Thermostat Housing Cap 5-94368589-1 8-94368589-0 For ISUZU D-MAX TFR54 NKR NHR 4JA1 4JB1 — OEM-Grade Billet Aluminum Die-Cast, Grey, 6-Month Warranty

The Thermostat Housing Cap with OEM references 5-94368589-1 / 8-94368589-0 is a verified replacement cooling-system cover for ISUZU D-MAX, TFR54, NKR, and NHR applications equipped with 4JA1 (2.5L), 4JB1 (2.8L), and 4JG2 diesel engines. This cap is the upper sealing cover of the thermostat housing, responsible for enclosing the thermostat element, maintaining the sealed coolant cavity against the housing body, and providing the mating surface for the upper radiator hose connection.

The part is manufactured from billet aluminum die-cast with a grey finish, providing dimensional stability and corrosion resistance in the hot coolant environment. It is a brand-new component, not remanufactured, supplied with a reference 6-month warranty and MOQ of 1 piece from the Guangzhou aftermarket factory. The cap is cataloged under the YIMI brand (model 4JB1) and is cross-referenced extensively with 8943685890, 8941525950, 8-94152595-0, and 8971397120 in multiple supplier databases.

• Part Type Verified: Cataloged as "Thermostat Housing Cap" / "Thermostat-Wohnungs-Abdeckung" / "پوشش مسکن ترموستات" across English, German, and Persian supplier listings — confirming it is the cover/cap, not the housing body or outlet pipe.
• Material Verified: Billet aluminum die-cast (铝坯压铸 / Billet-Aluminium Casting) — confirmed by Guangzhou Paqiben Machinery factory listing.
• Color & Finish Verified: Grey (خاکستری / Grey) — confirmed by the same factory listing.
• Warranty & MOQ Verified: 6-month warranty, MOQ 1 pc, supply capacity 100–1000 pcs/month — confirmed by YIMI-branded factory data.
• OE Group Cross-Reference Verified: 8943685890, 8-94368589-0, 8941525950, 8-94152595-0, and 8971397120 are grouped as interchangeable/related references for the same thermostat cap application across Clicparts, Partsk, and multiple supplier catalogs.
• Variant Reference Verified: 5-94368589-RA is confirmed as a variant reference applicable to ISUZU Engine 4JB1 — supporting the likelihood that 5-94368589-1 is a related market/packaging variant of the same cap family.
• Engine & Vehicle Coverage Verified: Confirmed for ISUZU D-MAX, TFR54, NKR, NHR with 4JA1, 4JB1, 4JG2 engines, and industrial applications including Hitachi DX40-C/DX45-C and Hyundai HDF15-3/HDF18-3.

• Coolant leak at the thermostat housing joint: Visible drips, dampness, or crusty residue (white/pink coolant crust) around the cap-to-housing interface — the most direct sign of a failed gasket or a warped/corroded cap sealing surface.
• Overheating or temperature fluctuation: A cap that does not seal correctly allows coolant loss or air ingress, disrupting thermostat operation and causing temperature instability, especially under load.
• Coolant residue on the engine block or under the vehicle: Dried coolant around the thermostat area indicates a slow weep that should be addressed before it becomes a sudden failure.
• Visible corrosion or pitting on the cap sealing surface: Aluminum caps can corrode at the coolant passage edges, preventing the gasket from sealing even when new.
• Cracked or deformed cap: Impact damage or over-torqueing of the mounting bolts can crack the aluminum casting, requiring replacement.
• Thermostat not seating properly: A distorted cap cavity can prevent the thermostat from sealing, causing continuous coolant bypass and extended warm-up times.

• Cap vs. Housing vs. Pipe — component scope must be confirmed first: This is the thermostat housing cap (cover). It is not the thermostat housing body (8971172430 / 8973729941) and not the water outlet pipe (8971397120). Compare the original unit's physical form — a flat/domed cover with a sealing perimeter vs. a housing with a thermostat cavity vs. a pipe with a hose neck.
• 5-94368589-1 vs. 8-94368589-0: The 8-94368589-0 is the mainstream catalogued reference; 5-94368589-1 appears to be a related market or packaging variant. Treat them as functionally interchangeable for the same cap position, but read the original unit's marking where possible, and confirm with the supplier that the shipped reference matches the ordered reference.
• Gasket is not included: The thermostat housing gasket is a separate component. Always use a new gasket when installing the cap. Confirm gasket inclusion at inquiry — a missing gasket means the vehicle cannot be returned to service until one is sourced.
• Aluminum sealing surface: Aluminum caps are sensitive to over-torqueing. Uneven or excessive bolt torque distorts the sealing flange and causes leaks. Use a torque wrench and follow the workshop manual sequence.

1. Confirm the required component first: Verify that the original unit is the thermostat housing cap (cover), not a housing body or water outlet pipe. Compare the physical shape and sealing perimeter.
2. Cold engine only: Allow the engine to cool fully before opening the cooling system. Drain coolant to a level below the thermostat housing.
3. Remove the old cap: Unbolt the cap from the thermostat housing. Note the orientation and any hose connection. Do not pry against the aluminum sealing surface.
4. Clean the mating surface: Remove all old gasket residue and corrosion from the housing face and the cap sealing surface. A clean, flat surface is essential for a leak-free seal.
5. Inspect the thermostat: With the cap removed, check the thermostat for correct operation. Replace if stuck open or closed.
6. Install the new cap with a new gasket: Seat a new gasket on the housing, position the 5-94368589-1 / 8-94368589-0 cap, and insert the mounting bolts.
7. Torque in sequence: Tighten the cap bolts evenly in a cross pattern to the workshop manual specification. Do not over-torque aluminum castings — excessive torque distorts the sealing surface and causes leaks.
8. Reconnect the hose and refill coolant: Reconnect the upper radiator hose to the cap neck (where applicable). Refill with the correct coolant mixture and bleed the system according to the ISUZU procedure.
9. Run and verify: Start the engine, bring it to operating temperature, and check for leaks at the cap joint. Confirm the temperature gauge stabilizes and the heater blows hot.

• Inspect the cap joint at every coolant service or every 20,000–30,000 km — look for crusty residue or dampness.
• Replace the gasket whenever the cap is removed — gaskets are single-use compression seals.
• Flush coolant according to the manufacturer interval. Degraded coolant accelerates aluminum corrosion.
• Replace the cap if the sealing surface is pitted or warped — resurfacing an aluminum cap is not recommended for this application.

• Standard packing: Neutral carton or wooden case; custom packaging available on request.
• Package contents: 1 × Thermostat Housing Cap. Gasket, thermostat, and mounting bolts are not included unless explicitly stated.
• Set options: Cap only / cap + gasket set — specify at inquiry.
• Suitable for: ISUZU D-MAX / TFR54 / NKR / NHR cooling-system repair shops, 4JA1/4JB1 engine rebuilders, light commercial fleet maintenance departments, spare parts distributors, and export wholesalers serving Southeast Asia, the Middle East, Africa, and Latin America.
• Related parts available: Thermostat, thermostat housing gasket, thermostat housing body (8971172430 / 8973729941), water outlet pipe (8971397120), and complete 4JA1/4JB1 cooling-system kits.