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Old 23-10-2016, 04:09   #1
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Perkins 4.108 erratic rpm's

Hi all,
I wonder if anybody can help me track down a problem that has just started with my Perkins 4.108 engine.
I am currently motoring along at about 6kts in the Ionian sea. Normally I cruise at about 2750rpm but an hour ago the revs keep doing little blips to about 2900. The malfunction lasts only a couple of seconds then returns to 2750.
I just tried lowering the revs to 2500 and it was steady for a few minutes then the blips started again.
I bought the boat, a 45ft Gulfstar sloop 4 months ago and this is the first problem I've had with the engine.
It has now done 800 hrs since a ground up rebuild in Swizerland.
I am assuming it is probably related to the injector pump? But really, although I have mechaniced all my own cars and bikes for 40 years, I know bugger all about diesel engines.
Any idea where I should begin?
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Old 23-10-2016, 04:29   #2
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Re: Perkins 4.108 erratic rpm's

I have a Gulfstar with a 4.108 too.
Just some random thoughts to begin with:

1) could be the beginning of an air leak or a clogged fuel filter - if the occassional rpm rise intensifies, and eventually your engine surges and dies, it's almost always an air leak in the fuel system.

2) could also be a weird wiring / set up of the alternator. On my boat for example, if the batteries are fully charged, the regulator will kick the alternator in and out of gear quite regularly - which results in the engines rpm fluctuating (load on and load off) - about ~200-300 rpm. (You could identify this problem by watching your alternator output while the engine does these "blips")

3) Are you running your Perkins 4.108 at 2700+ rpm continuously? That seems quite high revvs for this particular diesel engine.

Good luck, hope you'll find the cause quickly. Especially fuel system issues can be tough to come by.
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Old 23-10-2016, 04:43   #3
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Re: Perkins 4.108 erratic rpm's

Thank you Gypsy Spirit for such a rapid reply.
I too had just begun to think of fuel supply air entry and just went down for a little check. The flexible hose that goes to the filter has a loose hoseclamp which I just tightened but that hasn't cured it. I am about an hour away from my destination where I will be able to install a new fuel filter and give a good look over the fuel pipes...in fact I will renew all the flexible ones as I am just about to set off south for Crete and don't want these problems on the voyage there!
I'll did out my clip on ammeter now and see if the alternator is doing funny things.
What sort of Gulfstar is yours?
Cheers
Paddy
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Old 23-10-2016, 06:30   #4
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Re: Perkins 4.108 erratic rpm's

Quote:
Originally Posted by SV gypsy spirit View Post
...

On my boat for example, if the batteries are fully charged, the regulator will kick the alternator in and out of gear quite regularly - which results in the engines rpm fluctuating (load on and load off) - about ~200-300 rpm. (You could identify this problem by watching your alternator output while the engine does these "blips")....
We have a 4-108 in our boat with a fairly sizable alternator and have had similar experiences from time to time. In our case, turning on a few appliances that consume power seems to cure the matter.

FWIW...
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Old 23-10-2016, 07:02   #5
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Re: Perkins 4.108 erratic rpm's

Hey Paddy!

Quote:
Originally Posted by Tobaccoblender View Post
Thank you Gypsy Spirit for such a rapid reply.
I too had just begun to think of fuel supply air entry and just went down for a little check. The flexible hose that goes to the filter has a loose hoseclamp which I just tightened but that hasn't cured it.
Think about the whole suction side fuel system - it can be basically anything. Some boats have sludge on the bottom of their tanks which will sometimes clog the lower end of the intake tube. Some have a tiny crack in the intake tube that will occassionally suck in air when the fuel level drops in the tank and the crack becomes exposed. Some are dealing with clogged fuel tank vents and the engine has to deal with a building vacuum.

BUT before you drive yourself crazy with the fuel system:

Quote:
I'll did out my clip on ammeter now and see if the alternator is doing funny things.
If the fluctuations you are experiencing with your engine are actually pretty

- more or less regular
- not erratic
- just around a certain range of rpm (here: 2750-2900 and back)
- and your engine is not dying, nor vibrations, weird sounds, or loss of power,

I would strongly suggest to check the alternator and electrical system first.

Quote:
What sort of Gulfstar is yours?
A Gulfstar 37 aft cockpit.
With the same engine.

All the best,
Daniel
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Old 23-10-2016, 07:12   #6
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Re: Perkins 4.108 erratic rpm's

The simplest way to check if it's the 'Alternator Syndrome' would be to discharge your batteries quite a bit (running high loads such as radar, fridge, music, TV via inverter and such with the engine and other charging methods off) , then restart engine and see if the rpm problem persists.

If not, you've got the culprit.
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Old 23-10-2016, 07:49   #7
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Re: Perkins 4.108 erratic rpm's

Simply a confirmation post of probable fuel delivery ... Every wandering RPM fluctuation I have had was this...

The Alternator theory has a solid backing from the guys with experience...
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Old 23-10-2016, 08:04   #8
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Re: Perkins 4.108 erratic rpm's

Another thing to check is whether you are getting low on transmission fluid.
My velvet drive caused the same problem.
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Old 23-10-2016, 08:34   #9
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Re: Perkins 4.108 erratic rpm's

Quote:
Originally Posted by Wainui View Post
Another thing to check is whether you are getting low on transmission fluid.
My velvet drive caused the same problem.
I can see that!
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Old 23-10-2016, 08:40   #10
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Re: Perkins 4.108 erratic rpm's

my rpm blips on my latest misadventure with a newly rebuilt ka lunk were due to fail in both manual lift pump an electric assist--my injector pump was only fuel pump still functioning
i also get rpm blips from air in line and uhoh water in racor
other than those , before my runaway diesel i experienced some changes in rpms while running.
governor fail, then issues beyond my ability to resolve sola.
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Old 23-10-2016, 09:23   #11
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Re: Perkins 4.108 erratic rpm's

My 4-108 has this issue. After rebuilding injector pump, injectors, new Racor filter set up, fuel polishing, new fuel lines found the problem to a small crack on fuel line coupling nut at fuel pump. An inexpensive part cost me thousands before finding
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Old 23-10-2016, 09:49   #12
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Re: Perkins 4.108 erratic rpm's

On my yanmar 4jh3the same issue while back was secondary fuel filter beginning to plug up. Also check your tank vent. If you take the fuel cap off and hear an air hissing sound your vent is plugged.
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Old 23-10-2016, 09:52   #13
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Re: Perkins 4.108 erratic rpm's

You don't happen to have an inverter on that boat, do you? If cycling on and off, that can create just that kind of fluctuation.
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Old 23-10-2016, 10:07   #14
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Re: Perkins 4.108 erratic rpm's

That often occurs when there is a fuel blockage, air in the fuel or when you are about to run out of fuel. More likely that type of thing than a bad injector pump.
At 2700 rpm it's not an alternator on and off thing. At least in my experience with a 4-108 in a 44 ft boat.. You wont even notice it... at least with up to 125 amp alternator. Besides, your diesel engine should maintain rpm regardless of load right?
-If the boat is new to you, the idea of checking your tank vents for blockage is a good one, often missed. Little bees clog up the vent fitting. just open the fill cap while motoring and see if you get a rush of vacuum.
-Could be junk in the tank clogging the pickup tube too. It's starving for fuel.
-Did you by any chance change fuel filters and put a smaller micron one in like a 2 micron?
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Old 23-10-2016, 10:16   #15
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Re: Perkins 4.108 erratic rpm's

Just a thought but if you can record the engine sound when that happens the engine gurus here may be able to help you hone in on the problem.

I would think that if it is a load change that is causing the problem then there is something wrong with the governor. Also if it is a load change you should see it at a much lower RPM. Usually a load change results in a drop in RPM, I don't think I have ever seen it spike in RPM, especially at that high RPM.

I am leaning towards an air block in the vent tube but that is just an assumption.

Was there fuel in the tank when the boat went through the extensive refit? Could be crap in the tank.
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