I'm with SPO123 on this one...
The short and sweet of it is that bad glow plugs don't affect the idle once the engine is firing on all cylinders. My brother has a Kubota 3 cyl tractor with one glow plug that's been absent low these many years, (broken off in head, hole plugged with weld for several years), and once it starts and fires on all three it's just fine. However, last year during mowing season the air filter plugged with chaff, and it started running terribly rich! Wouldn't idle worth a darn, either. If they fixed your engine in two hours it was something simple - possibly valve lash or dirty filter. Could have even been a partly plugged injector. I don't think it was the glow plugs, although they might have tested them and found them to be in need of replacement. The thing to do is ask for your old ones back, then you'll know.
'82 300D/4.3L V6