# diesel engine loses rpms then recovers(?)



## raymilosh (Jan 12, 2005)

I have a 1973 4 cyl Westerbeke diesel engine in a sailboat. It stalled one day. It had badly clogged fuel filters and water in the fuel/water separator and in the next inline fuel filter. I cleaned and replaced it all. On Sunday, we motored for 2 hours at 1300 rpms and it ran fine. Then for the next *2 hours* it would run well for a few mins, then drop to 1100 or 1000 rpms for a few seconds, then recover. Made my heart sink every time (we were motoring a 30,000 lb boat in a heavily trafficked narrow channel). When I throttled back, it stalled, but restarted easily. It didn't *seem* to be running on 3 cyls during the rpm drop, but I could be mistaken.

The engine was rebuilt in 1995 and has 700 hrs on it. the two 40 gal tanks were filled 5 years ago. The engine has run only a few times a year. The tank was topped off last summer and twice this summer. 
There is already sediment, but no water in the clear acrylic bowl at the bottom of the freshly cleaned fuel/water separater. the next thing in-line is another fuel filter, then an electric lift pump, then the IP.

I'm assuming fuel starvation from either a filter in the tank or a filter in the IP that I don't know about, but the weird thing is about this recurring rpm drop for 2 hours without the symptoms getting worse. Maybe it is sucking in air?

Got any ideas?


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## Seth (Dec 3, 2012)

Old fuel. Drain the tanks, refill, change filters, then bleed the system while running. Stanadyne fuel additive would be a plus.


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## TnAndy (Sep 15, 2005)

Sounds like fuel starvation. Diesel will run on some real crap, so just being old probably ain't it. 

Had a similar thing in a tractor once, and it was a glob of junk floating in the tank. Every so often, it would float right over the outlet port, blocking it almost entirely, starving the engine for fuel. Picture a marble rolling around in the tank. 

My suggestion is remove all fuel from the tank and shine a light in IF you can see the outlet, OR (in the tractor case), remove the fuel line at the tank the next time it dies, and see if you physically have fuel flowing freely. If not, poke something thin enough up in the outlet and see if you can dislodge 'whatever', and get fuel flowing. If that's the case, you'll need to most likely remove the tank and clean it good.

Next suggestion:

Does it have a small booster pump between the tank and main engine ? When you turn the key on, do you hear a small 'tick-tick-tick' kinda noise ?

Had a Mitsubishi dozer once that used a small electric booster pump like this. In the back of the pump, and you had to remove the back part of the pump to find it, was a small (about the diameter of a 50 cent pc x 1" deep) filter that would clog from tank sediment, and starve out the engine. I changed main fuel filter twice with no effect, thinking it was a clogged fuel filter...which it was, I just hadn't FOUND it....and probably would not have if I hadn't starting tracing the fuel system in the part manual....which showed that filter. Looking at the pump, you wouldn't have guess there was a filter there.


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## LincTex (Oct 11, 2010)

If you found that much crud in your filters...... Just IMAGINE what is still left in the tanks, that you didn't draw up into the filters yet!!

I doubt the tanks are that easy to access... many are built right into the boat. You need to drain what you can, and access the inside by whatever means possible - likely through the fuel-level-sender plate. You can set up a 55 gallon drum with a vacuum line on one side and a suction hose (at least 5/8" ID) and "vacuum" out your fuel tanks. You'll likely get a nasty gelatinous blob that resembles a big jellyfish out of each tank. Search "diesel tank algae" to see what I mean. Gooey nastiness.... The presence of water ALWAYS exacerbates the problem!

Push clean fuel backwards through the fuel line to flush out the crud.

Diesel is expensive. Don't waste it. The diesel isn't toast... allow it to settle for a few weeks, siphon the water off the bottom, and filter it well through a 10 micron Goldenrod diesel filter and put it back where it came from. 

Add some "algaecide" like "Bio-Kleen" diesel additive or "Bio-Blast" diesel biocide EVERY YEAR to keep the "jellyfish" from forming inside your tanks again.


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## LincTex (Oct 11, 2010)

http://www.biobor.com/fuel-additives/marine-fuel-additives/

http://www.biobor.com/products/biobor-md-fuel-additives/marine-fuel-additives/
MD for "Marine Diesel"


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## idigbeets (Sep 3, 2011)

didn't see much listed in those additives as a fungicide... I'd also treat w/ that. An algae can grow and live in diesel fuel/tanks and wreaks havoc on filters, pumps, injectors etc...


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## raymilosh (Jan 12, 2005)

Thank you all for the replies. I'll check for a filter in the lift pump. (as well as in the tank and in the IP). The tanks do seem to be built into the boat. No drain that I can find. Fuel lines all come out the top. 
I am now thinking of vacuuming out the fuel, settling it and decanting off the good stuff from the top back in. I hear there is a service that I may be able to hire to do this for me. 
If not, Ill need two 55 gal drums, bc there is about 70 gallons of fuel in the tanks. 
I'm thinking the easiest access is through the fuel fill hoses which are 1.5" diameter and go fairly straight down into the tanks. 
And an algaecide sounds like a good cheap solution for the future.


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## Harry Chickpea (Dec 19, 2008)

Another possibility _might_ be a collapsing fuel line. If so, when the fuel flow gets going, the pressure in the line drops, a weak wall can then collapse blocking the line, which reduces the flow until RPM drops, and then it opens back up.


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## nosqrls (Jun 9, 2012)

Also check fuel tank vent. It should be topside It sound like it is letting in water and needs cleaning and or replacing.


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## raymilosh (Jan 12, 2005)

Nosqrls,
When we filled the tanks, fuel came out of both vents (we caught it with a No-Spill device). Can I assume they are open or are they more complicated than I realize?

Harry, THAT's the kind of thing that I'm looking for. The bad fuel/clogged fuel filters explanations are surely part of the story, but it doesn't explain the cyclical nature of the problem or the fact that it never improved or worsened for 2 hours. 

AHA... I could tee a vacuum gage in various places in the fuel line. It would tell me if a vacuum develops at all and if so, differences in vacuum in different places in the line would tell me where the blockage is. Now that's a potentially valuable diagnostic tool...

I've got an 89 diesel Jetta with a fuel starvation problem that has been plaguing me. I can't wait to try out my new trick. I bet it'll work.


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## nosqrls (Jun 9, 2012)

The fuel vent should allow air but no liquid to flow through. When filling this causes auto fuel shut off to trip handle. So I would change out or clean both vents. Clean both tanks check o-rings/gaskets on fills. Stop water from getting in. And put a water separator between fuel tank and engine or 2 auto works best senses water and dumps it in waste oil tank manual works but check it reg. I have 2 separators on my tractor because it will sit for long periods. Lucas fuel cleaner works wonders tractor was running like 5hit double dosed it, started it, ran it at idle for about 5min and idle changed from 500 to 540 rpm. If you want to hop up the jetta get bigger nozzles wife has 09 200 hp and 400lb/ft vw diesels are not vw they are bentley and porsche trans.


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## agmantoo (May 23, 2003)

I have observed something as simple as leaving the old fuel filter gasket in place and then installing a second new gasket with the new fuel filter creating a source of air to enter the fuel lines. You may want to check this also. I think we are in agreement that your issue is with fuel delivery, with air or water present. I would also consider new fuel in a clean can thus bypassing the current onboard fuel tanks. Gravity feed the new clean fuel using a long fuel line if necessary because of the location of the fuel pump.


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## JeepHammer (May 12, 2015)

I'm not familiar with the engine/fuel system, so I'm spitballing here...

Its VERY hard to get the crud out of a fuel system once the fuel has gone 'Rotten'.
Change filters frequantly, use injector cleaner in your fuel.

The other thing that springs to mind,
Most fuel pumps have a pressure bypass valve,
That valve usually consists of a plug/ball/piston of some kind in a bore or orifice/seat,
And a spring behind that piston or ball.

When the bore/ball or piston or spring corrode or get plugged up with rotten fuel or solid crud, 
The bypass can get stuck open, reducing fuel pressure and causing the engine to stall or run slower for lack of fuel...

There is also a screen or 'Rock' filter in the inlet of most diesel pumps.
This usually doesn't give you the 'Up/Down' RPM you describe, rather choking off fuel supply.

An exception is when something like a foil top from fuel conditioner bottle or stray piece of gasket/plastic gets into the fuel system and moves around...
Those foil seals have driven more than one mechanic crazy down through the years!!!
(Me included!)

A pressure gauge on the OUTPUT of the fuel pump will tell you pretty quickly of your issue is with the pump,
*IF* you are absloutely sure its not an inlet problem reducing fuel availability to the pump.


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## 1948CaseVAI (May 12, 2014)

Wow - rebuilt 20 years ago and has fuel that is 5 years old. May be some kind of record. At a minimum replace fuel and flush system, as others suggested. Use algae preventative Replace fuel lines (as others suggested) - old fuel line probably collapsing under suction. Check vent(s).


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## chuckhole (Mar 2, 2006)

Geesh. I haven't seen a Westerbeke in years. I feel your pain. I had a 45 hp Perkins in our 11.5 meter Beneteau (sold last year). We kept the fuel rotated and fresh every year and made sure that algae additives were used. And I also found that WD-40 makes a good starter fluid.

I had the same problem that JeepHammer mentioned with my truck. The fuel filter primer pump started sucking air. Old, cracked fuel lines will give you the same troubles.

Also, even 5 years ago, you could only get the 15ppm low sulfur highway diesel. Biodiesel is added as a replacement lubricant for the sulfur. But with water, it goes rancid too.

I would definitely flush and fill the tanks and then recover the old stuff after filtering and mixing it with newer fresh fuel. And spend a little more time in the channel under power or motor sail for while.

"Late at night, home to bed, on the right, leave the red".


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## raymilosh (Jan 12, 2005)

Two weeks ago, put the fuel line to the IP into a clean 1 gal container and ran the electric lift pump until the container was full. It took a couple of minutes, pumped clean fuel with no air bubbles and there was no decrease in fuel output during that time. I also removed the fuel pickup tube from the tank. It has no filter and no blockage. I sealed it and put the entire fuel system under 10 lbs vacuum and left it. it never decreased, meaning there are no air leaks. This test pretty much clears the everything upstream of being the problem, no? Not what I was expecting. It seems that whatever problem there was is now gone. 

It occurred to me the starvation problem could have been from the elec lift pump having failed when it got too hot(?). I couldn't check to see if it was running during the problem, because it is mounted to the engine which was hot and vibrating. 

I installed a fuel pressure/vacuum gage in line along with clear vinyl line just upstream of the IP so in the future, I can watch either fuel pressure (if the lift pump is working) or vacuum and any air if the elec lift pump fails or if the fuel pickup in the tank is temporarily blocked by a jellyfish. 

There is no water in the water separator. In my experience, fuel tanks here in the humid south ALL condense water during the winter, so water in the fuel happens in the absence of anything being wrong.

I do plan to clean the fuel and add additives and to check on the air vents, but I doubt they caused the starvation problem. 

I can't say I've figured out what happened yet, though. I'm guessing either an intermittent failure of the lift pump or an occasional blockage of the pickup tube or stuck tank vents (any of which will be diagnosed by the in-line gage).

JeepHammer. Interesting idea...I don't quite understand, though. Is the pressure regulator you're talking about on the IP? Where would I install a pressure gage?


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