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Old 08-09-2016, 10:51   #16
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Re: Engine suddenly drops rpm's

Check if your tank pickup tube has a mesh filter. It looks like you have the diesel fuel bug ( which in fact is not a bug) clogging up the fuel intake line. When engine is not running all the "bugs" drop to the bottom of the tank and only start clogging the mesh when there is fuel flow. Just my 2 cents.


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Old 08-09-2016, 11:11   #17
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Re: Engine suddenly drops rpm's

I've had this happen as well and after much work and expense found it to be a fuel issue.
We had the tank fuel polished and cleaned the tanks physically as well. Problem solved.
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Old 08-09-2016, 11:27   #18
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Re: Engine suddenly drops rpm's

Have you asked your mechanic if he has any ideas?
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Old 08-09-2016, 11:29   #19
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Re: Engine suddenly drops rpm's

Have you considered talking to the mechanic that did the work, since the problem started after he worked on it?
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Old 08-09-2016, 13:01   #20
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Re: Engine suddenly drops rpm's

You might wanat to check the tank vent for blockage. The lift pump might not be able to overcome the vacuum.
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Old 08-09-2016, 17:01   #21
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Re: Engine suddenly drops rpm's

I had a similar problem. Turned out to be a faulty ball valve in the fuel line. Handle opened all the way, but the ball only opened half way, allowing crud to build up and eventually plug inbound fuel. First rpm dropped, then the engine died. When pressure relieved, gunk dropped back into the fuel tank (most of it), allowing engine to run again and the cycle started over. Eventually the faulty valve totally plugged. Solution - replace valve and clean tank ( mine was removable).
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Old 08-09-2016, 21:01   #22
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Re: Engine suddenly drops rpm's

I've experienced that too. It was the packing nut on the propeller shaft not enough water going through to lubricate it.
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Old 08-09-2016, 21:58   #23
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Re: Engine suddenly drops rpm's

I had this problem with my Perkins 4.236. Changed fuel filters and the engine ran fine for another 30 hours. Then I found fuel in the bilge and discovered a broken diaphragm in the lift pump. Replaced the pump and all is well.
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Old 08-09-2016, 22:39   #24
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Re: Engine suddenly drops rpm's

You definitely have a small pinhole crack in your fuel line sucking in air on the intake line leading to the engine or crud in the fuel clogging up your filter. Changing the fuel line for new or cleaning out the crud should solve your problem, an easy do-it-yourself repair. It's also good to learn how to bleed the system, change your oil and impeller yourself, which takes less time than searching for a mechanic to do it for you.

Look for the easy, obvious stuff before the unusual expensive repairs.
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Old 08-09-2016, 23:54   #25
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Re: Engine suddenly drops rpm's

👍🏻👍🏻👍🏻 all the above posts would sit happily on the troubleshooting pages of engine manuals...give yourselves a clap on the back !
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Old 09-09-2016, 00:58   #26
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Re: Engine suddenly drops rpm's

I had a 4-108 and for the most part they are bullet proof but a little air will play havoc. I would bet two dollars that it's an air bleed issue. It's been a while but I believe there is a air bleed nut on the top of the injector pump. There might be a bubble there. My system had a gravity feed fuel system but I put an electric fuel pump in the system because the hand bleed pump leaves a lot to be desired. I also found that slightly cracking one injector connection at a time while the motor is running will let the air out.

I now have a Westerbeke that is the basic 4-108 block marined by Westerbeke. They have added a few goodies that helps the air issue. Treat that Perkins right and it will run for ever. They are great motors, they are heavy but I firmly believe that is what you want in a diesel.
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Old 09-09-2016, 01:21   #27
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Re: Engine suddenly drops rpm's

Maybe try a small tank set up to gravity feed the engine. It might also help to find the leakage point by presurising the system slightly.

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Old 09-09-2016, 04:33   #28
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Re: Engine suddenly drops rpm's

Same problem.

I discovered an almost invisible crack in a bend on a metal fuel line allowing air in.
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Old 09-09-2016, 06:19   #29
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Re: Engine suddenly drops rpm's

Had the exact same problem a few years ago on a 25 YO boat...changed and checked everything except the fuel lines because I knew they had been replaced a few years back. After looking at everything and replacing the fuel pump, filters etc. found the fuel lines were full of varnish like a gas fired lawn mower engine or carb would have after sitting for extended time. check or replace the fuel lines if they have plugged with varnish from the fuel.
After doing so the engine ran like new.
Best of luck
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Old 09-09-2016, 07:55   #30
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Re: Engine suddenly drops rpm's

If the engine restarts fine after a rest period I am more inclined to think fuel restriction than an air leak. If it were air, the engine would not restart without bleeding the injector pump.

Add others have mentioned, check for a closed fuel valve or a closed filter. If all else fails, run the engine direct from a jerry jug.
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