I have a 30' Tollycraft with twin Volvo diesels and unfortunately it has been sitting in the marina for 10 months without being run. Being new to diesels I have no idea if the fuel has gone bad or how to determine if it is bad.
Is there a way to determine if its bad or should I just pump it out.
There is really no reason to believe that 10 month old diesel fuel stored in a moderate climate is bad. It certainly could be bad, but it is pretty unlikely to be bad. My biggest concern would be that the o-ring on the fill went bad and it got water in it, not normal aging.
When it was me, I did what Delta says and ran it. Specifically...
-- Clean the primary filter/separator.
-- Idle at the slip for a bit and make sure everything seems okay
-- Advance the throttle to a high idle and let it run for a while and make sure the RPMs stayed stable. Surging is a sign of dirty filters.
-- Advance the throttle again to something more aggressive for a short bit, making sure that the RPMs stayed stable
-- Advance the throttle again to something as high as comfortable for a shorter bit and, again, make sure RPMs stayed stable.
-- Check the primary again for slime and water.
-- Make sure a helm-capable helper is aboard and take it out, again, keeping an eye that the RPMs stay stable and back off on the throttle if there is any surging or studdering.
If at any point I noticed surging RPMS, I'd clean the primary and replace the 2ndary.
If, at any point, any more than very minimal water showed up in the primary, I'd want to pump it out from the bottom of the tank rather than running it through and draining it out. Engines shutting down when the shut-off ball floats up are no fun. A simple, cheap, diesel-rated transfer pump from an auto parts store powered from the battery via alligator clips and several feet of hose and a bucket work fine.
If I noticed filter soiling of debris falling out, I'd clean the primary and change filter media more often, as often as needed and then some. If it were really bad, I'd consider using a biocide and then polishing the fuel (which, if done by a service, is as expensive as buying new, but easier than dealing with the old).
If the engine were somehow not performing to expectation, but not surging, e.g. low power, smoking, etc, I'd add a cetol-boosting, cleaning, cure-all diesel additive to the recommended ratio. If that didn't fix it, I'd consider pulling the injectors and having them pop-tested (usually free) by a rebuilder, listening for valve timing or checking/setting it with feelers, etc.
My very last resort would be to get rid of the fuel. And, if I did that, I would certainly not pay for its disposal. Plenty of folks from recreational marina neighbors to the charter folks, would gladly take it and use it, even if only diluted. I wouldn't have to ask twice, given that it is ~$6/gallon these days.
If it was me today, I'd do the same thing, but I'd pump some out from the bottom to check for water and slime build up 1st. It is easy to do and can only solve problems. If I found it, I'd just pump it out until I was getting stuff that visually looked good. The water and goop would probably not be too much. Less than a five gallon bucket. I'd guess. And, if more, well buckets are easy to come by. I'd have no problem pumping tons of water out myself. If I started to get a ton of slime, I'd probably get a professional to polish it or buy an appropriate filter and pump and do the same myself.
But, again, other than water leaking in via the o-ring or something, most diesel fuel should be good for a year or even more in a moderate environment. Condensation can add some moisture, but not really that much that fast.
. It should be fine.
