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Unread 03-11-2013, 01:51 PM   #1
Djsthe1
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2.5 to 4.2 swap.. need info

I have been searching but not had much luck on this swap. I have found a lot of swaps to a 4.0 but non to a 4.2. I know the 4.0 is a popular engine and most people recommend it but.. I think I wanna go the 4.2 route and if I don't like it I can do a swap later down the road. So I picked up a 4.2, ax-15and np231(23 spline) out of a 89 YJ, its going in my 91, what all is needed for this swap? I have read that you can reuse the motor mounts, is this correct? I also got the tank assembly in case I need that? The gauges and all seem to be the same cluster so they should all work? Any and all information is helpful..

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Unread 03-11-2013, 06:05 PM   #2
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**bump**
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Unread 03-11-2013, 06:39 PM   #3
Michaelgoesrawr
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You'll have to cut the mounts off of the frame a reweld them. The 4.2 is a good engine with good low end torque. You'll enjoy it over the 2.5 and you'll never know what you're missing in the 4.0. The 4.2 has a pump on the engine so you only need a pickup tube. Might as well swap the whole tank over unless it's a 15 gal.
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Unread 03-11-2013, 06:46 PM   #4
Djsthe1
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Thanks man, that is the reason I decided to go with the 4.2. After having the 2.5 anything is an upgrade. I was curious if it needed a fuel pump or not, even if its a 15 gallon will probably swap it in. Does it need a return fuel line?
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Unread 03-11-2013, 07:05 PM   #5
Michaelgoesrawr
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Quote:
Originally Posted by Djsthe1
Thanks man, that is the reason I decided to go with the 4.2. After having the 2.5 anything is an upgrade. I was curious if it needed a fuel pump or not, even if its a 15 gallon will probably swap it in. Does it need a return fuel line?
Yeah it needs a return line. I only mentioned the pump because the one in your tank is not needed. I suppose you could use the pump anyway and regulate the pressure needed for the carb. I think anyhow.

Did you grab the skid too? I'm not entirely positive your 2.5 skid will work but you can always drill new mount holes. Typically how people swap motors is they bolt it all together and drop it in, get the trans where it needs to be and then burn the mounts in. I could get you measurements from my 4.2 as a basis on where your mounts need to be if you'd like.
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Unread 03-11-2013, 07:13 PM   #6
Djsthe1
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Yes I did get the skid, mine is rusted out and dented in. That would be awesome man, hopefully I can them off with out an issue, if not a buddy said he could fab up a set.
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Unread 03-11-2013, 09:39 PM   #7
cnorth5863
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i too am interested in doing this swap..please keep us updated as you do the swap.
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Unread 03-12-2013, 12:58 AM   #8
Michaelgoesrawr
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Quote:
Originally Posted by Djsthe1
Yes I did get the skid, mine is rusted out and dented in. That would be awesome man, hopefully I can them off with out an issue, if not a buddy said he could fab up a set.
Sorry I forgot about this when I got home. Tomorrow sometime for sure. It may be easier just to fab new ones. Maybe with a 1" lift to help drive line angles if you decided to lift it or of you decided to do a tummy tuck. You could always get brown dog motor mount lift though. Just throwing ideas out there.
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Unread 03-12-2013, 02:25 AM   #9
Djsthe1
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no problem man. I what's yours about lifting the motor mounts, I'm running a 4 inch lift with the transfer case drop and I keep replacing motor mounts. are you running an electric fan?
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Unread 03-12-2013, 11:05 AM   #10
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Quote:
Originally Posted by Djsthe1
no problem man. I what's yours about lifting the motor mounts, I'm running a 4 inch lift with the transfer case drop and I keep replacing motor mounts. are you running an electric fan?
I think motor mount lift would help but I'm not sure. You're getting into sye territory now. No I don't have an electric fan. It's in the plans someday. An alternator upgrade plus wiring upgrade is needed. There are plenty of threads if you need more info.
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Unread 03-12-2013, 11:11 AM   #11
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Quote:
Originally Posted by Djsthe1 View Post
Thanks man, that is the reason I decided to go with the 4.2. After having the 2.5 anything is an upgrade. I was curious if it needed a fuel pump or not, even if its a 15 gallon will probably swap it in. Does it need a return fuel line?
the pump is on the side of the block... is mechanical.... yes it has a return line.... this is a dumb swap especially if your not carb swavy..... if your gonna do it and take the time and money do a 4.0 even if the 4.2 was free.... find a 4.0 and build a stroked version and swap that.....

to do this ans swap a 4.0 later when you dont like the carb is pointless, your going FI to carb back to FI...hmmm 3 times the work seems to me

unbolt the 4.2 from the trans and tcase u have and boplt a 4.0 up to them
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Unread 03-12-2013, 12:13 PM   #12
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The electric pump should work fine, you will need to regulate it down to about 3psi.
The elctrical is way different. I don't know what they gave you for the engine wiring harness but 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.

The coil and ignition setup is way different but if you got the complete setup then it is not that bad.

Giggity is right about the 4.0L swap being much easier as far as the wiring goes but the rest of the stuff from the 4.0L to the 4.2l are going to need to be done anyways; motor mounts,tcase, trans, and shafts would all need to be swapped on the 4.0L. So it is the wiring that is different and if you start by running the nutter bypass and maybe a HEI setup it is fairly simple wiring. The other thing he mentioned is the carb. The stock carbs are problematic. The nutter bypass gets rid of the computer control but if you are lucky enough the engine you got has one of the after markets on it already.
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Unread 03-12-2013, 12:41 PM   #13
Djsthe1
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I understand all the issues and reason people go with the 4.0 but if I upgrade I don't see me staying with a inline 6. As for it being a carb, I understand that going FI to carb is backwards but I would rather have a carb. If I upgrade after the 4.2 I would stay carbed, maybe a 4.3 or a small block. I could always swap in the 4.2 and if I don't like it swap in the 360 out of my J10
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Unread 03-12-2013, 12:53 PM   #14
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The carb on the motor is not the original, Im not sure which one it is I haven't started researching it yet.
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Unread 03-12-2013, 01:50 PM   #15
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Those two are good help too. I'll go get those measurements when my phone is more charged. I'll add them to this post.

Passenger and driver. Hope it helps. The end of the tape is touching the firewall at a level spot. Probably not exact.
image-3042381764.jpg   image-3007930400.jpg  
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