How to Determine if the Expansion Tank Cap is Faulty?
2 Answers
When the expansion tank cap is faulty, coolant will spray out from the radiator cap. After the cap is damaged, the flow of coolant into the engine decreases, leading to a drop in coolant pressure. This prevents the heat generated during engine operation from dissipating, causing the engine to overheat. In severe cases, boiling over may occur. Below are the relevant details: 1. Radiator: Most automotive engine cooling systems use an automatic compensation closed-type radiator, primarily featuring an additional reservoir. The reservoir cap is equipped with a steam valve and an air valve. This setup reduces coolant loss in the cooling system and enhances the radiator's cooling performance. 2. Coolant: Coolant is a critical component of the automotive cooling system. When the engine runs, the coolant continuously circulates through the cooling passages. If the internal pressure becomes too high, coolant may leak out, potentially causing pipe bursts. The coolant reservoir has maximum and minimum level marks. When adding coolant, ensure it is filled between these two marks.
With over a decade of experience in auto repair, I've frequently encountered issues with failed expansion tank caps. The diagnosis is straightforward. First, check for coolant leaks—if the engine heats up and the radiator area appears damp, or there are seepage marks around the cap, the seal has likely failed. When the engine warms up, an unstable cap pressure can cause the coolant temperature to spike, triggering the dashboard temperature warning light. If you stop to inspect and notice bubbles rising or coolant spraying from the tank, it confirms the cap can't maintain pressure. A pressure test is highly recommended: use a specialized tool to seal the tank opening, apply a few psi of pressure, and observe for leaks. If the pressure drops rapidly, the cap is definitely faulty. During routine maintenance, always inspect the cap for cracks or deteriorated, deformed rubber seals. These caps typically need replacement every 2-3 years, especially before long trips—address the issue promptly to avoid overheating and breakdowns mid-journey.
