Short correction first: the Toyota 5L‑E is a diesel engine — it uses glow plugs, not spark plugs. Below is an ordered replacement procedure with the theory at each step and how the repair cures the symptoms.

1) Theory reminder
- Diesels ignite by compression; glow plugs heat the combustion chamber/air at startup so injected diesel auto‑ignites more readily, especially when cold. Failed glow plugs give long cranks, white/grey smoke on cold start, rough idling after start, higher emissions or a glow‑plug fault code.

2) Preparation (why)
- Tools: deep glow‑plug socket or correct hex, ratchet + extension, torque wrench, multimeter, small wire brush/penetrating oil, replacement OEM glow plugs, optional anti‑seize.
- Theory: correct socket prevents rounding and breakage; OEM plugs ensure correct length/heating characteristics.

3) Safety & initial steps (why)
- Work with engine cold. Disconnect negative battery terminal to prevent shorts while handling electrical connectors.
- Theory: prevents accidental power to glow circuit and reduces burn risk.

4) Access the glow plugs (why)
- Remove any obstructing components (engine cover, air intake ducting, wiring brackets, possibly intake runner components on some fitments) to expose the top of the cylinder head and glow plug leads.
- Theory: glow plugs are screwed into the head, usually near the injectors; clear access prevents connector damage and allows correct torque application.

5) Label and disconnect electrical connectors (why)
- One at a time, disconnect the electrical lead to each glow plug (or the rail harness) and label if necessary.
- Theory: prevents wiring mixups and allows bench testing of the specific plug if needed.

6) Loosen and remove glow plug (how + why)
- Apply penetrating oil to each base if corroded and let soak. Use the correct deep socket and turn steadily counter‑clockwise. Remove by hand once loose.
- Theory: glow plugs can seize; slow steady torque reduces risk of snapping the plug (a broken glow plug tip in the head is a costly repair).

7) Test the removed glow plugs (why)
- Multimeter: check DC resistance — glow plugs should show low resistance (single‑digit ohms; will vary by type). Bench test by briefly applying 12V to confirm the tip/heater element glows/hot.
- Theory: a failed plug shows open circuit or very high resistance and/or won’t heat; this confirms which plugs actually failed vs wiring/relay faults.

8) Inspect wiring and supply (why)
- Check the harness for damage/corrosion and measure supply voltage at the connector during the glow cycle (key on, or while cranking if applicable) — you should see battery voltage controlled by the glow relay/ECU.
- Theory: many “bad glow plug” symptoms are actually bad wiring or a failed relay/ECU output. Replace wiring/relay if they fail.

9) Install new glow plugs (how + why)
- If using anti‑seize, use only a light film and reduce final torque accordingly (follow OEM guidance). Thread new plugs in by hand to avoid cross‑threading, then tighten to manufacturer torque. (Manufacturer torque is low — typically small Nm; use the spec in the service manual; if not available, tighten gently with a torque wrench — avoid over‑torque.) Reconnect electrical leads.
- Theory: correct seating ensures good thermal contact and consistent heating; over‑torque can strip threads or break the plug.

10) Reassemble and reconnect battery (why)
- Reinstall removed components and reconnect negative battery terminal.
- Theory: restores engine to serviceable state and re‑enables diagnostic checks.

11) Functional check (how + expected result)
- Turn key to glow position and verify glow indicator behavior (if fitted). Start engine cold and note cranking time, smoke, and idle. A correct repair will shorten cranking time, reduce white smoke on cold start, and give smoother idle immediately after start.
- Theory: new plugs raise combustion chamber temperature during the pre‑heat cycle so injected fuel ignites reliably and uniformly across cylinders; that fixes cold‑start misfires and incomplete combustion (the symptoms you had).

12) If symptoms persist (diagnosis)
- If new plugs test fine but symptoms continue, check glow relay/timer, ECU control, battery voltage/charging, injector spray patterns, and compression. Any of these can mimic glow‑plug failure.
- Theory: glow plugs only address pre‑heat. Poor fuel atomization, low compression, weak battery, or no control signal will still cause starting and combustion problems.

How the repair fixes the fault (summary)
- Replacing failed glow plugs restores the pre‑heating function so each cylinder reaches a temperature where injected diesel ignites at the proper time during compression. That eliminates the cold‑start under‑temperature condition that causes long cranking, white smoke (unburnt fuel), rough running after start, and related emissions/fault codes.

Notes/risks (brief)
- Avoid breaking a glow plug — extraction of a broken plug from the head can require cylinder head work. Use correct tools and patience.
- Always use the correct replacement plugs and torque to spec.

Done.
rteeqp73

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