Son is looking at a 97 Dakota reg cab 4x4 with 3.9.
I have zero experience or know anyone who has had one w/3.9.
Research brings up many spark knock issues. Some say intake gasket, some say PCM reflash, some say change thermostat to different temp.
Reason I ask is truck has mostly sat for last few years and only has 70k on it. Owner said that it starts hard but does run. We haven't went to look at yet. Price is actually pretty decent but just concerned if it may be a money pit.

Only problem I ever had with a 3.9(besides the lack of power and getting the same fuel mileage as the 5.2) was sludge build up on one. Was also a low miler, I think the problem was a lack of maintenance and the old man who owned it only ever used it for short trips around town. Feel the underside of the valve cover under the oil filler cap area for build up.

Drive one every day in my 2002 Dakota. 195,000 miles. Oil changes every 5000 with wix filter. Just basic tune up stuff. The starter and water pump have been changed. 3.9 Has been good to me.

Good engine. Most v6 trucks got 3.92 gears, most V8 got 3.55 gears so MPG was close (maybe 1 mpg better in the V6) in most of them, if you compare two trucks with the same gears the V6 will usually knock down about 2 more MPG unless you are hammering it all the time, drive it like a V6 not a V8 and they can get better MPG.

I have never heard any spark knock in my 97 V6 auto 4x4 with 3.92s. even with the auto and lower gears gets a smidge better than my 99 318 3.55s 5 speed 4x4.

The 5.9 is the one with the bad spark knock.

The 3.9 in my '93 Dakota was my first dealings with this engine.

I googled a lot of info and asked questions here before pulling the trigger on mine a year ago.

One thing I came across was that if you look down into the intake thru the throttle body and see wetness, the plenum gasket and/or the intake gasket is bad. Hughes sells an improved plenum kit that solves both issues. I had to do mine. Not hard to do, just a little time consuming.

I've also read that if its sucking oil because of this it can cause pinging.

If the plenum is bad, give it and the throttle body a good cleaning. Might also change the oxygen sensor.

I gave mine a tune up with what I thought were good wires from Advanced, and they were crap. I switched to wires from Summit and an MSD coil, and added platinum plugs. Runs like a new truck now.

Almost all of my driving is in town, and I usually drive it like a rental, and I get around 14 MPG. My one hwy trip I averaged 20.

My truck has 162K on it right now and burns almost no oil between changes.

Its not the fastest thing on four wheels that's for sure, but, its not a flat out pig either. I have no issues keeping up with (or blowing by) traffic.

If you plan on putting mondo mudders on that 4X4 you might want to hold out for a v8.

I'd buy another.

The 3.9s are good motors.
At 70K, I would be surprised to find the plenum gasket bad, but a look down through the throttle body will answer that question. If its hard starting, could be bad. I think I would also look at the condition of the fuel lines, and the brake lines, they had a tendency to rust through about at that age.

I would change the thermostat, just because its a cheap problem solver on may levels, but, don't buy the cheapest one you can find. Might not be a bad idea to back flush the cooling system & install new antifreeze. If you end up pulling the intake, do the thermostat and flush at that time.

I'd probably also start out with fresh plugs, cap, rotor & wires, pull & clean, and reinstall the tps, and replace the air filter & change the oil.
Reason for all this is because I believe a 70 K motor that is "hard starting" probably hasn't been very well cared for, and I'd rather do all this stuff before I started driving it. Gene

Looked at yesterday. Wouldn't idle at cold start. Feathered gas for 30-45 seconds then would on it's own and good warm. Guess is IAC or TPS needs cleaning or replaced.
Exhaust needs replacing, all of it. Heard low battery voltage could lead to this too.
Mileage is a legit 67k. 1 owner but now in hands of daughter, who is selling. Truck is pretty clean with the exception of both doors having a 2 inch rust problem along bottom that need addressing asap.
Best part is it has manual windows, locks and mirrors. Has a weird slop in steering going straight but is good around corners, is this normal?

And is the manly color of Fuscia or Magenta. I like it and above problems could be easily fixed, along with a complete tuneup.
Definetly been sitting 2-3yrs like they said. Has that scaly rust on leafsprings and frame. Sheetmetal is very clean underneath tho.
Asked them their lowest and didn't match mine. Did a little research and will probably undercut them a tad and see if they bite.