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1994 Dakota 3.9 to 5.2 swap questions

769 Views 25 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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Thanks for the excellent response Zaan!

One thing I should have mentioned - this truck has what appears to be a 2-2.5" body lift. I'm guessing the torsion bars are actually at or near the factory setting.
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With that in mind, I'll just measure the front height with the V6 in, then adjust the torsion bars to hit that height with the V8 installed. Torsion bars still sort of seem like magic to me!

Thanks for confirming my thinking regarding the electric fan and radiator shroud, and confirming the radiators are the same. I may still replace the radiator depending on how the coolant looks, but it's nice to know they're the same part and mount the same way. I still haven't decided what electric fan I'm using (I know they make aftermarket kits, but I'm leaning towards repurposing an OEM fan to keep this low-budget). Once I have that figured out I'll share it in this thread. I'll probably build my own fan controller.

Appreciate the tip on the AX15, I know they're popular with the Jeep guys so I'm surprised rebuild kits aren't available. Or did you mean rebuild kits aren't available for the NV3500? I do like the NV3500 (I have one in my '02 Dakota) but if the AX15 will hold I'll keep it. I'll also replace the clutch while I have everything apart. I don't have any service records and for all I know it could be the original clutch!

And regarding the engine, I may do some minor upgrades there too. I think that between the transmission and the overall weight of the truck it would be dangerous to add TOO much additional power, but... hmm, maybe dual exhaust off the OEM manifolds and a hand-ported OEM TB? Not really looking to invest in a cam, roller rockers, or any serious upgrades. 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!
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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.
Well, it will probably take me a year or so to get all the parts together and the 5.2 freshened up, so I'm not hoping for the 3.9 to fail right away. I also have two other project cars to finish up before I can really tear into this one. I'll still need the 3.9 for awhile!

I am aware of their reputation for longevity but from what I know of the person who owned the truck two owners ago I'm willing to bet a lot of regular maintenance was neglected. Just making sure I have a backup plan.
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.
Gotcha, that's good information to have. Mine seems solid right now, shifts very smooth and there's almost no play in the shifter. Hopefully it stays that way, but if not I wouldn't mind swapping to an NV3500 when I do the engine swap.

As an aside, I got my registration and plates today. The first batch of tune up parts has arrived (I'm doing oil/filter, air filter, front pads, wipers, plugs/dist cap/rotor, shocks, and a partial brake fluid flush). I'm also going to be reworking the stereo - it works, but one speaker is blown and the wiring to the other one only has an intermittent connection. I'll be hauling slash to the slash yard this weekend for sure though.

I've also noticed that the AC seems to work! I say "seems to" because the line under the hood gets cold, but I'm not getting cold air out of the vents. All the actuators work (I can toggle floor/blend/dash/defrost vents). I removed the cowl and sucked out some leaves and other debris, but I can't work the hose down into the area behind the dash. I'm thinking something might have built a nest around the evaporator, and air is being drawn in around the evaporator instead of passing through it. Am I on the right track here? Anything I can do to test that theory or clean that area from the outside? I'm looking for any option that would save me pulling the dash apart, I can do the work but MAN do I hate tearing into dashboards. I'd almost rather live without the AC.
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You say the tube is getting cold but is it sweating and when the ac is on do the vent blow as hard as the heater ? If you or short on coolant that core will freeze up and stop airflow. Just giving you an option to look at good luck hope u get it 😎
Excellent question on the tube, that steered me in the right direction! I do recall it wasn't sweating, but it's so dry up here I didn't think that was unusual. Went out with my temp gun today and the tube was about 48 degrees with the AC on for 10 minutes. I added half a can of R134a in small increments, and the tube is now about 38 degrees and sweating slightly. The vents are blowing cold at about 45 degrees, although it's only about 56 degrees ambient right now so that's not super impressive. Hopefully it will be warm tomorrow (haha just kidding, forecast is a high of 47).

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!
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Thank you both for the replies!

I don't know much about AC either, but all those cans of refrigerant always warn about compressor damage if over serviced.
immattj, thanks for the feedback there. I know overfilling is a risk, but I'm assuming I'll be safe with the refill kit I have as long as I stay in the green zone. It might be nice to get an actual numerical readout with a proper gauge, but I'm not seeing any in my price range (maybe next month after I get paid).

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.
Zaan, mine's R134a based on the truck's body sticker and the 94 FSM, so I think it's safe to say 94 is the cutover year. I am testing and filling with the engine running so no worries there (thanks for pointing that out though). If 3-5 seconds is a more "normal" on time, I'll check the time more accurately to see where I'm actually at. We still haven't had a really warm day up here though.

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
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2. Run wires from the new injectors to pins 17 and 18 on the PCM connector
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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.
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yes it really is that simple. crazy right?
So simple I can't justify putting off the upgrade any longer!

I've sourced a used 5.2 for $550 with ~200k. Complete from TB to oil pan, with the factory exhaust manifolds and injectors. Only thing I won't have are the accessory brackets - but I think those should be the same between the 3.9 and 5.2?

I'm also doing some research on common 5.2 issues and things I'll need to look for and address. The big ones seem to be the intake plenum gasket and camshaft position sensor. I'll plan to replace both of those, but is there anything else folks would recommend doing before I drop the engine in the truck?

I've blown through all my discretionary income for the next couple months, but I do have a source for a 5.2 MT ECU, which should be the last major piece I need (well, unless the clutch is shot). I'll use that time to pull the pan and intake and see if there are any major issue with the engine, and maybe get a start on the wiring (I don't see any reason why I couldn't wire the two new injectors with the 3.9 still in the truck so they're ready to go for the swap).
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i have two 94 5.2 auto for parts both run so if you need anything you can have it better to reach me through text so you'll have to pm me but one last thing if you have to do the clutch make sure you have a shop surface the flywheel or you will have a shudder when you release the clutch hope you get her goin like you want !!! sorry for slow response i only get on line at home after work
Thank you a ton for the offer! I'll go through the motor I bought and if anything's missing or damaged I'll reach out. It came off a running totaled truck, so I don't think I'll have to do much - but it's good to have a source of parts just in case.

Regarding the flywheel, that's one point I missed in my original post. Both flywheels have notches for the crank position sensor, but the design is different on the V6 vs V8 flywheel (six sets of notches vs eight notches). I'll need to swap to the V8 flywheel to make the truck run right. I have the engine and a 5.2 4x4 MT PCM, so I just need the clutch, flywheel, pilot bushing, throwout bearing, plenum gasket, camshaft position sensor, injector plugs (if you have any of those let me know), and fluids.

Now's a good time to talk efan swap too! I'll be using the following setup:

Flex-a-lite 16" 2200cfm fan: https://www.summitracing.com/parts/flx-105387
Flex-a-lite Adjustable fan controller: https://www.summitracing.com/parts/flx-121281
Inline sensor bung: https://www.amazon.com/dp/B018FY12CW/

I know I said I wanted to use an OEM fan, but after thinking it through I'd rather use new hardware (the mounting should be easier too). I've got plenty of scrap metal (including some 22ga sheets) to build a custom bracket and shroud for the fan. If this works well on my 94, I may do the exact same efan swap on my 02.
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i don't have any of the manual trans stuff but i do have the inj plugs as a matter of fact you could have the whole wiring harness and pcm all mine are carbed but you do have something i need . I have a 97 tj jeep and i want to put my 360 out of my rv in it but i need a bell housing off of a dakota to match to my trans so do you have an extra bellhousing????
Unfortunately I don't have an extra bellhousing, I'm sticking with the AX15 that came in my 94 so I'll need that bellhousing for my build. The AX15 should hold up to the 5.2 (I'm not doing any racing or offroading, more of a trash and slash truck) but if it ever fails I'll swap in a NV3500 and I'll save that bellhousing for you.

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)
How big was your fan? I'm going with a 16" that should move over 2200cfm, 2000cfm seems to be the recommended threshold for a stock 5.2 so I think that would be fine. I'm surprised you had overheating issues with an efan on the 3.9, unless it was a much smaller or worse-flowing fan?

I have an old data logger I may adapt and put on the truck before and after the swap to track temps, I'll let you know if I do that. That would give me concrete info on how well the electric fan worlks. I'll try logging:

1. 3.9 with clutch fan
2. 3.9 with efan
3. 5.2 with efan

hell you can have the inj and the plent i'm just keepin the motor for my 89 ram 2500 throttle body and granny gear it gets 21mpg but it's so ugly my kids won't ride in it ( a tree fell on the cab) but i couldn't bel the fuel mileage it gets amazing!
That's fantastic mileage! I can relate on my truck, everyone in the house thinks it's cool but nobody wants to be seen in it haha. I'm only getting about 18 in my 94 with the 3.9 right now, but all my driving is short trips around town.

If you want to send those parts my way I could definitely use the harness, that would probably be just as fast as repinning my existing harness and then I could swap back to the 3.9 easily if needed. I just picked up a PCM but I have no way to test it until I get the swap done, so an extra would also be handy for testing. Shoot me a DM and we can work something out!
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thanks for the fan info i'm not a ford guy do ya know about what year crown vic so i can make sure my salvage yard has one ???
Looks like that's the fan for a 2006-2011 Crown Vic, if that helps. I'm no expert, just peeked at the compatibility list at that link (there are some Lincoln and Mercury models which use that fan as well).

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