My friend has a 1987 300TDT. For the past few weeks it has idled poorly, even after the engine was warm. It also billowed/belched smoke out the tailpipe as it surged (at idle only). Everyone thought it was either the air filter/fuel filters etc. The repair shop fixed in 2 hours. The problem was 5 out of the six glow plugs were shot, only one plug was working. MY QUESTION: how does 5 non-functioning glow plugs affect the idle of a diesel? I always thought they only contributed to a hard start/no-start when burned out (that was never the case with this car though). After a diesel is running, isn't the job of the glow plugs finished?