Rough start up when cold. 3l diesel

Hi guys. Recently picked up this 94 supercustom. 3L diesel, 4wd, 247000kms. For the most part it's excellent, but in the last 3 days it's developed an issue which I'm looking for some advice on!

The last 3 mornings it's really struggled to start. It turns over and takes a bit of persuasion, and then when it does start it starts with a big puff of black smoke and then I need to keep the revs up or it'll stall. The smoke stays back until the temp has lifted off the bottom of the temp gauge.

I'm thinking it may be glow plugs. The light flashes on when I turn the ignition on but goes out pretty quickly. Is there a relay for that I can check? I've tried glowing it a few times but it doesn't really help. Any other suggestions as to what may be the issue are welcomed!

This is my first diesel so I'm not that clued up. Thanks

I think black smoke is most likely from the fuel not burning properly from having the wrong fuel air mix so could even be something simple like the air filter, you could try putting some fuel additive in the tank to clean the injectors too.

However it can also be a compression issue which is more serious like a piston ring so check you're not losing/ burning oil that could be an indication of something not so simple to fix.

Might be worth checking for vacuum leaks too. See if a local place will do a smoke test,

Thank you. I'd just like to stress it's only first thing in the morning when it's been sat over night. It starts multiple times during the day a absolutely fine and I've also had a mate drive it up a hill and there's no back smoke when it goes up a hill

This has a whole load of stuff about glow plug testing.. https://www.workshopservicemanual.com/m0072151-toyota-hiace-h100-1989-2004.html

Put all new glow plugs in, don't forget a little anti seize on the threads. If you still have problems have the injectors rebuilt. I could go farther but sounds like GP's