really rough idle
 

hi new to the forum and could really use some help with my 88 mk3 n\a i just bought a aftermarket temp gauge and i installed in into the thermostat housing where the allen plug was and my old one was broke off so i just plugged it with a bolt i also removed my egr then put it back in then the car would not crank eventually got to were it would crank easily but now it has a very rough idle almost dying like idling from 600 to 400 blinking the service engine light once i get going down the road though it does fine just when idling what could this be?