Manual engine has no power in third fourth and fifth gear when driving up an incline.

This is not a problem. This is perfectly normal. Unless your car has immense power, you will need low gears to climb steep inclines.

The higher the gear, the lower the effective torque available for acceleration. This is part of the trade-off gears give you:

• Low gears give you acceleration/torque so you can pull away from a stationary start and tow loads
• High gears give you a higher top speed

If one gear was perfect in all situations all cars would only have one gear.

First of all This is not a problem with your vehicle its general physics and applies to all vehicles

• When your vehicle is in neutral there is no load on the drive train thus allowing the RPMs to reach its limiter easily, when you are in gear , the drive train has an opposing force of the road which is working in the opposite direction thus restricting the RPM climb. (If you have a Supercar or Superbike then the RPM will touch the limiter but in an average auto mobile it does not have enough power when in gears to reach high RPMs)
• No matter what you do your car will not climb an incline in the 3rd or 4th gear as much as it can when its in the lower gears. That is the whole point of having a gear system. Lets dig into some science for further explanation.

In a vechicle there are two things which determine motion "Power and Torque".

Horsepower determines how fast a vehicle can travel, torque at the wheels determines how quickly that speed can be reached. The greater the torque figure, the faster the acceleration.

The easiest way to picture what torque is, is by loosening a wheel nut. Try to loosen the nut by pulling on the spanner at the same end as the nut (very little torque) and you'll struggle to loosen it. Pull on the spanner at the end of its shaft (maximum torque) and you'll loosen it quickly.

So , gears 1,2,3 have higher torque but less power allowing you to move from a stand still gears 4,5,6 have higer power which help in rolling acceleration once you hit a particular speed.

• Answering your question : while climbing a hill you need more torque thus being in the 1,2,3 gears allows you to climb more efficiently compared to highers gears which require you to be in at least 40mph to work efficiently.

You DO NOT NEED TO CHANGE ANYTHING on your car to make it go faster while on an incline just be in the lower gears, reach a proper speed then shift up and you will be fine, This is by design and working principle of an auto mobile and not a flaw.(Unless you have like 700+hp)

The question does not specify what speeds are being attained or what grade of incline is being climbed, so this is a generic answer;

If the vehicle cannot accelerate anymore it is due to a lack of power. Basically the resistances against the vehicle such as the weight of the vehicle against gravity on the incline, and aerodynamic drag have used all of the engines available power.

The only solution for more speed is to reduce resistance or increase power.

Check the flowmeter. If it is faulty, it gives wrong message to the fuel intake and therefore not enough fuel enters the engine.

Try downshifting for more torque = acceleration... this is typically an issue w/ALL cars when going up an incline... you'll need to be in a lower gear in order to accelerate regardless, cause you're powerband (your MOST efficient power/torque ratings) @ 2800 RPM (4cyl) / 3400 RPMS (6 Cyl) to accelerate

As for other 4-Runners & cars passing you, are they also almost 30 years old w/far lesser HP & torque ratings as well? Do they have the same engine and/or know what gear they're also in? Cause the issue may just be that your '91 4-runner doesn't have the same power available as newer cars to begin with. If yours is the 4 cyl, well then you've only got 115 HP & 140 lb-ft torque @ 2800 RPM to begin with... which isn't much for a heavier 4-runner up an incline (OR w/the V6 not all that much better @ 180 lb-ft torque @ 3400 RPMs)

So sounds to me like the issue is what gear you're in when going up the incline. Especially if it's the 4 cyl @ 4000 RPMS, as you've well surpassed the 2800 RPM powerband for most efficient power & acceleration. Do you have an issue accelerating anywhere else when driving normal, or just on inclines? If it's just inclines, then I can almost guarantee it's your gear selection