For 91 - 94 models (and 95 Type-R only) that are non-DBW, the fast idle target is around 1,500rpm depending on the coolant temperature.
Once fully warmed up, the target idle rpm is about 800rpm for MT.
At lower rpm range, it is fairly normal to have around 100rpm offset on our gauge cluster due to the calibration done through the pot.
It’s cross coil vector based with needle having its own weight so depending on which way the needle approached, there is always some reading difference.
Also, there is no temperature compensation circuit.
If you have very stable idle rpm after fully warmed up, just use that position as the 800rpm (MT) instead of the indicator lines on the dial face.
With the cold engine, you should see somewhere around 1,300 – 1,500+ rpm on the [ 1st ] fireup of the day unless you live in extremely hot tropical weather all year round.
The Fast Idle Valve (FIV) is not electronically controlled.
Instead, it’s just mechanical relying on the expansion/contraction of the wax element.
The wax element is held inside the bottom chamber where I’m holding it with my finger.
It’s submerged in the coolant.
At the very top, if you remove the brass coloured flat plate/cap (should replace o-ring if opened), you will see white outer sleeve with the brass plunger inside.
In this photo, you can see the oval shaped opening at the 12 o’clock position.
(Better view in the 1st photo.)
This opening is connected to the extra air channel located at the TH body.
Same channel used for the EACV as well as the base idle air adjust screw.
If you have ever cleaned the TH body, you will notice small circle opening at about 11 o’clock position when looked at the TH body from the air box side.
That’s the entrance of the extra air channel and the air is pulled by the vacuum of the intake manifold.
Back to the FIV.
When the engine is cold, there is a small gap (just like in the 2nd photo) between the brass plunger and the white outer sleeve. Thus, the extra air from the oval opening can enter the intake manifold (via the circule opening - 1st photo) through this gap under vacuum.
As the coolant gets warmer, the wax element expands and pushes up the plunger towards the white sleeve.
Eventually, the plunger meets the internal wall of the white sleeve resulting in good seal.
No more extra air sucked into the manifold through the FIV and the EACV is the main controller for this air channel.
[ [ You are not supposed to disassemble the FIV any further. ] ]
I did it at my own risk.
The white sleeve is actually threaded into the FIV metal body.
So, by rotating it, you can adjust the initial amount of the ‘gap’ against the plunger under the cold wax element.
Thus, this will change the target fast idle rpm.
You can’t test the gap adjustment if the coolant is even slightly warm so most likely, forced to wait after parking overnight before trying the new adjustment.
So, again, don’t disassemble FIV unless you know what you are doing.
The workshop manual states DO NOT disassemble.
Unlike EACV, once the engine is warmed up, there is no air flow inside the FIV so fairly clean even after so many mileage.
However, seen some failure with it in the past.
Though, normally, it’s the other way round and the idle rpm stayed extremely high even after fully warmed up.
Instead of taking any risk, if you are happy to replace the o-ring, just remove the top brass flat plate from the FIV while the engine is cold and look for the tiny gap between the white sleeve and the centre brass plunger.
If you start the engine under this condition, you should be able to feel the vacuum air flow by covering the gap with your finger.
If no vacuum, that means the plunger got stuck closed (very unlikely as it’s against the spring load) or the wax element failed.
Kaz
