4AGE 20V vaccum location

1. Brake vacuum from side of throttle 1 behind butterfly (big hole)
2. Fuel return to tank (entry) [from regulator]
3. Fuel return to tank (exit) [to fuel tank]
4. Brake vacuum from side of throttle 4 behind butterfly (big hole)
5. Idle Speed Control Valve (ISCV entry)

The brake vacuum is taken from throttle 1 & 4 via a side port. They are summed by a T-joint before connected to the brake servo. Since throttle 4 is far from brake servo hence it is connected to another pipe then to the T-joint (not shown).

1. Brake vacuum
2. Fuel input
3. ISCV input
4. Pressurized output for air-cond idle up valve
5. Pressurized output for power steering idle up valve at steering rack & pinio
6. From point 11 (hidden behind point 3) pressurized
7. From point 10 vacuum of throttle 4 - going to Throttle Opener
8. To Charcoal Filter
9. Fuel return to tank (exit) [to fuel tank]
10 vacuum to point (suspect to point 7

In order to get very stabilized "balanced" vacuum, drill extra hole pointed by the circle below for optimum performance.

Individual throttle body vacuum matching / balancing via mechanical fine tuning through valve opening. The valve limits the amount of air that by-pass the butterfly.
There are two holes behind the butterfly.

Idle Speed Control Valve - output then finally into intake via the red circle below.

Notice the old 4 throttles (shown below) does not have the opening (shown above) and that the injectors output spray is significantly hindered when comparing 4 throttles from generation year 91 against year 93 June till 20vMAP edition. The year 93 unit is so much better.
But this does not mean the old 4 throttle does not have all 4 ISCV holes. They are hidden from current view, but shown by the 4 purple arrows.

1. Brake vacuum from side of throttle 1 behind butterfly (big hole)
2. Fuel return to tank (entry) [from regulator]
3. Fuel return to tank (exit) [to fuel tank]
4. Brake vacuum from side of throttle 4 behind butterfly (big hole)
5. Idle Speed Control Valve (ISCV entry)

Red color

1. ISCV
2. ISCV
3. ISCV
4. ISCV

Pictures below courtesy Mr. Wong Yee Yong (forum member www.zerotohundred.com)

ISCV holes from 1st generation ITB.