THEY will be cheaper, but won't last as long. Platinum-tipped spark plugs are used for their longer life and are fitted mainly because America has laws about service intervals and the platinum-tipped plugs help meet the laws.

IT SOUNDS as if you have a bent valve. The Sonata engine has to be correctly aligned when the belt is changed and it's difficult to do. The other thing to check is the balance belt -- it should also be changed, and needs to be correctly aligned when the new cam timing belt is fitted. It's easy to get the balance belt 180 degrees out.

THE Sonata uses regular unleaded, but some Hyundai mechanics suggest running PULP occasionally to aid clean running. For the flat spot, check that the spark plugs were changed at 90,000km as recommended. The Hyundai uses platinum-tipped plugs and, because the inlet manifold has to be removed to change plugs in the three rear cylinders, mechanics have been known to leave them. They should have been replaced at 90,000km. Also check the plug leads.