I have a couple questions concerning glow plugs that I had a hard time locating concrete answers on. Car in question is my '79 240D. I have the old (original?) series glow plugs. My car normally starts fine, only a couple seconds of cranking. We've had a few kind of cold mornings here, high 20's F. It still starts OK but is a little grumpier about it. I glow cycle it twice, seems to help a little. I have noticed though that I have to give it some pedal to keep it going though, for about 20-30 secs when cold. It seems to be missing on one cylinder, but then it smoothes out and runs nicely. Don't have to throttle it to keep it idling. This sounds like a dead GP symptom, but I have series plugs, so here's my questions.

1. How do I test these SERIES, not paralell GP's?
2. Can one GP die and the rest be OK in a series setup? I wouldn't think so (electrically I know this would shut off all plugs).

The glow plug light seems to be fine. Glows for ~30-45 secs and then shuts off. The dome light does dim, so they are drawing power.

I want to know what this might be due to, I don't want to have a bad starting car come this winter when it gets cold.

__________________
'79 240D 4 speed manual 105k miles