My 240d became really hard to start in the AM. I usually had to heat them up, turn off the key and then heat them again before it would start.

Glow plugs seemed to test OK so did not change them for a while and figured that something else must be wrong.

Then, one Saturday, I did change them and it now starts immediately. Had a new set on hand that had been in a drawer for a few years.

Cannot figure out why the old ones tested OK though because at least one must have been bad.

They are so cheap, they are worth changing if you haven't done it for a while. Mine had been in for 10 years or so I think.