I did some mods to my 1998 3.9 engine , starting with a good tune-up, plugs, wires, distributor cap and rotor, the usual stuff, and then I kept going. I did not want to hot rod it , or add turbos, but more reliability, and possibly more power, with good starting in winter weather, was the plan.
I had replaced the plenum gasket on my 5.9 truck, so I thought I would replace the gasket on the 3.9 as well. I removed the intake plenum, and then got carried away. I decided to replace the timing chain, since I had the engine half apart already. The quick tune-up turned into a 2 day performance, oh well. I ordered up the gaskets, a timing chain set, and a chain tensioner, and pulled the front cover. The chain was a bit stretched, but not too bad, and I installed the new tensioner. The tensioner is a great part, some of these engines come with one, but they all need one.
I replaced the chain and put the front cover back on and took a look at the intake plenum. It was very oily inside, and the gasket showed signs of movement with an obvious leak. I used an aluminum plenum replacement plate, it is stiffer then the thin stamped steel plate. I bolted this up with the new gasket, and replaced the plenum. Goodbye leak.
So far not very expensive, just lots of work. I cut my own plenum plate out of 1/4 inch aluminum scrap metal, and the gaskets and timing set are cheap. Speaking of cheap, I had a V-8 throttle body, from a 96 truck, so on it went. I transferred the parts from the V-6 body to the V-8 part, and hooked it up. I got it all together and started the truck up.
It ran fine , but not really any different, so, I decided to try one more mod. I removed the timing sensor from above the flywheel, in a hole in the bell housing, and slotted the holes, to advance the timing. This advances the timing about 2 degrees, possibly more, but I did not want to make the holes too big. This sensor is fairly easy to remove, but hard to get back in, because you cant see up in there that well. This mod you could sure tell made a difference, more power.
The timing advance gave it some more low end, and the throttle body should let it have some more air at higher revs, and it is better starting.
After I drove it about a week, no problems, no leaks, I gave it to my daughter, and she has run it now for 3 months, very reliable, starts easily, doesn't use any oil, not bad for 360,000 (kms).
I only spent about $ 200, and got a better running truck. These trucks are easy to work on, just like a 318 with 2 cylinders missing.
It is running much stronger now than when I bought it, and the price was right.