Improper spring and control of spring alignment.
The Sonnax end plug with a centering nub keeps the redesigned spring centered.
The OE cooler check valve spring does not remain centered on the end plug. When the valve strokes, the valve stem can contact the spring and pinch the end coils against the end plug, breaking them off. This will prevent proper function of the cooler check valve, and lead to reduced cooler flow or drain back conditions. Additionally, the broken spring parts can migrate through the rest of the valve body, potentially lodging in other valves. Sonnax has developed a kit with a modified end plug and spring. The end plug has a nub at the inboard end, allowing the spring to stay properly centered on both ends. The modified spring works with the end plug to re-establish the OE operating forces.