I've got this 1992 Ranger extended cab 4.0 automatic that has 100,000 miles on the engine and transmission. It runs good and drives good no major issues. It's a good strong truck. It gets 18.5 MPG. Since I got it I've done plugs and wires using Motorcraft parts because it felt like it didn't have any power at all. Then I did a fuel filter, fuel injection service. Then I changed the PCV valve, o2 sensor, and changed the quiet flow muffler to the 17730 Dynomax muffler that comes in the Dynomax cat back kit. It helped a lot. Then I followed that with a BBK 66MM throttle body and that made the most difference. This thing will take off and run like a small V8. It pegs that 85MPH speedometer with no problem. Although the computer doesn't know what to make of the TPS and MAF values. Sometimes it flashes for literally like 1 second and goes off and then you can drive it 1000 miles and it never comes on. No one makes a cold air intake kit that I've been able to find so I'm just dropping in a stock replacement high-flow K&N type filter. I know there is headers available but I didn't want to have to mess with the exhaust since all the pipes are new. What about this AGR power steering pump that is suppose to take up less power? I do need a pump, mine is whining. Any pulley kits? High flow MAF? Any good chips available? Before I added the throttle body my 98 2.5L could out accelerate and outrun this truck. Now I don't think so. I know on my 92 4.0 the 3.08 8.8 rear doesn't help off the line much either. So what kinda crazy things are people doing to the 4.0s?

 

Ok, back up right now.

The initial things you did are all good. Maintenance is the best thing you can do.

Then you get on to the throttle body. A bigger throttle body might make a little difference, but thats a deception. Basically the bigger throttle body gives you more throttle opening with less throttle, which gives you faster response. However a stock 4.0L motor as is can't flow as much as the stock throttle body, so running a larger one does not actually give you more power, just more response with less throttle. Now in theory that can be to your advantage, because since the TPS values and actual throttle opening are no longer lined up, then the computer thinks you are under less load, and will use more timing. The downside is your tendency to ping has increased, a problem the 4.0L OHV is known for, and the overall engine longevity has been reduced.

On the cold air intake, the vehicle came stock with a well designed one. It outflows the motors potential, so again, all an aftermarket one will give you is more growl because the sound reading qualities are removed. So no performance gains. You can run one if you want, it is a matter of preference. However, I would take that K&N filter back immediately. Not only will a good quality paper filter last longer before it has to be changed or cleaned, but it outflows the engines needs. The K&N filters firstly, don't do a very good job of keeping dirt out of the intake. Thats the whole point of having a filter. Secondly, the oil from the filter is known to foul the MAF sensor, which will cause a decrease in performance and economy. The cost of cleaning and replacing the MAF sensor will make sure you never save money either, since between the cost f the K&N, the cleaning kits, and the damage it causes, you will find you spend MORE money than just buying a quality disposable paper filter.

Headers will buy you a little. If you get shorty headers they should bolt up like stock. I don't think its a very good gain for the money, but they do outperform stock manifolds.

On the power steering, drain the fluid out and replace it with MerconV. That should make it quieter. You won't see much gain running the AGR pump, but it might be a worthwhile swap if your old pump is leaking.

The stock MAF is high flow, again the stock intake components outflow the motor.

There is nothing chip wise that will produce more power without severely reducing engine longevity and increasing pinging potential. Ping will kill these motors. If you do want a way to tune, you can go with Jaybird or Quarterhorse. I would prefer the Quarterhorse, since it will let you extract the stock tune, then alter it, and it lets you tune in realtime.

If you want to build power with a 4.0L you have to modify the engine itself, since its biggest limiting factor is the heads and how much they flow (or don't flow). Things like cams, porting, etc. are what is required to produce any meaningful amount of power over stock. There are a couple cam options out there. I think cams can be swapped without removing the heads, but you have to remove the motor, or at least remove enough from the front of the vehicle to get the timing cover off and be able to pull the cam out. I would pull the heads anyway, since at your mileage, the heads are probably about toast. They have issues with receding valves, and the solution is to get inserts installed before they start to crack. The valve inserts solve many of the engine longevity issues. i would think with the proper internal upgrades, you can build a 4.0L to the 220 - 240 hp mark. Stock is 145-155 hp for your year range. Again, it boils down to money, the 4.0L is not a cheap motor to make power with. Thats why most people who want Rangers that move either do V8 swaps, or 2.3L turbo swaps. Much better return on investment.