Yanmar fuel issue

Ken Textor · Post by **Ken Textor** » Jun 4th, '08, 06:39

Hello all,

My Yanmar YSM-8 is exhibiting somewhat unusual symptoms and I wondered if we have a good diagnostician out there who can help.

Periodically, the engine loses rpms, 400 or 500 at a whack. When this happens, I open up the throttle to prevent stalling, she fiddles around some, adjusting rpms, moves up to a higher rpm, then moves back down, returns to normal, then going down and then up again, and we play with each other like this for half an hour at a time or so. Then, she completely settles down and holds at a specific rpm for an hour or more at a time, no problems whatsoever. Symptoms cannot be replicated at will. So far, here?s what I think we can rule out, mostly:

Clogged filters: Happens with brand new filters, primary and secondary. No crud observed or syphoned off at the water separator bowl. Fuel is therefore clean, clean, clean....or so the evidence indicates. Pickup tube at tank is a suspect, but why would it sometimes pick up crud, then not? On some days, I have run for hours without a problem, start to finish. Pickup tube seems unlikely but still possible.

Air in lines: Have bled and re-bled lines many times. Have owned diesels for many years and know how to do this. No air is observed in the lines whenever bleeding is done. Air could still be the culprit, I know. But from where? Also, all delivery hoses are new, as is the tank air intake hose. Unless there is a tiny pin hole somewhere, one that opens and closes periodically, air in the lines seems a low probability.

Prime Suspect #1: The engine ran flawlessly last season for over 100 hours. This past winter, the only thing I did to the fuel system was install a manual, diesel-specific primer bulb between the tank and first fuel filter to make air bleeding easier. Could it be the source of the problem? Removal of the primer bulb is on the list of potential solutions but I?d like some more input before spending a day griming around the nether reaches of the engine room.

Comments?

Ken Textor, s/v Marie Rose (CD 27 hull #139)
Arrowsic, ME
________
Suzuki sx4

Boyd · Post by **Boyd** » Jun 4th, '08, 08:24

I suspect a clogged tank vent. Spiders and mud daubers love to make nests in all the holes in a boat. Pull the hose off and check it thoroughly.

The other possibility is as you suspect a hose or bulb that collapses periodically.

Good hunting and let us know what you find.

Boyd
s/v Tern
CD30 MkII
Fort Lauderdale, Fla.

Ben Thomas · Post by **Ben Thomas** » Jun 5th, '08, 13:15

If the tank vent is clogged another indicator is when putting fuel in tank the inlet will burp back up. Air escaping tank via inlet not the vent. Ben

Mark Yashinsky · Post by **Mark Yashinsky** » Jun 5th, '08, 19:51

Fuel tank inlet screen that is partially clogged. Did you dump extra biocide in the fuel tank? Bad batch of fuel, that had water and let the critters reproduce?
Water in the fuel tank, and rocking causes some to be picked up.
Lots of water in the fuel water separator. See previous.
Tank vent. Try taking the fuel fill cap off, when you lose RPM's.
Dirty air filter, or something partially plugging the inlet feeding the air filter. You need fuel AND air to produce power.

tmsc · Post by **tmsc** » Jun 5th, '08, 20:38

I have seen primer bulbs installed without problems but that may not be the case with yours. Tempo said the material they use is okay with diesel even though they are mainly seen with gas.

Since you have not been able to reproduce the problem, I am curious if it seems to happen when the weather is a little rougher and the seas choppy or rough. If this is the case, you may have sludge in the bottom that is getting stirred up with the boat movement. This may be clogging the pick up or a something else. If this is the case, I suspect it is clogging the pick up since you have not had a filter problem. I have seen similar problems with gas and diesel boats, and this is one of the reasons that I never recommend that anyone shock treat the tanks on a boat especially one that they just bought. Shock treatment can create a sludge problem. If you don't want to have problems, make sure the tank is clean before playing with the chemicals if all possible. One suggestion, you could by by-pass the tank and use a portable one for a test tank before you take drastic measures. At least this would narrow down the search area.

Good luck,

Ken Textor · Post by **Ken Textor** » Jun 6th, '08, 06:24

Good replies, gentlemen. But the tank vent hose and its screen were both replaced last year and have a very low probability of being the source of the problem. I will try the filler cap trick anyway when/if the problem returns. And that's part of the difficulty. I can't duplicate a sequence of events that touches off this problem. Rough waters doesn't do it. Changes in atmospheric pressure don't seem to do it. The air intake is fine, completely unobstructed because I've operated since last year without an air filter in there. (Yes, I know the potential consequences of that. Just haven't gotten around to getting a filter.)

What I have not done is check the primer bulb when it happens. It seems to me the primer bulb would contract as fuel is sucked into the system with a blockage somewhere between the tank and primer bulb. Will keep you all posted on this mystery.

Ken
________
Marijuana Vaporizer

rtbates · Post by **rtbates** » Jun 6th, '08, 09:22

hello Ken:

The only advice I can offer, free and worth every penny, is what my dad once said to me after having problems with my car. Ask yourself, "What was the last thing you did?" Primer bulb, right? Get an inline fitting and by pass it. easy to try.

good luck

Mark Yashinsky · Post by **Mark Yashinsky** » Jun 6th, '08, 20:45

Are you SURE all your primer bulb and its hose connections are tight and air leak free. Was just a primer bulb installed or was it a primer bulb and attached hoses. Any factory made connections? Those should be suspect, along with any you made. Check the primer bulb for cracks or breaks in the rubber.
Intermittent problems are very hard to solve. Be methodical and logical about problem solving.
Dont forget, KEEP US INFORMED.

dfautohaus · Post by **dfautohaus** » Jun 9th, '08, 13:51

Ken,
If you havn't solved the fuel issue over the weekend, we had a similar issue with a Yanmar 8 in a Cape Dory 27, years ago when the boat was new.
This one would stall out completly after several hours under way. We tried everything imaginable and finally re-routed the fuel return directly to the tank rather than the factroy set up that returned to the injection pump. The thinking was that any system could suck in a minute amount of air; thus that spot of air would go round and round and evently be large enough to stall the motor. bleeding would get it going again. We simply rerouted the return and the issue never happened again
maybe this helps
dave flagg

The Cape Dory Board