Diesel Technician Practice Test 2026 – The All-in-One Guide to Master Your Certification!

A pressure cap on a cooling system elevates the boiling point of the coolant by increasing the pressure within the system. The standard boiling point of water is 212°F (100°C) at atmospheric pressure. By raising the pressure to 10 psi, the boiling point of the coolant is increased significantly.

Each additional psi of pressure raises the boiling point of water by approximately 3°F (1.67°C). Therefore, with a 10 psi pressure cap, the calculation for the new boiling point is as follows:

Starting boiling point: 212°F

Additional elevation for 10 psi: 10 psi × 3°F/psi = 30°F

New boiling point: 212°F + 30°F = 242°F

This makes 242°F (117°C) the correct temperature that an engine can achieve with a 10 psi pressure cap, effectively preventing boil over in the cooling system. Understanding this principle is crucial for maintaining engine efficiency and preventing overheating.