4108 Revs Up While Running

[gulfstar37]

Looking for thoughts and opinions. We have a Perkins 4018, 1978 vintage, and noticed starting this season while running engine for a period of time the engine will periodically rev for a very short time then return to previous operating speed. This happens usally when motoring for for several hours. Maybe 2 to 3 times an hour. I had the injectors rebuilt about 3 years ago. Normal change out of filters and oil ect.

Please no jokes about sailboats are not suppose to use the engine. Sometime it just happens.

Thoughts to what could be causing and what to check for would be appreciated.

Thanks

[mlydon]

I had the same problem with a westerbeke recently.

The problem is an air leak in your fuel supply.

Check EVERYWHERE for leaks. I believe my culprit was a hardened o ring on the engine-mounted filter, but in the process I removed my lift pump and filter completely, opened both up, cleaned both out, and resealed everything carefully.

No more problem.

Matt

[btrayfors]

My 4-108 did the same thing.

Two good possibilities:

1. air getting into the fuel line....could even be a tiny amount; or

2. fuel starvation....clogged intake, defective diaphram in lift pump, lots of other causes, ranging from very simple and cheap to not-so-simple and expensive.

Mine turned out to be a pinhole in the copper fuel line from the tank. It ran thru the bilge and was underneath the fuel line, so not visible. Only found it by hearing a slight hiss when I used the lift pump manually. Ran my finger beneath the fuel line and, voila! A short length of fuel hose and a couple of good hose clamps solved that one.

Also, if you've recently changed your filters...primary or secondary...it could be that one isn't seated correctly.

Good luck,

Bill

[gulfstar37]

Thanks for the responses
I can look for a air leak but help me understand why a air leak would cause the engine to rev. I would think this would make the engine sputter or stop. Isn't this the reason you have to bleed the fuel system?

[seahag]

Is your prop getting worn down? That could be something to check.

[shakey doug]

It causes a leaner fuel burn similiar to an outboard that gives a rev up prior to stopping when you turn the fuel supply off.

[capttman]

Sounds to me like a fuel restriction. Change all your fuel filters clean the bowls and blow out the lines to your tanks. Good luck

[Dynamo1181]

Originally Posted by capttman

Sounds to me like a fuel restriction. Change all your fuel filters clean the bowls and blow out the lines to your tanks. Good luck

+1
A

[Tia Bu]

I had the exact same symptoms on a summer cruise a year ago. About two days later the motor died and would not restart.

It turned out to be a clogged fuel pickup in the diesel tank. I also found some loose hose clamps on the fuel lines that appeared to be allowing a little bit of air to be sucked in as the engine started starving for fuel.

The only way to find out: carefully inspect and retrace the fuel system from the injector pump all the way back to the tank.

[neptunesjester]

Having fought similar problems, I'm leaning towards an air leak that is dependant on the engine space temperature. After a couple hours of run time, everything warms up and a piece of rubber somewhere in the fuel system becomes pliable allowing air through a crack.

I had a similar situation with the cooling system on my port motor. I could fire it up and run for 4 hours then it would mysteriously overheat. I would shut it down, allow it to cool overnight, fire it up the next morning and run for 4 hours before the problem returned. After rebuilding the pumps multiple times, I finally heard the problem. The rubber hose between the seacock and the water pump">raw water pump started to whistle. A hairline crack that didn't weep water would open up once the engine room got warm and the motor would suck just enough air to overheat without a noticeable loss of discharge from the exhaust. Replacing the hose fixed the time related problem that plagued me for about 900 miles of a 1300 mile trip.

Regards,
NJ

[JulianGullsway]

I had a similar problem with my 4108, it turned out to be dirty fuel and a poorly maintained system, I cut a hole in the the fuel tank and put in an inspection hatch, and found it full of bug and sludge, tightened up all compression fittings, then bleed the bugger like hell from the side screw in the injector until all the bubbles disapear, I still had problems starting it so if you remove fuel feed on injectors 1 and four, fire the engine over then as it fires tighten up the lines, it will fire on one or the other so don't just undo them both, hope this helps good luck and dont give up on it!!

[Ex-Calif]

Originally Posted by shakey doug

It causes a leaner fuel burn similiar to an outboard that gives a rev up prior to stopping when you turn the fuel supply off.

And because the lean burning engine initially runs at a higher rpm you get more "suction" from the lift pump which could be clearing the air bubble.

I am with the pinhole leak crowd. Water in fuel could also be a factor. Water will separate from the fuel in the tank underway it could be getting stirred up and mixed in.

Although fuel system maintenance is also not a bad idea depending on your system.

[Bowman Sailor]

My 4-108 displayed similar symptoms - momentary RPM increase followed by a return to normal RPM. I traced my problem to a blocked air inlet screen. I cleaned the screen with diesel fuel, dried it well, re installed and never had the problem again. Let me know if this works for you.