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Jeep YJ 4BT Cummins build!!

106K views 218 replies 62 participants last post by  theend2008 
#1 ·
hey guys thought i'd jump on jeepfourm and show y'all my yj build.im new to this site so bare with me.ive got a 1988 jeep yj it was 100% stock when i bought it at age 16.ive been slowly buying things for it,my plans are to drop a cummins 3.9l 4bt into it.so ill post some pictures of my build and how im going to do it.feel free to comment and give suggestions along the way :cheers2:
 
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#160 ·
Got steering hooked up,and throttle cable fabed up.yesterday nothing was going my way.it was one of thos days where a 15 min job took over and hour.but ill be working tonight too.hopefully I can get more done.hoping to be able to test drive it next week at the latest..I need to weld front perches,put shocks on,bleed brakes,put radiator in,make some power steering tubes,hook up boost gauge,and water temp.and then it should be done,long night at the shop tonight and I may be able to get all that done..
 

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#161 ·
Dont put your return line there. I put mine there and I could not keep it from pissing air into the tank. I tried extending it also. I had to put it in the neck so it could roll down the filler tube and let the air get out of the fuel before it hits the fuel in the tank. There is alot of air in the fuel from the pump in the return line and it just amplifies the air in the tank the longer you run it.

I had my tank down three times before I decided to do it like that.

The air in the lines would make it hard to start after sitting all night.

I have a p pump so you may or may not have the same return pressure.

Also when its like you have it in the pic when you get low on fuel it is pissing the fuel and air mix right at the pick up and makes it ten times as bad mine would start running rough when I got below a 1/4 of a tank.


Is that pinion bearing gona get lube up to it running that much angle?
 
#162 ·
Gaducker said:
Dont put your return line there. I put mine there and I could not keep it from pissing air into the tank. I tried extending it also. I had to put it in the neck so it could roll down the filler tube and let the air get out of the fuel before it hits the fuel in the tank. There is alot of air in the fuel from the pump in the return line and it just amplifies the air in the tank the longer you run it.

I had my tank down three times before I decided to do it like that.

The air in the lines would make it hard to start after sitting all night.

I have a p pump so you may or may not have the same return pressure.

Also when its like you have it in the pic when you get low on fuel it is pissing the fuel and air mix right at the pick up and makes it ten times as bad mine would start running rough when I got below a 1/4 of a tank.

Is that pinion bearing gona get lube up to it running that much angle?
Thanks alot.ill go back and change that when I get the chance..and I'm not sure I put a little more fluid then needed.hopping it might help a little,the picture makes it look like more of an angle then it is..even if it is a bit high,how big of a problem is that.and is there anything to fix it?
 
#163 ·
Worked a bunch this week got a lot done.unfortunately my phone camera is not taking pics like it should,they are really blurry.so didn't get to take any good pics of what I've done.ill up load some,just got to get around to getting the camera out and setting at the computer..ok so had some problems with clutch it was way to stiff and short,had a guy named stev'o come out and look at the jeep.he told me my master cyl ARM was at a weird angle,so I straightened that out,and it works a little better now,then got the rest of my wiring done,intake air filter fabd up.radiator brackets made and installed.radiator hoses connected,front spring perch welded,shift boot and floor plate made up..so I've basically only got,shifter,front drive shaft,bleed brakes,put rear shocks on,and throw the fenders on intill I get some tube fenders..other than that its ready to roll some coal.! Test driving it Monday or Tuesday.ill keep you updated.here's one pic that wasn't to blurry.

Motor vehicle Electrical wiring Automotive exterior Vehicle Gas
 
#165 ·
FINALLY!! the cummins lives and drives.it is a beast.1st is like wow,2nd is like a jet,and 3rd is just plan insane..so much power and torque..deff need new gears tho I have 3.07 gears with 37" tires..going to break something if I keep driving like that.but any ways.so much fun.had ppl across the parking lot just stare with a Wtf look lol..ill post a video 2maro!
 
#166 ·
congrats!

Take it out on the highway before you make a decision about those gears! Mine screeeeams at 60 mph, I cant wait to re-gear it, and I am probably going to go with 3.08 and 35s
 
#167 ·
youngjeeper11 said:
congrats!

Take it out on the highway before you make a decision about those gears! Mine screeeeams at 60 mph, I cant wait to re-gear it, and I am probably going to go with 3.08 and 35s
Yeah your probably right.ill take it for a spin first..I'm thinkin 4.10 gears and 37's.. just cuz this is going to be my dd and weekend rig. I think 4.10 would be the best of both worlds.
 
#169 ·
Man, I must just drive a lot faster than anyone else haha, but the reason I want to re-gear so high is so it can be my DD if I want it to. Right now I have 4.10s, but only 31s, and I feel like I need two more gears on the highway haha.

I am going to put 35s on it before I re-gear it, but I just cant imagine ever needing more torque haha, so I would rather keep my RPMS low and get good mileage on the highway. We have 4Lo for a reason right haha.
 
#171 ·
Awesome build man!!! Jeep is looking good!! I wish I had some of the fab skills you do.
 
#174 ·
Gaducker said:
I have three fifty fives and thirty fives with a nv forty five hundrend and I swear i could run a hundrend and twenty mph with out a problem.I have had it up to 90 mph on the interstste and it had PLENTY left in it.
Aweesome! Have you had anything break or wear out with that set up.?cuz that's what I'm worried about with mine.I love the torque and speed but just havnt ever messed around with gearing and angles before.not sure what to expect.
 
#178 ·
Man that is a huge undertaking. You've definitely got a well built machine there, makes me want to tackle a 4bt swap one day. Keep up the good work!
 
#180 ·
CumminsYj88 said:
Yeah either going to make some or,buy a pair. If you know of any good looking tube fenders let me know,most of the big brands I don't really like for some reason.
Check out C & C fab they are a small shop in Az and you can buy them from the shop or on eBay. I have their hood line front tube fenders.

Edit: sorry for the double pics, it won't let me delete one of them.
 

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#182 ·
I want to do this very badly. Question, you mentioned buying a " bread truck" earlier in the build. Can you give me a brief background on that, I'm a newb with little experience when it comes to rebuilds. The thing is defintely bada**! my buddy at work is all about putting a cummins in his now too.
 
#183 ·
You buy a bread truck so that you can have a donor that has the 4bt in it already, and possibly a transmission to use as well. It is good to keep around until your swap is complete because it may have little odd parts here and there that may be hard to find. It eliminates some of the foot work of having to track down all the parts, and they're usually aluminum bodies so when you're done with it you can scrap the body and make some of your money back from buying it and doing the swap!
 
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