Okay, here is the story. About a month ago my car stated vibrating like mad. It wasn't at all speeds/rpms, but only at idle. From about 600 rpm to 1300 it shakes like there is no tomorrow. Any other speed/rpm and the car is fine (or at least I don't feel it). The rpms stay real steady - no fluxuation, so it isn't a fuel problem (I don't think). Anyway, it was about time for a tune up so I changed the plugs, wires, cap, rotor, filters (air, oil, fuel), and the pcv. Started it up and there was no change. I then cleaned the carb (some of that spray in/on crap) and retuned it. Again, no change. The engine vibrates a little, the tranny a little more, and the exhaust a lot. But they are all real secure - exhaust is welded in, tranny is bolted down, and engine mounts look good. I think the exhaust and tranny just vibrate a little more cause they are lighter. The car runs just like it always has and the clutch still feels real good. There is no difference in gear or out, clutch engaged or disengaged. This vibration is beginning to make me ill!! (Literally, it shakes me up real bad) but I can't figure out what is causing it. Does anyone have any ideas?

 

Vacuum leak? REALLY low idle? Bad choke? Bad EGR?

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89 iroc-z 305 tbi
k&n filtercharger, open element air filter. nuffin' else

 

Interesting that you finally mention it has a manual transmission. When I got my 81Z (350 w/4 spd - yup, not original) it had a vibration up around 2400 to 3200 rpm. Finally the transmission began to grind a little going to 4th gear. After it go annoying enough I dropped it out to see why. Bearings on the input shaft/4th gear gone (chewed up) and the pilot bushing in the back of the crank had a hole big inough to put my thumb in. The input shaft was just flopping around in the back of the crank. Rebuilt the tranny and installed new pilot bushing - vibration gone.

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Z's r Best

 

Check your firing order also. If you got just two of your plug wires on your distributor cap switched, it can cause a vibration like you are saying and will smooth out when the engine speed increases.