Short answer first: If your Toyota B / 2B is the diesel B-series, it does not use spark plugs — it uses glow plugs. Replacing the correct type of plug (spark or glow) restores proper ignition/starting and combustion. Below are ordered procedures with the theory for each action and a short explanation of how the repair cures common faults.

A — If it’s a petrol engine with spark plugs (general procedure and theory)
1) Preparation
- Tools: socket set with appropriate spark-plug socket (usually 5/8" or 13/16"), extension, ratchet, torque wrench, spark plug gap gauge, dielectric grease, compressed air, anti-seize only if manufacturer allows.
- Theory: correct tools and clean working area prevent damage to plug threads and foreign debris falling into the combustion chamber.

2) Cool engine, disconnect negative battery terminal
- Action: work on a cold engine; disconnect battery negative.
- Theory: thermal contraction avoids seizing/stripping threads; battery disconnect prevents accidental arcing while removing ignition components.

3) Clean around plug wells
- Action: blow out dirt with compressed air around each plug well before removal.
- Theory: prevents dirt from dropping into cylinder when the plug is removed (which would cause scoring or hydraulic lock).

4) Remove ignition lead/coil
- Action: remove one coil-on-plug or spark plug lead at a time (do not mix).
- Theory: keeping replacements one cylinder at a time preserves firing order labeling and prevents swapping leads/coils between cylinders.

5) Remove old plug
- Action: use plug socket and extension; back out slowly and evenly.
- Theory: allows inspection of plug condition (color, deposits) which indicates combustion health (rich/lean/oil/coolant intrusion).

6) Inspect old plug and cylinder behavior
- Action: look at electrode color: tan/gray = normal; black sooty = rich/fouled; oily = valve guide or rings leaking oil; white/ash = lean/overheating.
- Theory: plug condition diagnoses root cause; replacement fixes symptoms but you may need further repairs if plugs show oil or coolant.

7) Check and set gap on new plugs
- Action: verify correct gap spec for your engine (typically ~0.7–1.1 mm for many Toyota petrols); adjust gently if needed.
- Theory: correct gap ensures optimal spark energy and timing; too wide causes misfires, too narrow lowers efficiency.

8) Install new plug — hand-start then torque to spec
- Action: thread by hand to avoid cross-threading, then torque to manufacturer spec (typical range 18–25 Nm / 13–18 lb‑ft; check manual).
- Theory: proper torque ensures thread sealing and correct heat transfer; under- or over-torquing can damage threads or reduce heat-sinking.

9) Reinstall coil/lead with dielectric grease on boots (if recommended)
- Action: apply a thin smear of dielectric grease inside boot, reconnect coil/lead, reconnect battery.
- Theory: grease prevents moisture ingress and makes future removal easier; correct boot seating ensures good electrical contact and prevents arcing.

10) Start engine and verify
- Action: run engine, listen for smooth idle, check for misfire codes with OBD scanner.
- Theory: replacement restores consistent spark delivery to each cylinder; solving misfires improves idle, power, fuel economy and emissions.

How replacing spark plugs fixes faults (summary)
- Faults: misfire, rough idle, poor economy, hard start, higher emissions.
- Cause: worn electrodes, fouled insulator, incorrect gap or intermittent electrical contact reduce spark energy or timing.
- Repair effect: a new plug restores proper spark voltage and timing at the correct gap; combustion becomes complete and stable, eliminating misfires and their symptoms.

B — If it’s a Toyota B / 2B diesel engine (glow plug replacement; diesel theory)
1) Understand the difference
- Theory: diesel engines compress air to auto-ignite fuel; glow plugs preheat the combustion chamber for cold starts. They are NOT spark plugs.

2) Preparation
- Tools: correct glow-plug socket (often deep 8 or 10 mm), torque wrench, ratchet, penetrating oil if corroded, battery disconnect optional but recommended.
- Theory: glow plugs are in the head and can seize; penetrating oil and correct tools prevent breakage.

3) Cool engine and disconnect battery (negative)
- Theory: cold engine reduces risk of snapping and battery disconnect prevents shorting while working with high-current circuits.

4) Clean area and disconnect electrical connector
- Action: clear debris and unplug the electrical connector from each glow plug, one at a time.
- Theory: prevents contamination and ensures you can trace which plug is replaced.

5) Remove glow plug
- Action: unscrew slowly; if stuck, apply penetrating oil and allow time. If a glow plug snaps, special extraction is required.
- Theory: avoiding breakage prevents cylinder head damage and costly repairs.

6) Inspect and test new vs old
- Action: measure resistance of old and new glow plugs (old should be low ohms for good plug); compare. Install new plug by hand, torque to spec (typical ~20–30 Nm; check manual).
- Theory: glow plugs heat by electrical resistance; high resistance or open circuit means poor heating and hard starting. Proper torque ensures sealing and contact.

7) Reconnect electrical and repeat for each cylinder
- Action: replace one at a time and reconnect.
- Theory: prevents wiring mistakes and enables correct diagnostics.

8) Test start-up and glow operation
- Action: turn the key and observe glow plug light/operation or check with scan tool; engine should start more easily in cold conditions.
- Theory: new glow plugs reach temperature quickly and evenly, allowing reliable cold ignition and reducing white smoke and excessive cranking.

How replacing glow plugs fixes faults (summary)
- Faults: long cranking, cold-start smoke, hard starts in cold weather, rough cold idle.
- Cause: worn/open glow-plug elements fail to preheat cylinders, leading to incomplete combustion on startup.
- Repair effect: new glow plugs supply required heat for fuel to ignite at cold temps, restoring reliable starting and reducing unburned fuel smoke.

Final practical notes (concise)
- Always replace one plug at a time. Torque correctly. Work on a cool engine. Use correct parts and gap/specs from the service manual. Inspect removed plugs — they give diagnostic clues and may reveal underlying issues that replacing plugs alone won’t fix (e.g., oil or coolant in cylinder).

Done.
rteeqp73

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