Probably 3 didnt go bad at once. It just started ok with 1 bad, then 2 bad. The same thing happened to me 2 weeks ago. The part store I use only sold them in fours !! So my glow plugs are 80% replaced.

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1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes