Clean and inspect valves and pistons
WARNING: This page is about a different car, the 2004 Volvo S40, 2003 Volvo S40, 2002 Volvo S40, 2001 Volvo S40, and 2000 Volvo S40. However, it is still accessible from the selected car via links, so may be relevant.
Clean all metallic components thoroughly in a cleaning fluid such as gasoline, paraffin or trichloroethylene. Dry with compressed air or lint-free rag.
Check that valve spools are not abraded or worn.
Check that there are no abrasion or wear marks on the valves and that they move easily in their bores. If necessary, the valves can be carefully dressed with extremely fine emery cloth.