This only started happening after the new spark plugs were installed? Did you use the correct spark plugs? Where applicable, what about the plug wires?
The basics for a problem under load is to check for a blockage of some kind, fuel--air--exhaust. How long since overall tune-up? Can you check for applicable trouble codes? Some of the national brand auto stores will check for free, if there are any?
Wednesday, May 6th, 2015 AT 7:04 AM