Here is how you identify a faulty glow plug
Moderator: RichardW
Here is how you identify a faulty glow plug
If you measure the battery voltage with the car off..say 12.8 volts, then every glow plug drops this voltage by 0.25 v when the plugs are on preheat. So your voltage should read 11.8 v and then rise to about 12.3 after the relay times out. If one of your plugs is faulty, this change is measurable by this method and is very reliable. It helps to have your readings for a good set but the above is nearly right.