Q:

2008 Honda Accord turns over but wont start

I have a 2008 Honda Accord 2.2 Diesel which turns over but won't start. There is 240,000 km on the clock. This issue first occurred about 9 weeks ago. I got in my car to go to work and it kept turning over but would not start. There is not a problem with the battery.
A mechanic came to tow my car away, and dismiss any small problems, and he sprayed some starting fluid near the engine while I tried to start it. The car would start but would not stay running and would then cut out.
The mechanic took the car away and fixed it by doing something to fix an issue with the car taking in air somewhere. I am unsure exactly what this fix was because in my eagerness to get home I did not clarify exactly what the problem was and what was fixed.
My car broke down six weeks from the initial fix again in a completely different location, with the exact same problem, which necessitated a fix by a completely different mechanic. I contacted the first mechanic to see if they could remember exactly what they addressed the first time but they could not recall the exact problem and therefore I could not pass this information onto the next mechanic.
The second mechanic fixed the car again but this fix only lasted two days this time  as opposed to six weeks. The fix that this mechanic made was related to fuel injectors. The car is back with the same mechanic again as he is a competent mechanic but for my own peace of mind I am putting this question up and seeing if anyone can pinpoint the problem.
Does anyone know what the problem could be?
Edit:
Still having the problem. The mechanic has put in a new diesel pump, one new injector and a new pressure switch. The problem is still occurring and I have returned the car to him.
Possible fix found:
My mechanic got a second opinion from a Honda specialist who was also stumped for an hour or so before they concluded that there was a problem with a weak release valve that was releasing excessive fuel back into the fuel tank. The car is currently starting sharply and I will be picking it up tomorrow.

A:

Since this car is a diesel, there are only 3 things required to make it run - fuel, air, and compression. As the problem is intermittent, it's probably not the compression (though that should be checked anyway). That leaves fuel and air, which must mix at a certain ratio to detonate in the cylinders.
So, you either have a fuel system problem causing you to not get enough fuel (or too much fuel in some cases, but probably not enough based on your comments), or you have a problem with unmetered air entering the engine (intake, hoses, other leaks in the system). Diagnosing it would go something like this:

Check for air leaks. This is done by a visual inspection, and can be tested with either a smoke test, or if the engine runs, by spraying flammable substances (starting fluid, carb cleaner, brake cleaner, etc) around the engine and seeing if the idle changes. Fix any leaks by replacing hoses, gaskets, and any broken parts.
Check for fuel pressure. The fuel pump pressurizes the system, and there's a specific pressure rating defined by the manufacturer that must be present in order to spray the right amount of fuel. If you don't have enough fuel pressure, check the fuel pump, relay, wiring, fuse, fuel filters, release valve etc.
If you have enough fuel pressure, next is the injectors. Make sure they're firing. If they're not, the injectors could be bad, or the computer (or mechanical system in some cases) controlling them could have failed. If the injectors are firing, they could still be partially or completely clogged, or even stuck open dumping too much fuel in. They should be removed and tested - whether or not you or your mechanic can do this depends on the injectors, they may have to be sent out to a specialist.

From your story, the first mechanic addressed an air leak, and the second one addressed fuel pressure. It's hard to say if those were the actual problems, but I'd suggest looking for more air leaks first since that seemed to fix the problem for longer.

