Hello, I have a 1989 firebird, 305TBI, T5, and it vibrates really bad at idle.The vibration is consistent whether in gear or in neutral. The vibration is noticeable when sitting in the car, the hood rattles, the shifter flops back and forth, and I can see the engine wobbling back and forth if I open the hood. The vibration mostly goes away if I rev the engine over ~2000rpm or If I get driving.

To me it looks like the engine is imbalanced, I pulled spark plugs and they are all fine except 8, which is all black. I pulled the valve cover for cyl 8 and I don't see any problems with the valvetrain, but then again I don't know if I could see a bent pushrod without taking it out.

I'm wondering if it could be a bad harmonic balancer, but I don't know how to test that.

 

None of that is likely to shake the engine except the black spark plug.

I"m guessing "misfire", probably of #8 and maybe others.

In my driveway, it'd get a cylinder-balance test, and then a compression test and leakdown test of weak cylinders.

If it's been misfiring long, the catalyst is toast.

 

Thanks, I'll try that when I can.

 

Compression test results: I tested each cylinder multiple times and all tests on all cylinders fell between 148 and 155 psi. I don't have the tools to do a leakdown test, but I'll do a cylinder balance test when my new spark plugs get here. Maybe see if cylinder 8 is the problem.

 

The only time I experienced "vibrates bad at idle" with my 92 305 TBI was when I did a tune up to resolve a No-Start condition... and crossed a plug wire causing a major misfire as described by Schurkey. Crossed a wire by chance?

 

It runs like a wire is crossed, so I double checked that after I did tune up, but it wasn't the problem.

 

Okay, I figured it out. I cleaned off all the plugs with acetone and a wire brush, put them back in and the problem is gone. Seems to have picked up more power as well, I guess cylinder 8 is firing now.

 

Good Deal!