I had the same problem when I first bought my 77, it has those same filament type glow plugs. I bought all new Bosch glow plugs, but that didn't do the trick, what was actually the problem for some reason is that my fuel filters needed to be replaced. I got a new primary fuel filter, and I looked at my secondary fuel filter. My secondary fuel filter was all black and gunked up, because I guess algae was growing in my tank since the last owner didn't drive it that much. Well anyway, I replaced the fuel filters and everything has been great since. I would also recommend looking at your glow plug relay behind the dash, because that goes out sometimes, and it is worth checking just to be safe

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1981 240D/297K/4-spd/OM617 (mine)
1982 240D/241K/4-spd (wife's)
1979 240D/291K/4-spd/OM617 (father-in-law's)
1983 300DT/240K/4-spd (brother-in-law's)
1985 300DT/???K/5-spd Intercooler conversion (brother-in-law's)
1985 280TE/160K/5-spd (father-in-law's)
1963 UNIMOG (brother-in-law's)