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1994 Dakota 3.9 to 5.2 swap questions

607 Views 23 Replies 4 Participants Last post by  cwkerr007
I've recently become the new owner of a '94 Dakota. 3.9 V6 M/T 4x4. For a truck that's approaching 30 years old, it's in great shape! However, it has 230k+ miles on the original 3.9 V6, and I'm seeing some leaks and hearing some noises that hint at an engine rebuild in the future.

Or... after doing a bit of research, it looks like the 5.2 V8 is a pretty easy swap! I have several other projects, so this will probably be a gradual "grab parts as they turn up" process, not a sprint to completion. However, I wanted to run through what I (think I) know now, and maybe get some feedback from folks to help plan this out.

1. 5.2 engine. We have a few decent yards nearby and I can get a running engine from a wrecked truck for pretty cheap. Ideally I'd just freshen up the top end. It looks like anything from 92-96 will work? Since the 3.9 and 5.2 are very similar blocks, can I retain all my accessories and accessory brackets from the 3.9?

2. ECM from a 5.2 M/T 4x4 truck (PN 56028263). I've seen a few of these remanufactured for about $300. I'm comfortable re-pinning my harness for the two additional injectors (I've built a few harnesses from scratch). I have a 1994 FSM on the way for wiring diagrams.

3. Exhaust. I'll probably pick up a set of factory manifolds with the engine. I can fabricate the rest of the exhaust myself. Will probably do a y-pipe, cat, muffler, and single exit - as close to factory as possible (also means I can use the same hangers).

4. Front suspension. Here's where my lack of knowledge about torsion bar setups shows. I know on a truck with front springs I'd need heavier springs for the extra weight of the V8. Do I need to swap parts with a torsion bar setup, or just adjust the bars?

5. Electric fan, radiator, and shroud modification (or removal). I think an efan is the way to go (makes for better throttle response). I believe I'll need a radiator from a V8 truck (but please correct me if I'm wrong). Also, I'm pretty sure the V6 radiator shroud won't work without modification, but I can figure that out.

I know some folks might chime in with suggestions for more power (cam, 1.7 rockers, dual exhaust with headers, etc etc) but I'm planning to retain the original transmission (which should be the AX15). I'm not trying to put down huge power numbers or hit the strip/track, so the AX15 will do fine behind the 5.2. I like wrenching, and I figured for a bit of extra time and money I could have a neat project AND a more capable truck.

Thanks in advance for any feedback or info!

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So for the torsion bars I can say they use the same piece. Just need to adjust the tension some although your truck looks like its been pretty close to maxed out. They don't normally ride that high from the factory, its already been adjusted. I'm curious to see how they fixed the camber at that height because I'm looking at a fully adjustable upper control arm from SPC to fix mine (93 dakota) I'd like it that high but there isn't enough adjustment in the control arm to bring camber back into a decent spec.

The radiator for the 3.9 and the 5.2 is the same part .
The fan shroud however is longer on the 3.9 because the engine is obviously shorter so you'll have to trim it back or remove it. This is easy if you going electric fan as you can just remove it. Going to an electric fan is a big benefit for low end power and something I plan on doing to my own soon, just make sure you get one that moves enough air to cool properly.

The AX15 is a pretty decent trans and will hold up to a non modified 5.2 just fine you could still do exhaust and a high flow throttle body for a little more OOMPH and it will hold. Any more then that and you'll probably need to upgrade to the NV3500. You will have issues finding a replacement or rebuild kit for that trans though. I lost 1-2 gear in mine (split my fork at the pin) and took me a year of searching (part time) before I found a place that would even look at one. Wish I could remember the name of the place....it was 5 years ago. Now I'll have to go look through my files when I get home.

Everything else it seems you have a handle on.
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Although I did buy a set of 1.7 RRs for the 3.9, maybe I just want it to fail faster so I can seriously contemplate this swap!
You might be hard pressed. Those 3.9 would go forever. When I worked at the dealership in town I saw more of those old 3.9l trucks with 300k+ on them (some 400K) and still running like a top they just don't like to die.
Appreciate the tip on the AX15, I know they're popular with the Jeep guys so I'm surprised rebuild kits aren't available.
Whereas they are both AX15 in the core the gear ratios are different, but the biggest issue is the tail housing is different. You can get bearing kits for them but that's about it mine needs a shifter shaft bushing that simply isn't available anywhere. Drive-able but lots of play in the shifter. The tail housing on the jeeps don't have the same mount studs for the 4x4 shifter.
I did note the compressor is only running for about 2-3 seconds at a time - is that normal for these trucks? I didn't have time to drive anywhere, and I'm thinking ambient temp is so close to the desired vent temp that the compressor just doesn't need to run much to hit that temp. On the other hand, I also know the compressor won't run for very long if the refrigerant charge is low. I only have one of the "refill" gauges (no actual numbers, just green/yellow/red zones). I'm in the bottom quarter of the green now - think I need to add more, or is now the time to actually invest in a proper gauge? I'm not well versed in AC systems, so any input is appreciated!
The AC on these old trucks are fun. Mine still has the old R12 as its a 93. 94 is when they went to 134a???? I'll have to look. If yours had R12 in it to begin with it won't get very cold the 134a just isn't as efficient and less so if its in an R12 system. Those DIY refrigerant gauges work OK as long as you don't over fill it into the red. More refrigerant doesn't necessarily mean colder.
The compressor will cycle that's normal 3-5 seconds would be normal ON time though 2-3 seconds means there's an issue (low refrigerant or too much). Also those gauges are supposed to be with the engine running.
Also, I know this got a bit sidetracked from the subject of an engine swap, so I'll redirect a bit: I've got my FSM in, and I'm going through the wiring diagrams. Based on what I'm seeing here, for my additional two injectors I need to:

1. Tie into the ASD relay with the other six injectors
View attachment 112084

2. Run wires from the new injectors to pins 17 and 18 on the PCM connector
View attachment 112085
View attachment 112086

Is that really it? Seems almost too easy, right? I know I'll need a PCM for a 5.2 MT truck, but it looks like the hardest part will be repinning that connector for the two additional wires.
yes it really is that simple. crazy right?
Seems like you have it all sorted.

Have a fun summer
i put an electric and a 4 core rad in my 88 3.9 i changed the fan 3 times and it still gets to hot with the ac on in stop and go traffic let me know if it works i nput a hayden on and it wasn't much better than the cheap one. So i'm going dual fan with custom shroud when i put the magnum in it .(y)
Try using a fan off a crown vic. Cheap and effective. They move a lot of air and are usually readily available for cheap at junkyards.
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