at 2k. I sprayed cleaner around the base and a few place where I though I might have a vacuum leak and found nothing. Think it's time for a new air sensor? Or any other ideas before spending the money?

thanks for any ideas.

cb
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Unread 02-20-2013, 06:48 PM   #8
Que89YJ
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Did you do the battery reset?
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Unread 02-20-2013, 07:41 PM   #9
az_zoner
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Yes.
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Unread 02-20-2013, 07:46 PM   #10
Que89YJ
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Then its the iac.
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Unread 02-20-2013, 09:36 PM   #11
az_zoner
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Thanks, I'm thinking I agree with you. I'm going to try putting the 2.5 throttle body, housing and iac back on if it idles high then that should be solid proof.

thanks all for you input

I'll let you know how it comes out.

Craig
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Unread 02-20-2013, 10:01 PM   #12
0verkillYJ
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Quote:
Originally Posted by az_zoner View Post
Thanks, I'm thinking I agree with you. I'm going to try putting the 2.5 throttle body, housing and iac back on if it idles high then that should be solid proof.

thanks all for you input

I'll let you know how it comes out.

Craig
if you throttle body came with an iac try that one. Otherwise hit up auto zone a get a 4.0 iac. If it doesnt work you can return it there easy enough.
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95 YJ, 2.5 AX15 conversion, 4.0 TB, SOA +1.5", 33" DC Crusher Tires, 8.8 Rear w/welded tubes + LSD, Spartan Locker HP30 Front w/solid axle swap, Rocky Road OTT kit, JKS Disconnects, AA SYE, Teraflex Shock Bar in Rear, 1/2" Boomerang Shackles, RE Add a Leafs, Homebrew Traction Bar, Monstalined in&out, LED Tail lights, 1700 Watt Stereo
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Unread 02-20-2013, 11:22 PM   #13
Drew_CarreyAB
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For me, I swapped the 2.5L stuff onto the 4.0L TB and it had the high idle. I took off everything and carb cleaned it, re-installed and had the high idle. I took the TB off and noticed the idle screw........hmmmmmm, gave it a turn out and installed again, dropped the idle quite abit ( fiddled with it till it got to about 8-900RPM and it's been there since. I also stuffed rags in the intake and dremeled the intake to match the gasket (which BTW, is same same for either the 2.5 or the 4.0 TB). More noticeable throttle response for sure, but I also put on a home brew CAI and K&N as well.
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Unread 02-21-2013, 01:25 PM   #14
az_zoner
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Quote:
Originally Posted by Drew_CarreyAB View Post
For me, I swapped the 2.5L stuff onto the 4.0L TB and it had the high idle. I took off everything and carb cleaned it, re-installed and had the high idle. I took the TB off and noticed the idle screw........hmmmmmm, gave it a turn out and installed again, dropped the idle quite abit ( fiddled with it till it got to about 8-900RPM and it's been there since. I also stuffed rags in the intake and dremeled the intake to match the gasket (which BTW, is same same for either the 2.5 or the 4.0 TB). More noticeable throttle response for sure, but I also put on a home brew CAI and K&N as well.
Where is this idle screw your talking about? I don't remember seeing anything like that on either of the throttle bodys I have.

cb
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Unread 02-21-2013, 03:02 PM   #15
0verkillYJ
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You have to be carefull messing with that idle screw. That is not a fix if your IAC is not working properly. It even tells you in the manual and on the box not to manually adjust it because the computer will do that if it is properly working. The reason is actually extremely important if you dont want to kill your Jeep super fast.

Your IAC lets air bypass the throttle body to control the idle. This will rev the engine for you when it is cold out and adjust the air moving through the throttle body while it is warming up. If your idle is working correctly you will notice when you stop at a light the idle is higher for a second or two. That is because your IAC motor is in the process of closing the air bypass. It makes sense that it would be open. When you are driving down the street going 35mph at 2500 RPMs, you may not have your foot on the gas so the throttle is closed, but your engine needs more air than it takes to idle at 800-900 rpms. This is where the computer tells it to open to let your engine breath. If your IAC is not working correctly, and you manually adjust the idle using that screw to fix it then it probably is not getting the amount of air it needs to run when your foot is not on the gas and your engine is higher than the idle speed. I am not a mechanic, so I do not know exactly what happens when your engine has fuel and a spark without enough air over a period of time, but my Jeep is not going to be the guinea pig to find out.

BTW, I figured this out after manually adjusting my IAC so that I could drive my Jeep around until I properly fixed the problem. I noticed that if I went for a long drive it would screw the idle up again. The reason why is because the air was getting sucked past the IAC so hard when the TB was closed is was screwing up my adjustment. Sorry to be long winded, but I know people will go adjusting IAC's all day if no one says why that is a bad idea.

EDIT: I should also add that if your IAC is too short to close the idle all the way and you unscrew it a bit to make it longer and your idle is working fine after that, that is no problem. It is only when your IAC is not working (pulling in and out to adjust the air, or not setting the idle correctly) that screwing with it can damage your engine.
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95 YJ, 2.5 AX15 conversion, 4.0 TB, SOA +1.5", 33" DC Crusher Tires, 8.8 Rear w/welded tubes + LSD, Spartan Locker HP30 Front w/solid axle swap, Rocky Road OTT kit, JKS Disconnects, AA SYE, Teraflex Shock Bar in Rear, 1/2" Boomerang Shackles, RE Add a Leafs, Homebrew Traction Bar, Monstalined in&out, LED Tail lights, 1700 Watt Stereo
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