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Old 02-08-2015, 10:50   #1
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engine wont throttle up?

Greetings folks. I own a 50 ft Cat with 2 x Volvo Penta 55HP engines. They have 980 hours of use. My port engine runs smoothly for a while but then suddenly fails to throttle up. It stays at idle and even then, runs a bit rough and occasionally will die. If I manually pump the fuel pump a few times, she immediately responds and runs fine for a while. We have checked the lift pump, replaced the O rings and replaced the rubber diaphram. I suspect she might be pulling a bit of air so next step will be to check the fuel lines from tank to engine. Any ideas on what else can cause this? Your assistance is appreciated. Regards Mike
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Old 02-08-2015, 12:56   #2
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Re: engine wont throttle up?

Partial clogged fuel line? Kink in the fuel line? Collapsing fuel line? Dirty filter? Is there a screen on the tank pickup? If so get rid of it.

You don't give us much information to go by. How old is the system? When were the tanks last cleaned? etc.

Have you read any of the 100's of threads on similar issues?
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Old 02-08-2015, 19:26   #3
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Re: engine wont throttle up?

Vaccum leak is another possibility. Was any fuel system work done before the problem started?
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Old 02-08-2015, 20:08   #4
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Re: engine wont throttle up?

So it runs fine most/some of the time then it gets the little stubborn fits?

To me, erratic behavior is often caused by a clog or partial blockage. For a start I'd disconnect the fuel line at the lift pump and blow compressed air through it backwards. If your engine is diesel and has a return line blow through that as well.

To check for a vacuum leak, remove the line between the lift pump and carb/injector and replace it with a hose that goes into a glass jar. Then crank the engine and see if you get bubbles in the fuel when it starts pumping. If you can use a piece of clear line you can see the bubbles travel through the hose which can give you a good idea how many there are.

You can also narrow it down by putting a hose on the intake side of the lift pump and running it straight to a can of fuel. If it fixes the issue then you know the problem is in the tank or line coming from the tank.

A somewhat half assed way of dealing with this issue if you are away from home is to put an electric fuel pump inline between the lift pump and carb/injector if you have tried everything and still can't figure it out. It is not right and if you have a partial clog there is the risk it turns into a total blockage. However, if that happens you can get enough hose to run the intake of the new pump and run it straight into the tank through a fuel fill. While this is a haphazard approach, it can get you home or to a competent mechanic somewhere. Just make sure if you put an electric pump inline that it goes between the lift pump and the carb/injector. If you put it in front of the existing pump there is the risk that if the lift pump diaphragm leaks/ruptures it will fill the engine oil sump full of fuel. This is especially dangerous with diesels as it can cause vapors to blow by past the rings and the engine becomes uncontrollable and won't shut off.

Anyway, good luck and hopefully you can just shoot some air through your fuel lines and your problem will go away.
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Old 03-08-2015, 09:26   #5
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Re: engine wont throttle up?

Sounds like a clogged fuel pickup from growth in fuel tank. Especially if it happens when traveling through rough water. As fuel sloshes around debris gets lodged in pickup causing problems like yours. After engine has been shut down for a while, debris will slide back to tank bottom. You can clear it immediately by connecting short fuel hose to tank and blowing it out. As stated by others, there should never be a screen on a diesel fuel pickup. Growth cannot be removed by polishing fuel using tank pickup. You must polish/clean through an access hole in tank.
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Old 03-08-2015, 09:37   #6
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Re: engine wont throttle up?

I had what you are describing occurring to my 4JH3hte Yanmar. What ended up curing the problem changing the garden hose vent hose Moorings repair handy man had replaced original hose with ,that was kinking. While you are at it check your vent hose for kinking.
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Old 03-08-2015, 09:53   #7
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Re: engine wont throttle up?

when was the fuel filters replaced, do you have a 12vdc fuel pump, some have filter screens in them, if you have replaced filters agree on checking fuel lines.
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Old 03-08-2015, 10:28   #8
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Re: engine wont throttle up?

Greetings everyone. Thank you so much for your valuable replies and input. I will systematically work through the suggestions made. The comments regarding fuel quality are interesting as I was wondering if the fuel contamination could be a factor as that certainly is a possibility. The filters have been changed frequently, more often than is normally required. The quick diagnostic methods suggested could save much time and effort. The knowledge, experience and willingness to assist on this forum is wonderful and is much appreciated.
Regards
Mike
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Old 03-08-2015, 12:03   #9
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Re: engine wont throttle up?

Another common problem is a clogged vent. Mud Daubers love to make little mud nests in them. If there is no screen over the vent this is a good possibility. Also, installing an electric fuel pump before the filter is an easy way to find leaks plus it makes it easy to prime the line & will often keep the engine running if there is a clog or mechanical fuel pump failure.
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Old 03-08-2015, 12:36   #10
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Re: engine wont throttle up?

Thanks again scout 30.
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Old 03-08-2015, 16:31   #11
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Re: engine wont throttle up?

On any diesel it can be air sucking in past the O ring seals in a fuel filter. I'd just replace the elements while you are at it and rub a little grease on the new O rings that come with the new filter element. Then put it all together again. Lightly greasing the O rings help them "feel" their way into place. Don't over tighten. Like a boiled egg; just right.


It's a process of elimination going through the fuel connections but a filter air leak can often be the culprit and doing that will at least provide you with a new element. Anything you do check the running before trying something else.
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Old 03-08-2015, 16:40   #12
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Re: engine wont throttle up?

Regarding my previous comment. I'm referring particularly to the fuel filter/s between the tank and the lift pump as with a leak they can possibly drip when the motor is not running and suck in air when it is running. Any leaking filter after the lift pump will usually only leak fuel out but still worth checking after everything else.
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Old 03-08-2015, 16:55   #13
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Re: engine wont throttle up?

In my experience with an older MD7A with similar behavior (quit shortly after launching), it was the fuel pickup in the tank that had a clogged screen. Ran fine on the hard, but wouldn't continue to run undeer load.
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Old 03-08-2015, 17:13   #14
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Re: engine wont throttle up?

An easy check on the fuel line blockage is to blow back down the line from the filter to the tank. If you can't blow by mouth use one of those small compressors that come with battery boosters for car tires and you should hear the air bubble back into the tank when clear. Then you know that you require a tank clean out.

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Old 03-08-2015, 19:55   #15
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Re: engine wont throttle up?

I always try to follow this saying used in the medical profession, ''If you hear the sound of hoof-beats, don't look for zebras''. Another words, check the obvious-most likely first. Has served me well.
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