6 seconds has never been enough here in the cooler climate on a cold start with any idi I have run with any brand of plug or any harness. Usually 8 or 9 seconds will do it and 10 or 11 on a really cold morning. I do run White Rogers relays on Idi's along with PSDs, Harness's are in good condition or custom made in some cases. One of the trucks in question has 390-402 psi on all cylinders, 10,000k Baby Moose pump and injectors, timed just a hair under 9degrees, no either, no overheating or exhaust temps over 1100.
Glow plugs are a year old, (maybe 250-300 cold starts) and I have lost 3 plugs so far. They just stop working. No swelling, no pitting, no weird ****. They do nothing on a bench test. Not even get warm.
On another truck, lost 4 in 2 years maybe the same amount of starts. ZD-9 in a 7.3. Factory controller, timed at 9.something degrees. Failing the same way with no distortion. Old pump and injectors but no signs of dripping injectors or timing way off or either damage.
So....what im getting at is....I think there might be a bad batch of plugs that went around for a while, OR perhaps quality may be starting to slip with these also....like so many other products.