DBrowns 4.0L Swap

This write up is intended to give general information to everyone planning on swapping a 4.0L into his or her Bronco II.

When doing this swap I did a lot of research before jumping in. Everyone that reads this should understand that this is designed to work as a guide, not an instruction manual. This swap will go a lot easier if you have help from someone experienced in pulling a motor or tearing one down.

Usually the ideal thing to do would be setting the trucks side by side. This will allow you to notice differences in the placement of everything. In my case it was not possible so I had to take a lot of pictures with a digital camera, mainly of the wiring harness. Take as many as you can. These pictures will help you to recheck the harness after you alter it.

I would also recommend doing some research on the years of the motors you are looking at. There are some differences in the 4.0L over the years. I would recommend getting a motor made in or before 94. I think that is the year that Ford changed the system to dual O2 sensors.

Here are some other things you need to think about before tearing in to the swap.


The first thing I did in this area was check to see if the stock fuel system could support the needs of the 4.0L. In my case it could. I was swapping my 4.0L from a 93 Aerostar mini van into an 86 Bronco II with a stock 2.9L. The 4.0L needed a range of 80 to 100psi from the fuel pump. The 2.9L.s fuel system was rated at 85 to 100psi, so the only problem I had to worry about was the fuel lines. This is a very important point of safety. Make sure that when you make these connections that they do not leak. I did some .creative hose clamping. but you might want to grab the fuel lines from the donor to make this easier.

Clutch / Flywheel

You have a couple different options in this area. You can use the 2.9L flywheel. If you chose to use the 2.9L flywheel you must use a 4.0L starter for an automatic transmission. The 2.9L and 4.0L flywheel for the automatic are the same width and the same number of teeth. You MUST use a 4.0L starter. The 2.9L starter will not work. If you chose to use this flywheel you will loose some of the advantages of the 4.0L. The flywheel for the 4.0L is larger so it handles the torque better. This also goes back to the year motor you are swapping. They changed the number of bolts used on the flywheel in 1997 from a 6-bolt pattern to an 8-bolt pattern.
2.9L flywheel


There are a lot of people that say you can use the 2.9L Y-pipe but in my case I couldn.t so I had to have one made. I think this is only a possibility if the motor comes from an explorer or a ranger with the 4x4 option.
4.0L exhaust

Wiring / Computer

This is where every one gets scared, but really it.s not that bad. The first thing every one should do is get a Chiltons manual for both the donor and the truck you are swapping the motor to. The Haynes manuals have the wiring diagrams but they are hard to fallow because they fail to give you wire colors and they are just all around hard to read.

When pulling the harness and computer on the donor, take everything! You don.t know what you will need. Once you have the harness out you can start searching threw the manuals and make the connections necessary for the motor to run.

The best way I could tell you to start this is handle one system at a time. Start with fuel, then go to main power, and so on and so on. Once you have the motor control part of the harness set up then take out the wires you don.t need. I.m talking about the headlights, turn signals, and stuff like that. Your harness should now be coming together.

Finally go threw and double check every wire, grounds, and clips. Another helpful hint is to take masking tape and label the clips so you know exactly where they go, and don.t for get to remember the layout of the under hood! This could cause you to have a clip that doesn.t reach its destination. Another thing that you need to make sure you do is solder the connections and heat shrink them to make sure you don.t end up with a bad connection later. I would give you specifics but I.m afraid that if I do people will just start plugging wires so if you have any questions please submit them to the GENERAL 4x4 TECH part of SHOPTALK.
4.0L wiring harness

Motor mounts

When swapping the 4.0L into the Bronco II that had a 2.9L you can use the 2.9L motor mounts. This is the choice I made because I didn.t want do and altering of the cross member they rest on. When using the 2.9L motor mounts on an Aerostar motor you must cut the passenger side mount to allow the exhaust manifold to work. Here is how I did mine Weeks before I started the swap I researched this completely. I did this so I would have a good idea of what I was getting into before I tore the truck apart and realized something wouldn.t work.

First day

Pulled the 2.9L, cleaned out the doghouse, and started on the harness.
Pulled 2.9L

Second day

Found out the motor hasn.t been pulled from the Areostar. With plenty of time to kill I dropped off flywheel to be machined, picked up new starter, and a new clutch. I then worked on the harness for around 2 weeks waiting for the motor to be pulled. Learn from my mistake! Wait till you have everything in front of you before you start. I had a hoist on rent and a borrowed truck for this entire time. That was money I didn.t have to spare.

About two weeks later

Swapped the flywheel, installed a new clutch, and pulled stock exhaust threw the doghouse. I basically pulled the catalytic converter and factory y-pipe but left the high flow muffler. I also relocated the new battery in the back to hold the 4.0L. Then, Finally, the motor went in!
battery box

Next day

Pulled the clutch fan, installed the altered harness, installed new starter, and made the new exhaust. The next thing I did was because the motor was sitting for awhile. I pulled the plugs and squirted a little oil on top of the pistons, and put in new plugs. This gives them a little lubrication for the first start. Remember my motor sat for 6+ mo. I then turned the key on a few times and let it stay on for around 5-6 seconds each time to let the fuel pump bring up pressure to the fuel system. Then for the first time I started the beast!


From this point on I have been cleaning up the doghouse more and making small changes in the on board air. The wiring on mine was not perfect the first time but it was close enough to run so don.t get mad if you have some gremlins in the harness. If you have any questions please divert them to the GENERAL 4x4 TECH forum in SHOPTALK so everyone else can see the question / answer and learn more. Good luck on your swap!
Bronco II 4.0L conversion