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The tachometer cable may be faulty. The tachometer sensor, on the side of the engine might not be functioning properly.
might be bad motor mount / or electrical short
possibly a bad ground check your wiring
Can be engine miss or broken motor mount.
Have it checked for codes, that should give an indication of possible causes.
What causes the color to become hot
No! it only controls the air/fuel mixture at idle - it is by-passed when the engine revs-up. Generally, when there is a problem with the air control valve it causes the idle to become erratic and 'hunt' and maybe the engine keeps stalling.
what scientific causes something to become putrid
Good question, I'm interested in this myself as I know that flour is a fuel and its combination with heat causes this erratic behaviour.
i suggest changing the distributor. i had erratic timing for a while, during cruising speed the emgine would chug here and there. One day I decided to replace the distributor cured my erratic timing, and stopped the chugging.
replace the ignition coil. There is a resistor inside the coil that fails over time and causes 2 problems. 1 the transmission may fail to properly lock the torque converter and 2 will cause the tachometer needle to fluctuate wildly, esp when it is hot outside.
The fuel sending unit is sticking, or the wiring to the gauge is loose. These are the two most common causes of an erratic gauge.