kick in the ass cold start


hello guys i have a 1986 daihatsu rocky 2.8 diesel non turbo and its a pain in the ass when start from cold. now correct if i am wrong but i am almost a 100% sure is bad glow plugs or the relay wich i will change shortly depending on what it is. now my other question is (just in case) what if i change the glow plugs or relay or both and still doesn't start from cold, what could it be?
because once it has started it runs smoothly and you can turn it off and turn in back on with no problem as long as it still hot. any help will be highly apreciated.

glow plugs etc

Problem like this I think got to be methodical and work through one thing at a time.Had similar problems on a number of diesels. I took out glow plugs and tested each one by using a wire directly from battery pos to to top of plug whilst holding body directly on neg. battery terminal for a short time. if OK will get hot real quick. In my experience all need to be OK for good starting . Test relay with a 12v bulb on wire to plugs and to earth to check if power is going through from relay. If engine restarts OK when hot then 99% will be one or the other. Hope this helps.

thanks for the help

yeap am pretty sure that one of those two is the problem.