I've noticed that my Tach starts to read up to 5000 rpm while glowing the engine for start up. What causes this?

I don't have a clue in any case, but your question cannot be answered without knowing the year, model, and location of the vehicle.

Sorry, it's a 1987 Mercedes 190D and the location is Delaware.

I would suspect a short or faulty ground

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2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 231K (mine)
1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)