Cooling System Troubleshooting Guide

Toyota Prado 90 Series
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Introduction

This guide provides troubleshooting steps for cooling system issues, specifically addressing overheating under load, coolant overflow problems, and diagnostic methods for head gasket or cylinder head failures.

Problem

The engine temperature gauge rapidly rises into the red when driving uphill or under load at highway speeds (around 110 km/h). The system pushes coolant into the overflow tank but does not draw it back into the radiator when cooling. The temperature drops quickly to normal when idling and drops low when coasting downhill.

Potential Causes & Diagnostic Steps

  1. Airflow Obstruction: Check for obstructions in front of the radiator, such as debris, bent panels, or incorrectly fitted accessories (e.g., winch, lights) that might be deflecting airflow at high speeds.

  2. Cooling System Bleeding: Ensure the cooling system is properly bled. Turn the heater to maximum heat during the bleeding process. Fill the system and coolant reservoir, run the engine at 2000rpm for 10 minutes. Let cool and check the coolant level.

  3. Radiator Cap: Inspect the radiator cap seals (both the neck and top seals) for damage or deterioration. Verify the hose to the recovery bottle is tightly clamped and sealed.

  4. Thermostat Installation: Check that the thermostat was installed correctly, with the bleed “jiggle valve” (if equipped) at the 6 o’clock position.

  5. Viscous Fan Hub: Inspect the viscous fan hub for proper function. When the engine is hot, the fan should be roaring and very noticeable. Consider checking/adjusting and refilling the silicon oil within the hub:

    • Internal adjustment screws should be approximately mid-way in their thread.
    • Refill with approximately 18ml of silicon oil.

  6. Water Pump: While the water pump was replaced, ensure it is functioning correctly. There is no easy test without disassembly.

  7. Head Gasket/Cylinder Head Leakage:

    • Pressure Test: Perform a cooling system pressure test.
    • Cylinder Leakdown Test: Remove glow plugs, apply pressure test, and crank engine to see if water squirts out of the cylinders. This indicates a head or gasket issue.
    • Bubbles in Coolant: With the radiator cap off, run the engine and look for bubbles in the coolant, which indicates combustion gas leaking into the cooling system.
    • Overnight Pressure Test: Check for pressure in the radiator in the morning before starting the engine. Pressure indicates an exhaust gas leak.

  8. Aftermarket Temperature Gauge Install

    • Mount sensor under the bolt that holds the engine diagnostic plug on.
    • Located on the top radiator hose.
    • Expected temperature readings:
      • General cruising: 73 degrees Celsius
      • Longer hill climb: 83 degrees Celsius
      • Hard driving in soft sand: up to 90 degrees Celsius

Solution

The described symptoms (overheating, coolant loss to overflow, bubbles in coolant, rough idle, white smoke) strongly suggest a blown head gasket or a cracked cylinder head.

Relevant Parts & Specifications

  • Aftermarket Cylinder head quoted at $1550 (bare).
  • Silicon Oil - 18ml Tube - Toyota Part number Unknown.

This technical summary was auto-generated based on recovered archives. Do not rely on its factual accuracy. Refer to the original thread. Source: View Original Conversation

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