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Old 10-06-2016, 09:51   #46
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

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Originally Posted by Jason Flare View Post
I called this one. (See post #14)

But I am not a mechanic so don't expect this to happen every time.

Me neither but with every troubleshooting I learn something. When you go out there its you and your boat.


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Old 10-06-2016, 10:09   #47
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

checked under the rocker cover? I had a perkins that the push rod popped off the rocker. It ran great! but smoked and gained fuel in the oil. (pushing raw fuel past the rings as the valve wouldn't open.) When we removed the rocker cover it was obvious the push rod was just sitting there along side the rocker.
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Old 10-06-2016, 12:02   #48
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

Problem solved. It was bad spring on stop lever


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Old 16-10-2020, 22:18   #49
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

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Perkins 4108 runaway.
My 1984 Perkins 4108 low oil pressure alarm went off on idle and after checking the oil and noticing it was "fine" I replaced the sensor. With the new sensor the alarm went away so I figured it was a bad sensor.

The next day as I was leaving the marina when in reverse and with the main sail up suddenly the engine began to accelerate and white smoke came out of the exhaust. It lasted for 5 to ten seconds. I was able to put it in neutral and shut it down. But the boat ended up on a reach sailing into the marina in a channel with slips on both sides and a concrete wall at the end.

By pure coincidence a friend had seen the whole thing and jumped from another sailboat. As I ran to lower the main he steered the boat and my wife looked after the girls. I was able to lower the sail but of course the boat was slowly moving towards the leeward docks full of boats. We decided to turn on the engine and were able to stop the boat and get enough reverse movement to put it in the slip. But it did runaway again for lets say 10 - 20 seconds intermittently. During the event rpms went up to 3000 but it never overheated. There was about a quart of an oil/fuel mix under the engine. Tracing it it seems like the oil came out of the valve cover cap.
After checking everything out I found that the injection pump was leaking badly into the engine through its main shaft. I removed 2.8 gallons of oil/fuel mix from the engine. Don't ask me why I didn't notice the high level oil in the dipstick. I guess there was so much diesel that the dipstick would drain and by the time I looked there was only fuel in the tip.
At this moment I have installed the rebuilt TAD fuel injection pump and engine runs great except for rpm's under load won't go past 2500 rpm. Clean propeller, freely turning shaft, it won't even smoke under load at full throttle. In neutral it revs up to 3800 rpm. In the past with the old pump under the same conditions it would rev up to 3000 rpm under load tied to the dock and black smoke would come out of the exhaust. Any ideas?
How did you find out it was the injection pump leaking?

We usually need to add oil regularly to our 4108 (but have not had to recently) and now find that the oil pressure is very low with the alarm on at low revs. This is not normal for our engine.

The oil dipstick shows full but have not added any oil for ages.
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Old 17-10-2020, 02:27   #50
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

There are several ways diesel can make it to the crankcase. First I removed the lift pump to check the diaphragm and it was fine. Then I removed the injection pump and it was evident immediately as there was diesel pouring out of the main shaft seal. I replaced the lift pump and got a rebuilt injection pump from TAD. You really want to figure it out. Run away engine was not a pleasant experience.
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Old 17-10-2020, 07:51   #51
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

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How did you find out it was the injection pump leaking?

We usually need to add oil regularly to our 4108 (but have not had to recently) and now find that the oil pressure is very low with the alarm on at low revs. This is not normal for our engine.

The oil dipstick shows full but have not added any oil for ages.
Well, it does sound like you are getting diesel in the crankcase. The 4-108 runs low oil pressure when idling normally, if you have thin oil from diesel in it, it probably makes the alarm go off.
There are a few things that get diesel in the crankcase: (and probably more than I have noted)
-leaking lift pump diaphragm
-leaking injection pump
-a bad injector streaming fuel
-bent pushrod. (I had a perkins that the push rod popped off the rocker. It ran great on 3 cylinders! but smoked and gained fuel in the oil. (pushing raw fuel past the rings as the valve wouldn't open.) When we removed the rocker cover it was obvious the push rod was just sitting there along side the rocker.)
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Old 17-10-2020, 11:01   #52
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

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Originally Posted by Cheechako View Post
Well, it does sound like you are getting diesel in the crankcase. The 4-108 runs low oil pressure when idling normally, if you have thin oil from diesel in it, it probably makes the alarm go off.
There are a few things that get diesel in the crankcase: (and probably more than I have noted)
-leaking lift pump diaphragm
-leaking injection pump
-a bad injector streaming fuel
-bent push rod. (I had a Perkins that the push rod popped off the rocker. It ran great on 3 cylinders! but smoked and gained fuel in the oil. (pushing raw fuel past the rings as the valve wouldn't open.) When we removed the rocker cover it was obvious the push rod was just sitting there along side the rocker.)
Yes - something is definitely not right. The other symptom I forgot to mention was occasionally - particularly at higher RPM - the engine with increase in RPM on its own - maybe 100-200 RPM - not often.

It really points to a fuel problem, and my suspicion is on the injectors. Will change oil and filter first.
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Old 17-10-2020, 11:24   #53
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

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Yes - something is definitely not right. The other symptom I forgot to mention was occasionally - particularly at higher RPM - the engine with increase in RPM on its own - maybe 100-200 RPM - not often.

It really points to a fuel problem, and my suspicion is on the injectors. Will change oil and filter first.
That rpm increase sounds like it's starving for fuel a bit, they do that just as you are running out of fuel too. But if the crankcase was over full with fuel it could be about to runaway!
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Old 17-10-2020, 11:41   #54
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

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Originally Posted by Cheechako View Post
That rpm increase sounds like it's starving for fuel a bit, they do that just as you are running out of fuel too. But if the crankcase was over full with fuel it could be about to runaway!
Re the runaway bit - it was after the RPM surge that I added more oil - this should have made a runaway scenario even more likely but the engine behaved for the rest of the day.

Will do oill change tomorrow and update.
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