Yes WD40 is flammable, but compared to starting fluid it's edible,LOL!
A diesel has about 20 to 1 compression ( twice that of a gas motor) and no spark plugs. It makes fire by smashing something flammable till it explodes. WD fits the bill.
Keep in mind that unlike a gas motor a diesel has no air intake regulation. The only rpm regulation is achieved by fuel
When using any spray while cranking the motor the motor will sound horrible! You will regulate the rpm by adding more or less WD. The trick is to not release the starter
too soon. Trust me the first time I show people this it is a little scary (for them)!
You've got your head next to what is all ready a noisy motor when running normally, Your spraying WD and thinking the thing is gonna blow.
Not for everyone, Not trying to sell you on it. Just real time experience.
Started many OTR trucks spraying 2 or3 cans at the same time! A little much for a 3 or 4 cyl. 1 can with the spray tube does the trick.
In reality you could skip the whole bleed process if you can live with the horrendous start up knock of WD40.
I often find that people are put off by any dry restart process. The motor will knock and smoke and jump around. It's normal.
I still recommend a low side pump
. Often they will make the WD40 unnecessary. Just remember to bring your throttle up and and let it crank a bit even while it sounds like it almost running.
Yanmars are in most tractor trailer refer units. Their prestart button simply runs the electric fuel pump
If your up for a little diesel 101, with your motor running normally at idle give just a tiny little wisp of WD40 down the intake. You'll hear a little knock and a little rpm surge. It will give you an idea what to expect if someday it's your only option.