What to do when the car feel sluggish and gear jerk

After years of driving suddenly you feel the car becoming sluggish and the gear starting to jerk especially when reverse or braking while on gear D. This is common behavior for most cars when it is getting old. So, what is first thing to check when it happens. 

The most basic thing to look at is the battery connection - make sure nothing loosely tightens and if so, tighten it accordingly. Loose battery connection can be the main culprit in sluggish car as the engine sensos such as.

MAF - Mass Air Flow Sensor

MAP - Manifold Absolute Pressure Sensor

IAT - Intake Air Temperature Sensor. 

TPS - Throttle Position Sensor

ISCV - Idle Speed Control Valve

ECT - Engine Coolant Temperature Sensor

CMP - Camshaft Position Sensor

CKP - Crankshaft Position Sensor

OCV - Oil Control Valve Solenoid

Knock Sensor

As well the spark plugs, fuel injector, fuel pumps

The sensors and components above require efficient electrical signal to communicate them between Engine Control Unit (ECU / ECM / PCM) effectively.

Meanwhile on the transmission side the gearbox solenoids and sensors require efficient current to communicate with the transmission control unit (TCU). 

Therefore, make sure the battery connection is tightly.

Redundant nipple valve on the K3 or 3sz-ve engine block behind the catalytic converter

Earlier I was wondering what the redundant nipple valve, sticking out of the 3sz-ve engine block. Even daihatsu forum has no answer for this so I've searched and found the mechanic manual book for K3 engine and it turns out that is the cooling water drain plug. Not sure what is the common function of the nipple valve since the coolant drain can easily be done via the radiator drain plug and for the record that valve rarely being used by anyone in the lifetime servicing their K3 engine.

Idle RPM not going up when A/C turned on and it cause the car to vibrate (shaking)

This is quite an intriguing problem that I've not yet do some experiment. The idle is running at 750 RPM without accessories on especially the air conditioning system. Now the interesting part is when the A/C is on the idle RPM refuse to increase and stays at 750 RPM that result in the car to shake (vibrate) feeling like almost stalling. 

The problem is becoming moe intense when braking on traffic light while on gear D and when the A/C compressor kicks in the vibration becoming even more because the idle speed stays around 750 RPM. Why won't the RPM also increase dynamically based on the engine load? 

Suggested solutions:

1. Idle Speed Control Solenoid - dirty / malfunction

2. Adjust the throttle body cable to get more air?

3. Air conditioning gas is over charged thus the compressor magnetic coil consume more power to engage

Will need to experiment this one by one and see the outcome.