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Smoking 3.9, just got a 5.2, have some questions

1555 Views 5 Replies 3 Participants Last post by  Deeblo
Hey all, I acquired a 94 Dakota extended cab Sport 5spd with the 3.9L. Well, I drove it home, fixed a few electrical gremlins in the dash, figured I'd check the radiator to see if it needed to be topped off, only to find the truck dumping it out. So, I moved the truck from my garage to the side of my house, and scored an amazing deal on a 5.2L from a 98 jeep grand cherokee. I also got the PCM from it, unfortunately the jeep had an auto trans, so I know I'll need a.. flexplate I think? for the V8. However, since the PCM is from an obd2 vehicle will it even work with my dakota?
Also, if not, I've read quite a few posts say something about wiring in the 2 additional injectors, but after scouring all 3 pages of posts on here I can't seem to find a definitive answer. This isn't my main vehicle, just one I want to get up and going to play with, so down time isn't a big deal. And on a level of 0 (novice) to 10 expert) I feel comfortable saying I'm solidly in the middle when it comes to mechanic work. I've built a chevy 350, swapped a hyundai motor and trans, replaced the clutch on an acura integra, and tinkered with a few other vehicles, so I'm not afraid of getting my hands dirty.
To do this swap, I've read the 5.2L bolts right in where the 3.9 was, but what else will I need to replace?
Since the engine is out already, I planned on doing all new belts, and probably pulling the valve covers off and putting new valve cover gaskets on. I know I also need to drain the oil, as it was tilted pretty far back to get it out of the jeep (not my doing, I bought it already pulled.) and likely has coolant mixed throughout.
Is there anything else I should be worried about?

Thanks all!
Pic of my Derpkota for attention.


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The camshaft and crankshaft position sensors will have to be swapped from the 3.9 to the 5.2, as the OBD-II sensors will not work with the OBD-I magnets. The PCM will also not work with the engine or trans, as the transmission is hydraulic actuated unlike the 98 Jeeps trans. The only electronic control on the trans is the overdrive and torque converter lock up.
There really isn't a wiring diagram to go off of to add the extra injectors needed to run the 5.2 wit the 3.9 harness but if you have access to a factory service manual or a Haynes manual, you could go off that. Or, try to find a wrecked 5.2 Dakota from 92-95 but these trucks are now over 20 years old and they are not going to be very common in the junkyards these days.
Yes, the 5.2 will bolt up to the 3.9 trans but IDK if the motor mounts are the same. And, the 3.9 trans was the 42RH, which is not as stout as the 46RH the 5.2's came with. You can run it but if you plan on having some real fun(drag racing, road courses, etc.), then you will need to find a replacement 46RH some time down the road if you don't have the funds available right now.
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Thanks for the thorough response.
What I have is the manual, so no worries on the torque converter or anything along that nature, though I've heard the auto's bolt up just fine.
In terms of the pcm, do I really even need it then?
I'm also lucky because there's a junk yard about 25 minutes away with a wrecked 94 dodge dakota 5.2 v8, so I'm sure I can get whatever parts I need off it, but I didn't want to chance a junk yard motor when I got this one with ~120k miles on it.
Would grabbing the pcm from the junk yard 5.2 94 dakota mean no changes in wiring, or would I still need to find a way to add the 2 wires for the injectors? Wiring isn't my forte, unless it comes down to audio stuff.

So one more thing I've noticed while trying to research this as much as possible, is that the oil pan from the 318 may not fit in my 94, any clue if that's true or not? I've already got the flex plate coming, I plan on ordering all the sensors as I take them out.
With the engine supposedly only having ~120k on it, would it be worth pulling the heads and doing new head gaskets while it's out of the engine? Never done head gaskets before, so another question would be how to do them properly.. can I just take the valve covers off, pull the heads off, clean the surface, put a new gasket down, and pop the heads back on? Or do I need to remove the springs and whatnot?
Thanks again!
The 5sp in your Dak is an AX5 which is a very weak trans and the 5.2 will toast it if you plan to do any real gear slamming. I killed one with the V6. The Jeep oil pan does seem to have too large of a sump area, the Dak's are very small. I would also suggest you get the PCM and complete wiring harness from the V8 Dak at the yard, Ive know a couple of people try to wire in the last 2 injectors and ended up grabbing a harness due to issues. Yes pull the heads and have them magnafluxed. Mag V8's are VERY prone to cracking between the valves. There was a class action lawsuit started shortly after Mag's came out but it never got anywhere. Ive pulled no less than a dozen Mag V8 and 6's apart and only had one good set of heads.
Thanks for the response FazDak.
I lost out and the dakota wasn't there when I went back.
However, I happened to be there last Friday looking for a mazda 626 for some parts for my daily driver, and they had another 94 V8 dak.
It had already had a lot picked off, but I was able to grab the entire intake manifold, fully assembled, so throttle body, injectors, fuel rails, etc all there. All the sensors and wiring, too.
However, there wasn't a PCM there, and it was an automatic, not the manual.
I don't intend to race the truck, so I'm not so worried about the ax5 holding up or not, though everything I've ran across online said 94 dak's with M/T's had a couple options, the AX15, and NV3500 are the most common, with the NV3550 being the next, so I don't know which trans I actually have, and it's been put on the back burner since I got a new project car (83 camaro z28) to work on.
So many vehicles my wife is going insane... but I sold her 98 grand cherokee, got the mazda 626, so until I get the camaro going, I get a 626 to drive around while she drives my 2011 jeep wrangler.
Once the camaro is going, I'll sell the mazda and work on getting the dakota going, and then switch between the 2 depending on the day. I figure the dakota will be peppy enough I won't need to roast the clutch, and with the camaro, I'll have a quick vehicle already... so the Dakota will be more just leisure driving... and truck stuff, no heavy offroading since I have a built wrangler for that.
Anywho, sorry for the wall of text!
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