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Old 11-29-2010, 07:45 AM   #1
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Small Surges in RPM Every Few Minutes.

We have just finished moving Skinny Dippin' from New Bern to her new home in Carolina Beach, NC. During the trip, I've notice a slight increase of the little 200-300 rpm surges. Every 7-10 minutes we get a little ZROOOM that last no more than a couple of seconds. Hardly enough time to even look down at the tach to see it returning to the priviously set rpms.

Any idea what this is and how to stop it?

(1986 Perkins 6.354M)

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Old 11-29-2010, 07:58 AM   #2
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RE: Small Surges in RPM Every Few Minutes.

Hiya,
** How clean is/are your fuel filter(s)?
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Old 11-29-2010, 08:15 AM   #3
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RE: Small Surges in RPM Every Few Minutes.

I would look for a small air leak in the fuel line between tank and lift pump. Have you had any part of the fuel system open recently?
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Old 11-29-2010, 08:56 AM   #4
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RE: Small Surges in RPM Every Few Minutes.

Fuel filter should be pretty clean, but that was my first thought too. I usually run a 10mic filter, but when I changed it last time, I only had a 2mic. then I topped off my tanks, so it could be a bit clogged.

As for the air. I could check for that. I haven't been in that end of the system yet (pre lift pump). I cleaned the screen in the lift pump about 6 months ago and there is a small fuel leak where the line goes into the top of the secondary filter.
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Old 11-29-2010, 09:53 AM   #5
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RE: Small Surges in RPM Every Few Minutes.

2 micron last time since you didn't have a 10 -- There you go! How about a filter vacuum gauge on the primary- do you have one? On my Perkins Sabres I run 30 micron on primary Racor*with Perkins on engine filters. Nary a hiccup*nor any increase in vacuum.
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Old 11-29-2010, 10:28 AM   #6
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Small Surges in RPM Every Few Minutes.

Hiya,
** I run 2 micron primary (Racor) and 2 micron secondary (CAV) on a 120 Lehman. Now*I know nothing about Perkins but flow rate SHOULDN'T be THAT much different than the Lehman (or maybe it is) so changing from a 10 micron to a 2 micron will probably not cause your surging.* I'd look to the integrity of your total fuel system before immediatly blaming the 2 micron and as Mr. sunchaser suggested, a vacuum guage is a good idea.* JMHO.* Another thought, could be a clogged injector but start with the simple things first.

-- Edited by RT Firefly on Monday 29th of November 2010 11:29:18 AM

-- Edited by RT Firefly on Monday 29th of November 2010 11:44:57 AM
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Old 11-29-2010, 10:56 AM   #7
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RE: Small Surges in RPM Every Few Minutes.

I ra a Perkins for many years and that sounds exactly like mine did when I had a fuel restriction thus starving the injection pump. That happened several times toi me until I got it all sorted out. In my case there were large particles clogging the 3/8 fuel lines, nothing clogging the filter as the large particles got spun to the bottom bowl.

Hope this helps.
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Old 11-29-2010, 11:08 AM   #8
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RE: Small Surges in RPM Every Few Minutes.

My Cummins 330 did that once near the end of a week long trip where I was cruisng at 1800 rpms for extended periods of time. I ran the engine wide open for a few minutes and the problem has not been back.

I either blew something out or un gummed something that was sticking I guess. I had no clue was was wrong but I knew that it was not a fuel filter because I had just changed it.
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Old 11-29-2010, 12:06 PM   #9
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RE: Small Surges in RPM Every Few Minutes.

Surging or hunting is about 98% of the time a problem with fuel restriction, either a blockage somewhere in the system, up to and including the pick up tube in the tank, or a vacuum in the fuel lines like a clogged filter. Chuck
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Old 11-29-2010, 01:05 PM   #10
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RE: Small Surges in RPM Every Few Minutes.

Okey-Dokie... I guess I should start looking around.

BTW... No vac gauge on the Raycor... It's on the list
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Old 11-30-2010, 04:18 AM   #11
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RE: Small Surges in RPM Every Few Minutes.

Fuel is always the first place to look, but the shift cables not fully placing the tranny in gear are also suspect, as is the tranny .

Oil level in tranny?
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Old 11-30-2010, 05:56 AM   #12
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RE: Small Surges in RPM Every Few Minutes.

Quote:
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Oil level in tranny?
Fine. I check it before every trip.
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Old 11-30-2010, 07:28 AM   #13
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RE: Small Surges in RPM Every Few Minutes.

Temporarily put a piece of clear tubing in the fuel line just before the lift pump and look for bubbles.
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Old 11-30-2010, 08:54 AM   #14
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RE: Small Surges in RPM Every Few Minutes.

Quote:
RickB wrote:

Temporarily put a piece of clear tubing in the fuel line just before the lift pump and look for bubbles.
That's a neat idea. THANKS!
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Old 11-30-2010, 09:34 AM   #15
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RE: Small Surges in RPM Every Few Minutes.

Ironically, I had this issue with a previous boat. The cause was in the vacuum gage for the Racor. The gage was remote so there was tubing that plumbed the suction from the filter to the gage....that is where the leak was and air was getting into the system there. A mechanic acquaintance found this for me....I never would have found it.
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Old 11-30-2010, 09:38 AM   #16
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RE: Small Surges in RPM Every Few Minutes.

I'm curious why it only happens every few minutes, though?
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Old 11-30-2010, 09:51 AM   #17
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RE: Small Surges in RPM Every Few Minutes.

The leak my be an xxpsi leak. It may require a buildup of xxpsi to cause the leak and then release air into the system....so it is somewhat intermittent....just a WAG on my part.
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Old 11-30-2010, 10:09 AM   #18
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RE: Small Surges in RPM Every Few Minutes.

Quote:
GonzoF1 wrote:

I'm curious why it only happens every few minutes, though?
If it is air, the flow through the system is not very fast and the air may collect in a high spot until it can bubble over into the main stream so to speak and then move into the high pressure pump.

The governor increases fuel delivery to maintain rpm then when the air has cleared the injectors it is a bit slower (because of inertia) to slow the engine down again. That is why you see a small rpm increase above the sepoint but nothing below it.

I don't think you will find much of a leak, it will be very slight.
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Old 11-30-2010, 10:38 AM   #19
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RE: Small Surges in RPM Every Few Minutes.

Will it be harmful just to leave it alone? If it's so small that I might not find it, perhaps I should just not worry about it unless it gets to be worse. Opinions?
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Old 11-30-2010, 11:21 AM   #20
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RE: Small Surges in RPM Every Few Minutes.

It can only get worse ... at the worst time of course.
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