Engine Problem After Running OK for Awhile

[Wayward]

I have a Perkins Prima M50. It starts ok and runs ok. Problem comes after running at cruising speed for a period of time, say an hour or more. When I go back to neutral, it idles rough and when I put it back in gear again, there is no increase in engine revs when the control lever is pushed forward.

The morse control cable is not the problem.

If the engine is stopped and left for 5 minutes, then the problem will go away, at least for a while.

Is this likely to be air in the fuel line, a faulty stop solenoid or a problem with the fuel injection pump??

[cdennyb]

is there any chance of a spot that gets really hot near a fuel line which might cause it to boil and thus let air into the inj pump? Other than that, perhaps a fitting that has a small crack developing which is fine when cooler then opens up to let air in after it gets hot. That would be on the suction side.

[donradcliffe]

The problem you describe is not likely to be the stop solenoid. It is most likely a fuel problem--either air inleakage or a partial blockage/vacuum in the fuel line. It could also be the governor in the fuel injection pump, but you should eliminate the fuel issues before you tackle the pump, as they are more likely to be the problem and less expensive to fix.

First things to check:

Blocked fuel tank vent--does the problem go away when you leave the tank fill plate cracked open??

Are you sure the fuel filters are clean?

[Chief Engineer]

The fuel lift pumps on these engines are notoriously poorly made.

I replaced three on one engine because the priming levers are a press fit and the convolutted way they hook up a plunger to it "wrings the lever off"

Sounds like an air leak on the suction side.

If the fuel return does not go directly back to the tank you are accumulating air in the secondary filter.

[Wayward]

Thanks for the quick response everyone.

The only pipes on the supply side of the injection pump that are getting any heat other than general engine compartment heat are the pipe from the lift pump to the engine mounted filter and from the filter to the injection pump. Neither of these are on the suction side so should not be the cause of an air problem.

I considered the blocked fuel tank vent, so tried opening the filler cap, but saw no obvious difference. I also changed both filters and then a few hours later changed the supply line filter again, though it appeared as new.

I've also replaced the fuel lift pump with a new one in case that was the problem. No difference.

And the fuel return line goes direct back to the main supply tank.

I'm thinking I may have to replace the complete fuel pipe from tank valve to primary filter. (I already changed the hose from primary filter to lift pump.)

I was also wondering if the engine breather system should be cleaned. Could this contribute at all?

[Boracay]

Can you tell us:-
1) How old is the engine? How many hours? Have the injectors been serviced?
2) After rough idling does the engine smooth out when the control is put into forward?
3) Do you have any useful power?
4) Any chance of the engine overheating?
5) Have all basic service items (fuel, air oil including gearbox) been done.

My guess is that there is a small air leak somewhere (worst case injector pump) that becomes worse when the engine is hot.

[Wayward]

1) The engine is 1992 with 1200 hours. I bought the boat at the end of last season. I do not know when the injectors were last serviced. Possibly not at all.

2) When put in gear the rough idling seems not so bad, but this may just be because of the load from the prop dampening it.

3) The prop turns at idle speed so there is some small amount of power, but not really useful.

4) The temperature gauge is steady at 82C or 180F which seems on the low side if anything.

5) Both fuel filters changed. Oil & oil filter changed. No air filter. ATF in gearbox changed.

How does one track down an air leak? Are there any tricks to making sure a connection is air tight?

[Boracay]

I'm not a mechanic so my suggestions should be taken with a grain of salt...

Go over all fuel lines, checking that they are in good condition and that all clamps are snug and not distorted.

Consider jerry rigging a can of fuel as high as possible over the engine with a tube running to the inlet side of the fuel lift pump. See if this improves the situation. If it does this could indicate if the problem lies in the fuel system between the tank and the engine filter. This is not a conclusive test.

[donradcliffe]

Some boats come with a screen on the intake in the fuel tank--bad idea, because if it gets gunked up your engine will be starved for fuel.

Another cheap test is to put a bulb type outboard fuel pump in the fuel line from the tank to the lift pump and try squeezing it when the engine is rough/no power. If the bulb is partially deflated and doesn't refill with fuel after you squeeze it, you have a blockage upstream. If it refills easily and squeezing solves the problem, you could have a partially blocked filter/line or a bad lift pump. The squeeze bulb also makes bleeding easier, and some people leave one in place but I am leery of the potential fire hazard and only use mine for testing.

[John A]

It dosen't sound like an air leak as any air that leaks into the system usually.would become an air bubble that when it became big enough to stop the engine would need to be bled from the fuel system before the engine could be restarted.

However, here is the way that I have tracked down air leaks on my Perkins:

Take about a foot of clear rubber tubing and attach one of those bubble pumps that are used to prime outboard motors from a detached fuel tank, use hose clamps for a good seal. Then attach it upstream to anything you want to check and pump fuel through it.

Disconnect the fuel line that runs from the primary filter to the secondary filter jwhere the line connects with the secondary filter and pump fuel thru the line. If there are bubbles of air in the line, move the test to before the fuel goes into the primary filter and try again. Any air leaks are quickly identified. Also check the metal fuel line that runs from the secondary pump to the injector pump for any leakage of fuel, keeping in mind that if fuel leaks out then air can leak in.

Because your problem seems to be tempreture related, I would think that the problem is with the injector pump and it needs to be rebuilt.

There is a possibility that the fuel line is not large enough to carry enough fuel thru the injector pump to keep the injector pump tempreture down. This usually is a concern when re-powering from one brand of engine to another, but it's something to consider.

Hope the solution is a simple one.

[Boracay]

Could the fuel return hose be blocked?

[Chief Engineer]

How long have the fuel filters been in there?

Whay micron size are you using?

[Chief Engineer]

Check and see if you have fuel to all injectors.

While engine is running, crack open each injector line at the injector, you should notice a decrease in rpm...there should be fuel spurting out...no bubbles.... If you notice no decrease in rpm...that injector is not doing it's job.

I would also check the linkage for throttle all the way fom lever to engine. Make sure all mounting clamps are secure and that cable is not damaged.

Check you stop cable also and make sure it is in the proper position.

A partially engaged stop cable can also be a problem.

Call Sherri at www.tadiesels.com

[Wayward]

Hi Boracay, I think you may have it. The fuel return line was completely blocked. I've cleared it now, but won't get a chance to confirm for sure that this has solved the problem until I can get a chance to go out and do some motoring at cruising speed. May be a day or two as there is a gale blowing outside just now (the Mistral!).

Before I reconnected the fuel return line, I let it run into a container for a while. The flow was pretty slow at perhaps half a litre in five minutes. Is that about normal would you think?