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Old 07-09-2016, 19:50   #1
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Engine suddenly drops rpm's

I would appreciate your thoughts on this. I have a Perkins diesel with about 150 hrs on it. I just had my mechanic drain and replace the oil and replace the fuel filters along with the impeller.

I took the boat out of the marina for the first time since the work was performed and after motoring for about 10 minutes the engine suddenly dropped rpm's. I increased the throttle so it wouldn't die on me and the motor came back to life for a few seconds, then dropped again. I was able to bring the rpm's back up with the throttle.

By this time I was able to hoist sail so I turned off the motor and continue under sail.

After a few hours of sailing, I retuned back home. Coming back to the marina, I started the motor and it ran strong for about 10 minutes before the motor started doing the same thing. I made it back to my dock without any more problems.

This is the first time that this has ever happened. When I bought the boat three years ago, the motor would die anytime I pushed the rpm's above 3000. I discovered that the fuel tank was full of crud which caused the fuel pickup to become clogged, starving the motor, so I had it cleaned. Problem solved.

But this is a new issue.

Thoughts?
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Old 07-09-2016, 21:43   #2
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Re: Engine suddenly drops rpm's

I'd bleed the system again and look at the fuel lines for loose connections or leaks. If no dice there, look at the filters. Correct micron replacement? Then look at the tank again. Maybe some remnant crud from the tank got stirred up? Gotta be something restricting fuel flow along the way.
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Old 07-09-2016, 21:48   #3
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Re: Engine suddenly drops rpm's

Also make sure the return line to the engine is not clogged / collapsed.

Though sounds like residual air that needs to be bleed out or air leak, perhaps around a filter that was changed.
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Old 07-09-2016, 22:02   #4
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Re: Engine suddenly drops rpm's

Probably a fuel issue or air in the fuel as has been mentioned. But I recently saw a similar problem due to a wiring fault that caused the stop soliniod to momentarily activate. The symtoms included the gauges flickering each time it happened.
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Old 08-09-2016, 01:18   #5
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Re: Engine suddenly drops rpm's

air or blockage.
Air; the mechanic maybe tightened some line nuts and broke a seal or leant on a pipe and broke a seal or cracked the pipe. This becomes obvious when the air finally gets to the injector pump, but is not enough to stop the motor.
Blockage: perhaps something in the tank or filter that is sucked towards an outlet and eventually partially blocks it. It moves away when the motor stops.

Solutions: bleed the pump. if you have to hand pump at all then air is getting in the line. start with joins that might have been touched and work back. You might have to reseat every join, but usually it's something that was recently tightened.
Inspect the tank (if you have a port). Look for stuff drifting around near the outlet. otherwise get the fuel polished. Dismantle and inspect primary filters/water separators and replace filters.

It might also be a loose throttle linkage that moves under vibration. Visually check throttle position when the motor drops revs and compare with operating position.
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Old 08-09-2016, 04:27   #6
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Re: Engine suddenly drops rpm's

When you figure out what it is let us know. The same thing can happen to our Perkins.
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Old 08-09-2016, 04:56   #7
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Re: Engine suddenly drops rpm's

start with the simple and check the water separator/filter
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Old 08-09-2016, 05:58   #8
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Re: Engine suddenly drops rpm's

It does sound like fuel starvation, either a blockage or air in the system.
What kind of filters? If CAV they can be tricky, easy to get a flat O ring slightly off.

When I bought my boat, same issue, ended up being crud that would block the fuel line, but never made it to the filter so filter bowl looked spotless.
You could also have a weak lift pump too, but as it just had filters changed, I'd start there.
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Old 08-09-2016, 06:32   #9
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Re: Engine suddenly drops rpm's

you have an air leak. mine did that when i wanted to move for storm named newton. couldnt restart--is air. check ALL your connections and replace the copper washers when you find em. have fun. is a neverending battle i am finding
each thing you do to perkie makes it seek more attention.
if you paid close attention to the noise, you would have heard a slight air knock then rapid rpms then reduced rpms. most associate only drop in rpms, but there is a lot more. i knew when i heard the rattle i would not be moving under my own power. oops. and damn.
15 min was fuel pick up. 10 min is prolly your fuel pump, if is electric, or a leak at the remote filter. happy hunting. mine is at my newly installed electric assist fuel pump. oops.
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Old 08-09-2016, 07:12   #10
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Re: Engine suddenly drops rpm's

Is your tank vent line clear. You are in an area where the wasps like to plug up tank vent lines. Run the engine and then loosen the filler cap when the engine looses speed. See if that helps.
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Old 08-09-2016, 07:17   #11
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Re: Engine suddenly drops rpm's

You say fuel filters got changed. Check the area ... leaks, air, bad filter, dirt, etc.

Sounds like something has gotten into the fuel line and possibly all the way to the hp pump.

Start at the filters/fuel lines though. Make sure everything is clean and tight. Blow the lines with air perhaps.

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Old 08-09-2016, 08:43   #12
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Re: Engine suddenly drops rpm's

Dose your motor have a turbo. If so it might need some repair
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Old 08-09-2016, 08:49   #13
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Re: Engine suddenly drops rpm's

Does the throttle handle move when the RPMs drop. Could be a slipping throttle cable. If the engine does not die, just goes to idle, it may not be a fuel issue.

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Old 08-09-2016, 08:56   #14
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Re: Engine suddenly drops rpm's

Check for a partly closed valve on the fuel supply. Perhaps valves at the fuel filters, possibly the emergency shut off got inadvertently pulled.
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Old 08-09-2016, 10:42   #15
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Re: Engine suddenly drops rpm's

I am with dpfrz Sounds as if the fuel pump works until vacuum builds up to slow the fuel delivery easiest thing to check. remove the fill lid and run.
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