Suddenly Hard Starting

[SPYDERLK]

Our old diesel skid steer had a small air leak in the fuel line like Spyderlk said.
It never leaked fuel but on a very quiet day, and just after running it, then turning off, you could barely hear a faint noise like a mosquito buzzing. New rubber hoses and no more problems.
It would lose it's prime and would be hard to start first thing until then.

Good example.
My experience was on my Mahindra 7520. -- I had long ago turned my low idle down to 450rpm to halve my minimum ground speed in sensitive light duty work. No issues, til yrs later it started to die if set down at minimum -- no problem at 800 or advanced under load. Next day normal starting of .5 to 1 sec changed to 5 sec or more continuous cranking. I found a rubber splice section in the metal fuel line. Coated it with grease and the problem went away for a day or two. Ordered some "soft" teflon tube and replaced the section. Should be good for a lifetime.

 

[sixdogs]

My JD 5420 (open station) will bleed down and lose prime if it sits a few days with less than half tank of fuel. There is a slow leak in the supply line that I have not taken the time and effort to repair. See if it happens if you keep the tank full. Try manually priming it (under the hood..takes 2 minutes) before you crank it to see if that fixes the problem.... as a diagnostic idea.
I also had to change the cold start module several years ago, but that was a No-start problem. You can unplug it..and it will start... to see if that is the culprit.

Now that you mention it, I did have a slow leak in the supply fuel line as well when I had my 5520. I just replaced the lines and problem went away.

One of the lines went under the seat and was squeezed between the floor and transmission. It took me several hard hours to change that line. It looked like the hose was just deteriorating and crumbling. I replaced with NAPA hose and all good.

 

[DarkBlack]

I see a lot of posts suggesting the fuel filter. I wouldn’t suspect that, since it’s a starting issue, not a running issue, which it would be with a restricted fuel filter.
Air leaks as previously suggested or faulty pre-heat/glow plugs are likely the issue