I have a 1990 Dodge LS 250 van The motor has 196 K miles. It is a 360. It runs strong I believe it has been well kept but at low rpm and up a slight grade the engine has a slight ping to it. It does not run on and the power seems ok. What if anything can I do to resolve this pinging noise? I tried a fuel additive that said on the label it would help the ping but it has not been in the tank long enough for me to tell. Help me!