I had awful gas mileage, lost of noises, lacked power in the peddle, and it just ran like crap & no fuel gauge.
I changed the plugs, wires, cap, rotor, switch plate, Oil pressure sensor. O2Sensor, PCV valve, TPS and coil and I adjusted the TV cable.
It got better but still failed to run as well as the sound of the motor and the power I could feel in it said it should. Then I discovered two TSB's 09-05-00 and 09-07-97. The intake plenum (the symptoms explain most of the problems I described previously) and Timing chain. Apparently the sheet metal OE plenum on an aluminum intake was a bad choice. It ends up leaking oil into the intake. The TSB fix is to change the gasket which leaves you open to the problem happening again. The permanent fix is to get a kit that supplies an 1/4" aluminum plenum with new bolts and gaskets. I had the timing chain cover and intake plenum tanked before reinstalling them and soaked the TB at home myself. They were black with carbon and I also got new intake manifold bolts.
I used Hughes Engines plenum kit and a double roller timing chain and the tensioner designed for the fix as well as a suggested gasket kit which comes with an extra/different WP gasket. The hardest part as you may know is getting all the stuff off the motor to be able to get at the intake and TC.
The timing chain was installed OE with no tensioner, this is corrected by adding a new chain and tensioner.

Now it has great power & no lifter or timing chain noises. Gas mileage started increasing a lot when I did the O2 sensor adding the IAC and MAP sensors when I did the intake plenum also did a lot to help mileage. Still haven't fixed the fuel gauge problem.

If you want to do this you need a full set of sockets & wrenches SAE and Metric. You should remove the entire fan (1 7/16" nut) and clutch on the 3.9 as the shroud won't come off over the fan clutch. and you need a steering wheel puller to remove the balancer a regular gear puller that grabs the outside will destroy it.