My buddies and I have taken an interest in paintball tanking (driving a "tank" around on the field while playing). We started with a riding lawnmower but it's time to go bigger.

I just picked up a 1987 Dakota 4X4 with the 3.9 (carbed). It seems like a solid little truck, but in the interest of making repairs and diagnostics easier, I want to simplifiy the drivetrain as much as possible. This means getting rid of the air/smog pump, all the EGR stuff, the electronic spark control, etc. I know this is a bad idea for a street driven vehicle, but this is going to be towed to and from the paintball field, and only driven in the summer.

I don't care about losing some horsepower/torque/drivability, I want this thing to be simple, basic, and reliable. It's going to spend most of it's life cruising around in 4-Lo and 1st gear.

Unfortunately, I'm not very knowledgeable about carbed engines. I haven't worked on one in many years, and spent most of my time learning EFI stuff instead. I wanted carb for the simplicity, but I've got some learning to do.

So first things first, the truck seems to run fin for about 5-10 minutes, then loses power and dies suddenly. It's able to be restarted immediately and it's good for another few minutes. I've done some research and it sounds like it could be the 9V battery for the emissions control stuff.

I will try swapping that out, but I'd like to just strip all that out of the engine bay. However, if I strip it out, will I have problems with the spark control computer or the mixture control solenoid on the carb? I haven't found which parts I could use to replace the distributor with a basic vacuum/mechanical advance one.

The carb is a Holley 6280, with the feedback system. The Holley 2280 is the same thing without any mixture solenoid. I've read that I can "convert" the 6280 to a 2280 by disconnecting the wires and screwing the 2 screws that the solenoid controls all the way in, is this true?

I also want to remove the evap canister, I don't think I'll run into any electrical issues doing that. It's just hanging loose under the truck asking to be ripped off by an obstacle.

Also, the oil light comes on at low RPM/idle sometimes. In my experience this usually means loose tolerances inside the motor, and time to replace rod/main bearings. Apparently on the Dakota it's fairly common that the sender is faulty? The motor seems to run and idle fine, but there's maybe a slight knock when you rev it up a bit and let it fall back to an idle, as the engine is falling I can just hear something. Is that normal for this motor? I'm more used to newer OHC motors, not sure if this is a regular thing for a cam-in-block style motor.

Thanks, and looking forward to driving this around!

 

The emissions CONTROL doesn't have a 9V battery. If it has it, the EGR MONITOR has a 9V for battery backup of the RAM (as opposed to later years using NVRAM).

I'd remount the carbon canister properly - or at least with big zip ties! - and keep that, it costs nothing really. If you ditch it, be sure to figure out SOME way for the gas vapor from a heated tank (in the sunlight) to get out!

It'll be hard to find a points-style distributor, since Chrysler never made one for the 3.9 - but some careful melding between an old 318/340/360 LA block distributor and the 3.9 one may make one that'll work for you.

As to the oil light - I'd tee the sensor with a mechanical oil pressure gauge (left under the hood, possibly mounted on the firewall or one of the inner fenders!) and see what pops out there.

Do remember that the water pump is driven off the AIR pump and/or the power steering pump - you may want to leave that on even if just for that.

RwP

 

Well, a bit of a long overdue update:

We pulled off all emissions related stuff and plugged all the vacuum lines we could find. Truck starts and runs fine, no more dying. Wouldn't recommend this on a road-going vehicle but it's fine for my needs.

Next issue: It was indeed a bad rod bearing causing the low oil/faint knock. We pulled the pan and replaced just the one bearing (cylinder 1) and hoped it would hold together till the end of the season, which it did. The crank was slightly scored, to the point where you could see it, but you couldn't feel the scores with your fingernail, which I've been told is borderline.

But shortly after replacing the bearing, the knock came back. So I either need a new crank/rotating assembly, or a new junkyard motor that runs a bit better. It's nearly impossible to find these 3.9 carbed V6s, but can I just grab the later model TBI LA V6, or a Magnum V6, and use my intake/exhaust manifolds? Even if I had to swap heads from my motor to the newer V6 that's not difficult. Just not sure if there's changes made to the blocks themselves that makes this impossible.

 

I "think" you can use a 87-91 engine and all will bolt up. The heads are the same on those years.
As far as using the magnum, the intake bolts differently. On V-8 conversions, some re-drill the heads to match the old style intake. The distributors will interchange, that is between the LA and magnum motors. Also, I think you will need to go with an electric fuel pump on the magnum motor as well as use all the magnum accessories.
Again, I have no experience in converting to the magnum but have been reading as much as possible on the subject. But my plans are to to go to a magnum v8 on my 87.

 

I did some checking on Rockauto, and the intake gaskets are definitely different between LA and Magnum, unfortunately. Head gaskets are listed differently, but it looks like LA/Magnum heads are interchangeable?

I could always just drop in the Magnum v6/v8 and use megasquirt or something, but I was really trying to keep wiring and complexity to a minimum. Magnum V8 is definitely the easiest one to find, and all engines cost the same at my local junkyard.

 

What I meant was the 87-91 engines (long block) are the same. But the magnum has a lot of differences. No fuel pump provision. Will need to have the heads re-drilled for the old style intake. I think the accessory drives are different, os if you go magnum, you want to make sure to get the accessories with your motor and try to get one from a Dakota. .
Just my opinion, but I would think you would be better off finding a LA motor (87-91) then trying to convert unless you are after the extra power of the magnum. There are going to be a lot of little things to modify, change, adapt, etc.
Have you considered a remanufactured motor for your truck? Just doing a google search, I found a few on there for $11xx.00 and up.

 

The GOOD news is that if you keep your carburator, most any of the 1988-1991 LA 3.9's should swap right out.

At worst, you'll have to swap the intake also. But I don't think so (although that "thinking" may have very little to do with the real world.)

To use the Magnum, the only three main differences are:

1) Electric fuel pump due to the lack of a fuel pump opening in the timing chain cover.

2) Redrill the Magnum heads with the proper angle to take the LA style intake (the angle of the bolts are different.)

3) Use the distributor out of your old motor, or a compatible one, instead of the Magnum one.

That just about covers it.

Down side, most of the extra power in the Magnum is due to the better intake ... although, a 1992-1993 Magnum 3.9 will also have beefier exhaust manifolds, so you can use the Y-pipe out of one of those and run the larger exhaust. That'll help SOME.

RwP

RwP

 

Thanks for all the info, the reason I'm asking is I found a '99 Magnum 3.9 for $250 locally, with 50k miles on it. Sounds like a good deal, I just don't want to get into a big project to install it. I'm thinking I'll try to swap it over to carbed using my original intake, as well as throwing on the distributor and a fuel pump. If that doesn't go well, I'll just use a megasquirt, as I've got several extra hanging around.

 

I'd go for the Magnum intake and the megasquirt then.

That extra power is a nice addition *grins*

RwP