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post #1 of 10 Old 07-06-2010 Thread Starter
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diesel engine surging up and down

After getting bounced around the other day in a storm our engine(volvo md7b) has started to surge up and down in rpm. I can set it to 2000 rpm and it will run for about a minute then go down to 1500 rpm for a minute then back up to 2000 and repeat. It does not do it at idle and will idle just fine. It also seems to take a bit of time after we start it before it starts to do it but after its warmed up it will do it as soon as it is started.

I was thinking maybe we got some bad fuel or maybe some air sucked in when we were bouncing but I bled the lines and the problem persists so it does not seem to be air. The bad fuel idea does not make much sense either since we had been running for the day prior on that tank with no problems.

So I am down to either clogged fuel filters or a worn out injector pump. Or if someone else has another suggestion I am open to that.

Would clogged filters cause the engine to drop rpms like that? I have a racor 30 micron external filter and the engine mounted filter. Its not a fun job to change them and I just did change them a couple of months ago so don't want to do it again if its not a likely solution.

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post #2 of 10 Old 07-06-2010
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Bad fuel!! basically the bottom and sides of your furl tank are covered in sludge/slime and the rough passage has shaken some loose and clogged your filter(s). Be prepared to have this continue, maybe getting worst with the engine stalling until you get the fuel tank cleaned out. There are plenty of threads on this...
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post #3 of 10 Old 07-06-2010
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Yep, clogged filter(s). Might be crud in the tank or water.

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post #4 of 10 Old 07-07-2010
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Changing your fuel filters is certainly a good place to start. When you sail in rough weather, all of the crud in your tank gets stirred up and has a tendency to clog filters. Even without problems like algae, a boat that hasn't had its fuel tank polished in a while and is normally sailed in calm weather will often exhibit this behavior when it gets rough.

When a fuel filter starts to clog, the engine will idle fine since not much fuel is required then but it will have trouble with loads since it is fuel starved. When you change the filter, think about cutting it open and seeing what is inside to see the condition of your tank.
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post #5 of 10 Old 07-07-2010 Thread Starter
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It was indeed a clogged filter. I replaced the Racor and it ran fine today. The fuel was kinda brown looking though so I think I have some more work ahead... Will have to see if I can figure out a way to suck that stuff out of there and clean it out.

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post #6 of 10 Old 07-07-2010
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Hire someone to clean the tank and polish your fuel.
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Originally Posted by huguley3 View Post
It was indeed a clogged filter. I replaced the Racor and it ran fine today. The fuel was kinda brown looking though so I think I have some more work ahead... Will have to see if I can figure out a way to suck that stuff out of there and clean it out.

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post #7 of 10 Old 07-08-2010
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Depending on how your tank is installed it may be worth removing it for cleaning. If it's covered with growth on the inside it may even be best to just replace it if it's a simple design. These steps may sound radical, but I've been through this process and if I had to do to over I'd seriously think about just replacing the tank if it's bad enough.
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post #8 of 10 Old 07-08-2010
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While removing the tank and cleaning is a good suggestion, there is really no reason to replace a perfectly good tank just because it has some growth in it. A good custom tank can be pretty expensive, and cleaning the tank, as long as it is physically sound, is probably a much better idea.

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post #9 of 10 Old 07-08-2010
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It totally depends on the installation, complexity of the tank, and how badly it is contaminated. I spent well over half the cost of a new tank (I only found this out afterwords) and way too much time messing around with an old tank. I'm just tossing the idea out there - I wish someone had done the same for me.
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post #10 of 10 Old 07-08-2010
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Fuel polishing only, with an obviously fouled tank ('brown/black' fuel) is a total waste of time, money and effort, as polishing will not remove the accumulated alkenes, etc. and fungal cellular fragments adhering to the tank walls. Traditional industrial, etc. remediation requires that the tank be thoroughly cleaned ..... THEN you recirculation filter polish to remove any residuals.

There are some recent 'improvements' to such remediation that may be of benefit - IF you have access to a transfer pump, an INDUSTRIAL filter housing (with a graded pore depth media filter). ..... AND one of the newer 'enyzmatic TANK CLEANER" Compounds. You want a large capacity pump (for the highest possible volumetric 'turnover' at approx. 3-5 gallons per MINUTE), 2.75" dia. X 10" length graded pore density depth filter @ 10-15µM (or larger). Add the Tank Cleaner, let sit a few DAYS, then recirculate the fuel for several HOURS/DAYS through the /pump filter set until the fuel becomes 'visually clear'. You may need to change to a 'fresh' filter several times (such filters only remove ~50 grams of 'crud'). The HIGH turnover rate through the filter for a LONG time will eventually remove the particulate; but, you 'may' need to 'final trim' the particle load with a 2-5µM depth filter. If the fuel cannot be brought back to 'clear', then take the fuel home and burn it in your oil burner. Add more fuel and more tank cleaner, etc. etc. until the fuel becomes 'clear' druing the recycle process. You should be able to rent such a transfer pump and filter housing from a tool rental. If you cant get graded pore density filters then go to Home depot and get some polypropylene 'spun bonded' ~15µM (and maybe one or two 2-5µM filters). This is going to be a long particle extraction process

Recirculation polishing a fouled tank will be of absolutely no benefit in 'cleaning' a tank; however, an onboard recirculation polisher (always operating when the engine is on) will quickly bring the particle population back under 'control' if/when the tank 'slugs' itself with particles breaking loose from the tank walls. Once a tank is cleaned the BEST method to keep it clean is an onboard 'constant run' recirculation polishing filter.

Recirculation polishing WITH an enzymatic tank cleaning compound will probably be of great benefit. Better to mechanically scrub/clean the tank .... THEN recirculation polish AFTER tank cleaning.

Tank HAS to be cleaned, either by mechanical scrubbing or by enyzmatic Tank Cleaners.

To prevent fouled tanks .... dont top-off your tanks, keep onboard ONLY the fuel amount you NEED plus some reserve, dont get your fuel from a 'low turnover' source such as a MARINA - get it from a high volume TRUCKSTOP etc., use an anti-fungal compound, clean out the tank on a routine maintenance basis, install a small onboard recirculation polishing filter.

Example of 'tank cleaner' .... http://www.starbrite.com/productdeta...&ProductSSCat=
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