91302-028-000 Honda SEE PART DETAILS - SUP; O-RING (15X2.5) (Honda Code 0295592).

This engine has a pressurized cooling system. A pressurized cooling system has two advantages. The cooling system can be operated in a safe manner at a temperature higher than the normal boiling point (steam) of water.This type of system prevents cavitation in the water pump. Cavitation is the forming of low-pressure bubbles in liquids that are caused by mechanical forces. It is more difficult to create an air pocket or a steam pocket with this type of cooling system.
Illustration 1 g00921815
Boiling point of waterRemember that temperature and pressure work together. When a diagnosis is made of a cooling system problem, temperature and pressure must be checked. Cooling system pressure will have an effect on the cooling system temperature. For an example, refer to Illustration 1. This will show the effect of pressure on the boiling point (steam) of water. This will also show the effect of height above sea level.The coolant level must be to the correct level in order to check the coolant system. The engine must be cold and the engine must not be running.Note: At operating temperature, the engine coolant is hot and the coolant is under pressure. Use caution when you work on a hot engine. After the engine is cool, loosen the pressure cap in order to relieve the pressure out of the cooling system. Then remove the pressure cap.The level of the coolant should not be more than 13 mm (0.5 inch) from the bottom of the filler pipe. If the cooling system is equipped with a sight glass, the coolant should be to the proper level in the sight glass.Test Tools For Cooling System
Table 1
Tools Needed    
Part Number     Part Name     Quantity    
4C-6500     Digital Thermometer     1    
9S-8140     Pressurizing Pump     1    
9U-7400     Multitach Tool Group     1    
1U-7297
or
1U-7298     Coolant/Battery Tester     1    
Making contact with a running engine can cause burns from hot parts and can cause injury from rotating parts.When working on an engine that is running, avoid contact with hot parts and rotating parts.
Illustration 2 g00876179
4C-6500 Digital Thermometer The 4C-6500 Digital Thermometer is used in the diagnosis of overheating conditions and in the diagnosis of overcooling conditions. This group can be used to check temperatures in several different parts of the cooling system. Refer to Operating Manual, NEHS0554, " 4C-6500 Digital Thermometer Group" for the testing procedure.
Illustration 3 g00286276
9U-7400 Multitach The 9U-7400 Multitach Tool Group is used to check the fan speed. Refer to Operating Manual, NEHS0605, " 9U-7400 Multitach Tool Group " for the testing procedure.
Illustration 4 g00286369
9S-8140 Pressurizing Pump The 9S-8140 Pressurizing Pump is used to test the pressure caps. This pressurizing pump is also used to pressure test the cooling system for leaks.
Illustration 5 g00439083
1U-7297 Coolant/Battery Tester or 1U-7298 Coolant/Battery Tester Check the coolant frequently in cold weather for the proper glycol concentration. Use either the 1U-7297 Coolant/Battery Tester or the 1U-7298 Coolant/Battery Tester in order to ensure adequate freeze protection. The testers are identical except for the temperature scale. The testers give immediate, accurate readings. The testers can be used for coolants that contain ethylene or propylene glycol. Making the Correct Antifreeze Mixtures
Adding pure antifreeze as a makeup solution for the cooling system top-off is an unacceptable practice. Adding pure antifreeze increases the concentration of antifreeze in the cooling system. This increases the concentration of the dissolved solids and the undissolved chemical inhibitors in the cooling system. Add the coolant/water mixture to the same freeze protection as your cooling system. The following chart assists in determining the concentration of antifreeze to use. Refer to Operation and Maintenance Manual, "General Coolant Information".
Table 2
Antifreeze Concentrations    
Temperature     Concentration    
Protection to
−15 °C (5 °F)     30% antifreeze and 70% water    
Protection to
−23 &d