tach jmps violently when accelr8tng then engine stalls

[BACKWOODSFREAK]

I have an '84 2m4 (4 banger, manual tans). I have recently had speed sensor codes and rich exhaust codes, so I replaced the speed sensor which i believe was giving me the rich exhaust. In turn it helped the car run better and smoother during shifting but the tachometer still bounced around like crazy. does anyone have any suggestions on what the bouncing tach is and why it isnt throwing out any codes. i am sure that somebody has had this problem before. please help me if you can,
-thanx bwf

[FieroBoy]

have you looked at installing a tach filter

[Ragtop_Renegade]

Do your ACTUAL rpms change as well? You haven't really given enough information to distinguish an engine problem vs a problem with the tach itself. If the engine rpm isn't actually surging, but the tach still jumps, there is probably a problem either in the distributor, (where the signal comes from) or in the tach itself. (Where the signal is converted to a visual indicator via the tach needle)

[BACKWOODSFREAK]

the motor actually cuts out when it jumps and then resumes rpm when tach stops jumping. whe the tach is jumping you can push on the gas and it does nothing but make it jump even more. when you let off the gas pedal it slowly returns to normal rpm and engine performance is restored. sometimes when you go around a corner and shift down the car jerks and then the tack starts its b.s. thanks for posting

[Ragtop_Renegade]

Sorry for the late reply (computer broken) but it sounds like the problem isn't the tach, it's the engine.... you seem to have some sort of ignition problem.

[FieroGT86]

its funny because i have the same problem. i got an 1986 fiero GT and its doing the same thing. i taught it was my old exhaust but it wasnt because i replaced mine and still doing the same thing. when I start my car and drive it for 10-15 mins the tach meter jumps up an down and when i press the gas the tach goes up violently. After a couple of mins later itll die n wont start tell i let my engine cool down. i really need help because i dont want to throw my ride away.

[84fiero123]

May simply be a bad ground on the engine, these cars are plaged with problems related to bad grounds. simply put a large gauge wire from a good clean spot on the engine to the firewall. Any gauge around 12 or better will work.