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)

Tom-

Hiya,
** How clean is/are your fuel filter(s)?

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?

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.

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.

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

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.

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.

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

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

BTW... No vac gauge on the Raycor... It's on the list

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?

FF wrote:

Oil level in tranny?

Click to expand...

Fine. I check it before every trip.

Temporarily put a piece of clear tubing in the fuel line just before the lift pump and look for bubbles.

RickB wrote:

Temporarily put a piece of clear tubing in the fuel line just before the lift pump and look for bubbles.

Click to expand...

That's a neat idea. THANKS!

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.

I'm curious why it only happens every few minutes, though?

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.

GonzoF1 wrote:

I'm curious why it only happens every few minutes, though?

Click to expand...

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.

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?

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

Yep...you don't want to let that one go...it will rear it's ugly head and bite you at the most inopportune time!!!

"Temporarily put a piece of clear tubing in the fuel line just before the lift pump and look for bubbles."


The better way to do this is to go to a refrigeration supply house, and ask for an "Armored Sight Glass".

This will come with a male and female fitting (ask for your size) and you can see bubbles in the fuel as desired.

About $15.00 the last time I got them.

It's actually easier to see small bubbles through a 6 inch or so length of plastic tubing that can be bent to form a high spot to collect them.

And for the OP, you don't have to run the engine. Shut off the fuel valve at the tank disconnect the fuel line where it fits the lift pump, attach a hand operated vacuum pump like you find in the auto parts store and pump.

Just make sure you keep the end of the fuel line above the filters and keep liquid in the tubing so you can see bubbles. It might take a few tries to get this right. If the vacuum stays constant for a few minutes with no bubbles, you have some other problem ... and that will make for a longer thread.

How do you keep from sucking fuel into the vac pump? To keep fuel in the line, wouldn't you want it below the filter level?

I'm having trouble picturing the process you are describing in my mind.

Are you talking about a pump like this:

http://www.autozone.com/autozone/ac...mp&fromString=search&itemIdentifier=2080_0_0_

Cheaper version:

http://www.autozone.com/autozone/ac...-25j2?itemIdentifier=70116&_requestid=1076515

-- Edited by GonzoF1 on Wednesday 1st of December 2010 07:45:38 AM

-- Edited by GonzoF1 on Wednesday 1st of December 2010 07:47:16 AM

Gonzo*

What size of fuel filters - Racor 500 or 900?
*
Have you changed your primary*filter from 2 back to 10 micron with new rubber O rings*? Have you recently changed your on engine filter with new O rings too?* Stop trouble shooting until you check off these two steps.

You may want to go to boatdiesel.com for hundreds of similar posts and solutions

Hiya,
** I think I understand what Mr. Rick B is suggesting but rather than putting that 6" of clear hose into the fuel line, shut off tank, disconnect the fuel feed from the tank side of the pump, attach a LONG (3' or so) piece of clear tubing to the open end of the fuel line and connect your vacuum pump (VP) to the open end of the plastic tubing.
** Hold the*VP as high as you can and pump the system down.* That 3' of height SHOULD give you a "buffer" to keep the fuel out of the VP.* Check for constant vacuum.
** To check for bubbles, allow the fuel to rise, say 1' up the clear hose and make a 360 degree loop in that fuel filled line close to where it attaches to the solid line (you may have to tape that loop to maintain it's*"loopy" shape).* Again apply vaccuum with the VP held high.* If any bubbles are generated they SHOULD collect in the top of that fixed loop.
** Clear, or still having trouble?* I wish I knew how to attach a diagram as I think it would be much easier to visualise my explination.

-- Edited by RT Firefly on Thursday 2nd of December 2010 05:51:50 AM

Yeah, that's the pump.

If there is no leak or a very small one (which it sounds like) you won't suck any fuel out of the line as long as the top of the clear plastic tube and the pump are above the filters. Picture sitting with the pump on the edge of the engine hatch with your feet dangling and a clear tube leading down to the fuel line at the lift pump, suck up enough fuel to mostly fill the tubing but leave an air space above the fuel level. When you pump with the tank valve shut it will pull a vacuum on the line from that point without moving any fuel.

Hope that makes it more clear. Make sure the filters are full or you may suck up some fuel as any air in the canister expands. I suspect that just doing that might reseal the filter housing(s) and cure the leak.

Ahhhh hahhhh! NOW it makes sense. Thanks guys (I'm assuming).

Mr.White, It's a Raycor500 and I have not changed out to the 10mic filter yet. I'm a "weekender" and probably won't get around to tackeling this problem for a few weeks. Although, come to think of it, I don't think I changed the o-ring last time.

There was a period of time in the spring where I was having some starting issues and replaced the filter more often than needed, so really, the current 2mic filter only has MAYBE 50-70 hours on it. Still, I'll get on trying to track it down sooner rather than later.

-- Edited by GonzoF1 on Wednesday 1st of December 2010 09:54:29 AM

GonzoF1 wrote:

Ahhhh hahhhh! NOW it makes sense. Thanks guys (I'm assuming).

Mr.White, It's a Raycor500 and I have not changed out to the 10mic filter yet. I'm a "weekender" and probably won't get around to tackeling this problem for a few weeks. Although, come to think of it, I don't think I changed the o-ring last time.

There was a period of time in the spring where I was having some starting issues and replaced the filter more often than needed, so really, the current 2mic filter only has MAYBE 50-70 hours on it. Still, I'll get on trying to track it down sooner rather than later.

-- Edited by GonzoF1 on Wednesday 1st of December 2010 09:54:29 AM

Click to expand...

"2 mic filter only has MAYBE 50-70 hours on it."
I had a complete engine shut down due to fuel starvation from a clogged 2micron fuel filter at 80 hours of use. And this was only two months after a 500 mile delivery run in which we were burning and buying new fuel frequently, so the fuel wasnt that old.

Do not trust the filter when you are having fuel issues.
R.
*


-- Edited by ralphyost on Wednesday 1st of December 2010 07:58:21 PM

www.mityvac.com/pages/products_hvpo.asp

This is a hand pump that will pull almost 28 inches.

Useful on board for priming a diesel, or every 2 or 3 years to replace the brake fluid in the family car as PM.