My '89 Cavalier Z24 has been shuddering when I'm driving it around 30-40mph. My plugs and wires were recently replaced last year to fix this problem, but did not work. I also replaced the EGR valve and PCV valve recently. Does anyone have some suggestions on what may be wrong?

Thank You.

May want to start with all filters and such. Also make sure you've cleaned out the TB, those 2.8/3.1 TB's get REALLY dirty after a few years, all black and gunked up.

From there, check the TPS (sensor on top of the TB) which would cause a shudder.

All of this could be for nothing, could you clarify if it's an "engine" shudder or a "suspension" shudder or you don't know yet.

It seems to be a engine shudder. I do have mostly new filters. So I suppose I should clean out the TB (Throttle Body, right?) What would be the best way to do that?

It seems to be a engine shudder. I do have mostly new filters. So I suppose I should clean out the TB (Throttle Body, right?) What would be the best way to do that?
Just unhook all the air intake stuff and get some torx bits to take off the TB. Of course you got a sensor or two and some vacuum lines, nothing big. DO NOT forget to go to the car place and get yourself a TB gasket before doing this.

Beyond that, I just took it out and went nuts with some carb cleaner and a bunch of rags, most of my buildup was on the back side of the valve. Have fun. Even if it's not the problem, it can never hurt.