This is about my damned 1981 240D. It must be on its road to perdition :-(.

I tested my glow plugs by removing them from the engine block
and laying them down on the valve cover so that each plug's body is in contact with the valve cover (as ground) and their center electrodes were still connected to the GP relay. Next, I turned the ignition to the #2 position. I saw that all 4 glow plugs become RED HOT for about 15 seconds. I retested again and again by going through the glow plug cycle for several times. The glow plugs always became RED HOT. If I touched their body (not the heating element), it's extremely hot.

However, after screwing all 4 glow plugs into the engine block and going through several glow plug cycles, I touched the outer body of each glow plug and they were all NOT even warm (same temperature as the engine block). The voltage across the battery did drop, indicating that the glow plugs were sucking energy.

How come? I don't think the glow plugs can dissipate quickly and completely all of its heat into the engine block. So why the glow plugs's outer bodies were not even warm when I touched them after going through several GP cycles while they're extremely hot
when tested outside the engine block?

Could sometimes try to touch the outer body of their car's glow plugs after going through 1 or 2 GP cycles? Hot, Warm or Cool?

Thanks a lot for your information.

Eric