I've been getting some rough starting lately, usually a sign of glow plug failure. However I come equipped for such eventualities, and have a DC clamp meter to stick over the glow plug feed cable. However, this showed 42A, indicating all four plugs were coming on OK. Bit of a head scratcher.
So after another week of lumpy starting, I decided to take the plugs out one by one and stick them across the battery, timing how long each one took to get cherry hot. First one, spark on contact, just a few seconds to glow up. Second one, nice spark, warmth generated but no glow. Disconnecting and reconnecting gave no spark suggesting it had gone open circuit. I let it cool, connected it again, spark, warmth, nothing. So I can only presume that when it gets hot it disconnects something inside. Wouldn't have found that with a multimeter measuring each ones resistance either!
What is interesting is that the other plug I took out had white deposits on, and this is after running through about 40l of bio when I needed bio for an unexpected trip and only had soapy bio to use. This is consistent with how my plugs used to look in my early bio-making days when I was regularly running on soapy bio. The deposits are quite hard - very much like calcium in consistency. So keep an eye on your glow plugs, I think they'll let you know if your fuel is too soapy!
