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Old 05-06-2016, 07:21   #1
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PERKINS 4108 LOW RPM UNDER LOAD...help!

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?
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Old 05-06-2016, 07:43   #2
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

Any chance your old engine is just worn out and that is all it will do anymore?
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Old 05-06-2016, 08:06   #3
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

Very interesting. I'm looking at a similar situation. Getting diesel into the crank via the pressure pump shaft. Prior to this we had a pressure pump leak (on the low pressure side). Had the pump reconditioned, and installed back. Just started testing and she fires beautifully and seems to work fine at the dock, but as I start throttling up past ~2000 the RPM response seems sluggish. The tack seems to stall around 2200-2500, then with a little time and more throttle the tack surges up to around 3000.

I haven't had it under load yet. And we would usually causing at around 1800 anyway, but I too am curious/concerned about this change.
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Old 05-06-2016, 08:18   #4
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

This engine was rebuilt and has less than 300 hrs. I would be surprised if that was the case. I could do a compression test but I doubt it since it used to rev up without problems. The fact that it doesn't smoke at full throttle makes me think there isn't enough fuel.


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Old 05-06-2016, 08:20   #5
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

Quote:
Originally Posted by Mike OReilly View Post
Very interesting. I'm looking at a similar situation. Getting diesel into the crank via the pressure pump shaft. Prior to this we had a pressure pump leak (on the low pressure side). Had the pump reconditioned, and installed back. Just started testing and she fires beautifully and seems to work fine at the dock, but as I start throttling up past ~2000 the RPM response seems sluggish. The tack seems to stall around 2200-2500, then with a little time and more throttle the tack surges up to around 3000.



I haven't had it under load yet. And we would usually causing at around 1800 anyway, but I too am curious/concerned about this change.

Interesting! Did you get a pump from TAD?


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Old 05-06-2016, 08:22   #6
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

Quote:
Originally Posted by Mike OReilly View Post
Very interesting. I'm looking at a similar situation. Getting diesel into the crank via the pressure pump shaft. Prior to this we had a pressure pump leak (on the low pressure side). Had the pump reconditioned, and installed back. Just started testing and she fires beautifully and seems to work fine at the dock, but as I start throttling up past ~2000 the RPM response seems sluggish. The tack seems to stall around 2200-2500, then with a little time and more throttle the tack surges up to around 3000.



I haven't had it under load yet. And we would usually causing at around 1800 anyway, but I too am curious/concerned about this change.

Mine goes up to 3800 - 4000 rpm in neutral but the issue is under load.


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Old 05-06-2016, 08:28   #7
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

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Originally Posted by avazquez View Post
Interesting! Did you get a pump from TAD?
No, it was our original pump rebuilt by a local pump shop. From all accounts they know what they're doing. Odd...
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Old 05-06-2016, 08:30   #8
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

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Mine goes up to 3800 - 4000 rpm in neutral but the issue is under load.
I have to get a new valve cover gasket before I can really put the engine to the test. I'll get that done tomorrow. After that I'll report back on our load tests.
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Old 05-06-2016, 08:38   #9
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PERKINS 4108 LOW RPM UNDER LOAD...help!

The thing is that when it happened I removed the lift pump, the injectors and the injection pump. There are two things I still have to verify: the injection pump stop lever and verify if there is an obstruction at the lift pump. Other than that I am clueless. I asked the guys at Trans Atlantic Diesels and they suggested advancing the pump timing a bit which I did. The engine runs smoother, doesnt smoke but still wont rev properly under load.


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Old 05-06-2016, 15:09   #10
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

Sometimes debris can block or partially block a fuel line and allow just enough fuel to let the engine to run steady at a lower rpm. Also, a piece of dirt stuck under a fuel lift pump valve can allow fuel to backflow and do the same. Check your return flow back to the tank.
An injector pump out of exact time also will restrict rpm. Partially blocked injector(s) can do it, too.
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Old 05-06-2016, 15:24   #11
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

Why take an old engine to 3800 rpm in neutral? Don't.
This pump works well and no smoke at a 75% power under load. Perfect.
Before the old pump leaked more fuel into the cylinders and you got what you got.
If it ain't broke don't fix it.
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Old 05-06-2016, 16:09   #12
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

Just wondering here but if you adjust the high stop for more travel, does that mean the no load rpm would go even higher? You would think it would but I'm not so sure. I have never heard of a Perkins getting over rated max rpm. I'm suggesting you turn in the stop screw and see if the load rpm increases. If it does then worry about the no load rpm. It might be limited inside the pump with some sort of governor?
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Old 06-06-2016, 08:09   #13
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

I wouldn't want to mess around with the pump settings other than the idle and anti stall as per TAD instructions. I am going to the boat this afternoon to check the stop lever and the lift pump fittings.


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Old 06-06-2016, 09:02   #14
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

Two days ago I changed the oil in my Perkins 4-108. Went to start it after and it turned over and didn't start.

Something's fishy. Checked the stop lever. Stop lever OK.

Tried it again. Turned over without firing. Moved the stop lever to no fuel, then back to fuel. Hit starter button. Started immediately.

Note to self. Something's fishy. I wonder when it will bite me in the behind if I don't address it?
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Old 06-06-2016, 09:15   #15
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Re: PERKINS 4108 LOW RPM UNDER LOAD...help!

@ avasquez
The condition high rev idle/neutral ok and too low rev with prop engaged suggests a clogged/dirty fuel filter.
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