My '90 300zx has a bad stumble/bogs down very badly below 3k rpms. This has been going on for a long time, and I have yet to solve it. It does it when it's cold and warm. Once I get to about 3k rpms it is like a switch is flipped and she runs great. I have the correct resistor attached to the knock sensor plug, but never noticed a change, & have a replacement also just haven't had a chance to relocate it yet. Sometimes when I start the car it starts off running fine as I am driving, then it will all of a sudden start to stumble as if something is telling it to. Also, I just had to replace the water pump so I did the timing maintenance as well and now there is a backfire sometimes when it's cold and at low rpms. Any help narrowing down what the issue is would be fantastic.
