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jr438234606 06-12-2006 06:05 PM

Runaway Diesel.. or not?
Hi All,

I have a Perkins 4.108 diesel aux. engine in my 44' cutter. I've noticed that after periods of extended running, the engine will begin to surge slightly. At first it will surge from 2100 RPM to around 2400 RPM, and come back down to 2100. Gradually, over a few hours, this "surge and return" routine will work its way ever further upwards. I can pull back on the throttle to keep it running normally for a while, but ultimately the RPMs get so high that even pulling back all the way on the throttle won't bring it down. At that point, I am forced to also slightly engage the kill lever to choke off additional fuel. In this configuration, I can keep the engine running smoothly indefinately. Once I kill the engine and let it cool off, it resumes normal operation.

I've looked at the idle and stall preventer settings. They seem fine and indeed the engine will run for hours and hours before starting this. I've heard of runaway diesels before, but I'm not sure this qualifies. My oil and filters seem fine and I change them often. The engine runs very smooth, is easy to start, and doesn't emit any smoke. Could this be a problem with the injector pump?

John Richardson

hellosailor 06-12-2006 06:29 PM

John, how is your oil level? If the lift pump leaks, fuel can be pumped into the oil, resulting in the oil level coming up too high, and some engines will then start to ingest oil and run away, because as far as they are concerned it is "more fuel" and they really don't care about where it is coming from.

If your oil level has been going UP instead of down, or the oil feels thin and slick (from the fuel)...that can do it.

It might be getting too much from some kind of injector/pump failure I guess, but I'd check the simplest thing (oil level) first.

jr438234606 06-12-2006 09:36 PM

I checked the oil. It does not appear to be thinned out with fuel. It does seem that I have had to add oil to the engine more than I would expect to have to. I filled it, ran it about 30 - 40 hours, and now the oil level seems a little low. I think I also have a small leak somewhere since I see some oil seepage under the engine. Not too bad though. Probably the oil pan.

Have a diesel mechanic coming this week. I'll repost to this thread.

DeepFrz 06-12-2006 10:19 PM

Surging is often a sign of fuel starvation. Have you checked your filters and fuel lines?


crzhors 06-12-2006 10:39 PM

sucking air
I've noticed on perkins that if they are sucking air,like before a clogged filter or pinched line,or suction pipe at tank, they act"lean" and get a surge.good luck,I'm sure you'll find it with good old common sense,usually"it ain't rocket science";)

jr438234606 06-13-2006 12:26 AM

Deep, glad you mention that. I have noticed that the surging also seems to take place a while after being in moderate/heavy seas. I thought, ok, the tank is stirred up and the filter is clogged. I should just change the filter-- and this does seem to help. But, resting the engine also seems to help. The last time it surged like this, I once again started to change the filter but noticed that the filter didn't look dirty, and the negative pressure gauge on the fuel didn't indicate any blockage. So, I just let the engine rest and everything was fine the next time I started the engine. Could it be a gradual build-up of suction?

As a precaution, I had my tanks cleaned. I have filled the tank with new clean fuel. I am using a 2 micron Racor filter. Still the surging persists. I have noticed some poor fuel hose connections on the filters and Algea-X device. A tiny dab of fuel is leaking from one. Could this small a leak cause such a major fuel starvation issue? Could tiny quantities of air introduced in the lines cause that? I'm also curious as to how fuel starvation could actually cause an increase in engine speed... seems like a contradiction!


Omatako 06-13-2006 01:58 AM

Hi There
Fuel starvation can also be caused by a blocked fuel tank breather causing a vacumm to be drawn in the tank. When the surge is happening try and remove the filler cap and see if it hisses as the cap comes off.
This could also be a simple issue (not DIY simple) of the governor in the injector pump playing up. If it is it will worsen with heat as the tolerances all expand ever so slightly more.
But as said elsewhere, check the simple stuff first.
Goos luck

pluscard 06-13-2006 06:33 AM

I'm also curious as to how fuel starvation could actually cause an increase in engine speed... seems like a contradiction
================================================== =======
Generally, a lean mixture makes an engine run hotter along with the surge. Sudden throttle may stall a lean engine. A rich mixture runs cooler, and has a mushy response to the throttle.

sailingdog 06-13-2006 06:38 AM

Resting the engine would allow the vacuum in the fuel tank to equalize, and if there is some larger debris blocking the intake pipe, it would allow it to settle away from the intake. So it is possible that either could be a cause of the surging. If you've taken the precautions of cleaning the tank, then I doubt it is the latter.

The poor fuel hose connections might also be the cause, as the leaks will allow the fuel line pressure to drop, but it doesn't sound as if the leaks are large enough for that. It would probably be a wise precaution to change out those fuel hose segments and properly tighten the replacement fuel line there.

DeepFrz 06-13-2006 02:02 PM

Probably not your problem, however I heard of one guy had his tanks cleaned and the workman left a rag in one of the tanks.

I had a partially plugged fuel line once and the engine would run for minutes before dying. Start up a few minutes later and it would run for a while and die. It all depends on how much fuel is getting by the obstruction. After blowing the line with a scuba tank I ran a wire through the line and then pulled some small pieces of cloth through to get all the gunk. This happened in the Yucatan Straights. ugh...<g>

good luck


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