I have no idea how long the rest of them will last. The air may have been a contributing factor, but that would have to happen over a long period of time, not just once like you were saying earlier.
Pulling the injectors is not that hard, once you get the valve covers off. On your van, I think the passenger side is blocked by the climate control and you have to remove that to get #5 and #7 out.
I may be wrong about that.
I've never pulled injectors on an E series, just my truck is all I have ever done with Fords.


I paid around $2100.00 for my set of eight from my wholesale warehouse.
It was just about to the dollar what it would have cost me to buy them from Diesel Innovations, including what shipping would have been and thier buy all eight, pay for seven policy.
.
For those of you that need a set of injectors, bite the big bullet and get the new ones.
I bought a set of rebuilts from a very well known supplier and the rebuilts ran worse than the rattling set that I removed. Mine were noisey, but the truck ran better than with the rebuilts.
There was a fluttering miss at 1100 rpm, 5% throttle that would not go away unless I sped up or gave more throttle.
And a popping noise coming out of the exhaust at WOT. Not impressed at all with that set of injectors.
So ran out of patience, finally, and bought a set of new Alliants and never had to look back at all.
Truck runs GREAT.

 

Is this something I should attempt on my own? Is it really a difficult job?

 

For me it is a very easy job, but then I have been a mechanic for 28 years now.
So my opinion on that is rather biased. I think nothing of pulling the entire front end off of a car to gain access and do a job.
How comfortable are you with digging into the engine? Just because it is a diesel should not throw you, it operates the same as a gasser with the only difference is that there is injectors instead of spark plugs. Nothing else is really that different.
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That has been a problem with my van for quit a while, low rpm fluttering accompanied by blue smoke. Right now with the economy and working two days a week the price of new or even rebuilt injectors are far out of the budget. I have been researching for new injectors but as it stands the DP tuner has worked as a quick fix to my problem, the fluttering has ceased and power has been restored. I have noticed an improvement in mpgs even with some poor driving habits that have come about now with all that extra power. I will see in a couple days when i take a 180 mile trip how overall mpgs on the highway are affected.