1977 300D glow plugs
 

How long are the glow plugs suposed to remain on after the dash light shuts off and you star the engine? Mine stay on long enough to cause two of the rails to gloe red hot before they shut off . The engine then smooths out considerably and the headlights get brighter. I have replaced the pre-glow timer/relay and it strill does the same thing. Help! I am burning out glow plugs and rails far too quickly.