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Go Back JeepForum.com > Models > Jeep Wrangler Forums > YJ Wrangler Technical Forum > 2.5 to 4.2 swap.. need info

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Unread 03-12-2013, 03:08 PM   #16
Djsthe1
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1991 YJ Wrangler 
 
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Another few things.. Exhaust. how is it ran on that model? same as the 2.5? Air intake, can the old air box be reused or just use a round breather?

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Unread 03-12-2013, 03:56 PM   #17
Djsthe1
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Quote:
Originally Posted by Que89YJ View Post
you will need to reverse the 5 to fire that the people going from the 4.2L use to make the MPFI work in the older models.
Reverse the 5?
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Unread 03-12-2013, 05:40 PM   #18
Que89YJ
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You can go to the round breather.
Exhaust:
Click to View Search Results for http://www.google.com/search?num=40&hl=en&newwindow=1&biw=1315&bih=623&q=site%3Ajeepforum.com%2Fforum%2Ff12++4.2+exhaust+routing&oq=site%3Ajeepforum.com%2Fforum%2Ff12++4.2+exhaust+routing&gs_l=serp.3...3264.16424.0.16869.21.20.1.0.0.0.114.1328.18j2.20.0.les%3B..0.0...1c.1.5.serp.1IdK_PYoh0g http://www.google.com/search?num=40&hl=en&newwindow=1&biw=1315&bih=623&q=site%3Ajeepforum.com%2Fforum%2Ff12++4.2+exhaust+routing&oq=site%3Ajeepforum.com%2Fforum%2Ff12++4.2+exhaust+routing&gs_l=serp.3...3264.16424.0.16869.21.20.1.0.0.0.114.1328.18j2.20.0.les%3B..0.0...1c.1.5.serp.1IdK_PYoh0g
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Unread 03-13-2013, 01:05 PM   #19
Djsthe1
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Quote:
Originally Posted by Que89YJ View Post
You can go to the round breather.
Exhaust:
http://www.google.com/search?num=40&...rp.1IdK_PYoh0g
Link doesn't work.
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Unread 03-14-2013, 06:56 AM   #20
daviddoughty1
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I just pulled the tub off my 87 last night to do the same swap. I found a 4.2 , T177 and NP 208 . I didn't measure from the firewall but was thinking of just matching the position of the motor / bell housing joint, would this work too?
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Unread 03-14-2013, 08:26 PM   #21
Djsthe1
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I would imagine so, I'm worried more about it being squared. I have an 8.8 with the offset pinion I wonder if I should line it up with that..?
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Unread 03-14-2013, 08:41 PM   #22
Michaelgoesrawr
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Quote:
Originally Posted by Djsthe1
I would imagine so, I'm worried more about it being squared. I have an 8.8 with the offset pinion I wonder if I should line it up with that..?
Nah. Then you'll have problems with the fan. You should save up for a sye and new driveshaft. Bolting the trans to the skid and the skid to the frame should be enough to let you get it straight.
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Unread 03-14-2013, 08:45 PM   #23
Djsthe1
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Yea I picked up a J231 with a slip yoke, my np231 I never had any vibes for anything so I hope I can get it right. If not SYE here I come!
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Unread 03-14-2013, 10:07 PM   #24
YJjeep258
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Well we did this swap on a 92 YJ and we just got it running the other day. If you do this swap I would recomend putting the HEI distributor and also putting a weber carb on it. When we did it we were able to use one of the mounts and we had to fabricate the other one. Also when you do the swap make sure to use the sensors off the 2.5 or your gauges won't work right. Also I'm wanting to know what color the wire is going to the tach and if you can hook it up with out having a computer?

Last edited by YJjeep258; 03-14-2013 at 10:52 PM..
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Unread 03-15-2013, 10:00 AM   #25
wanabejeeper
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Might want to check your state inspection. Many states do not allow going backwards with engine installs although as long as you pass emissions I dont see how they would know.
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Unread 03-15-2013, 10:26 AM   #26
YJjeep258
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Well they don't check emission where I live. If any one has any information on what wire goes to the tach and how to hook it up without the computer it would be helpful.
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Unread 03-15-2013, 10:37 AM   #27
Djsthe1
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YJjeep258, Im not sure on the tach wire but will find out when I pull mine apart. Also with the motor mounts, would it be a good idea to make a set the is a bit taller that the old ones so that it changes the pinion angle?

Wannabejeeper, I have historic tags so no inspection or emissions.
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Unread 03-15-2013, 01:48 PM   #28
Que89YJ
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Tach wire goes to the coil ground. Green with white coming from the bulkhead connector.
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Unread 03-15-2013, 02:55 PM   #29
YJjeep258
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ok thanks, so that green and white will give me a reading on the tach right? About the engine mounts we didn't raise ares which we probably should have because we put the borg warner T5 transmission which has the external slave cylider for the clutch with the flipped dana 300 and we have about an 1 inch of clearance between the front drive shaft and the slave cylinder. Now if you have the internal slave cylinder i wouldn't raise it but it is up to you. Also about the exhaust the original exhaust wouldn't work for us it was rubing the engine and to much angle down so I hope some of this imformation work for you.
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Unread 03-15-2013, 04:05 PM   #30
YJjeep258
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Ok I got looking at the connecter going into the guages there is a green wire and a green and orange tracer wire and comming out of the firewall there is a green and grey tracer wire and a green and orange tracer wire. I havn't seen a green and white tracer wire so could one of those wires be for the tach signal.
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