3GM30F Intermittent power loss

[ScuzzMonkey]

I've had a couple of concerning incidents in the last few days with our Yanmar 3GM30F losing power. Briefly, not long after starting up and putting a load on, the engine will drop 300-500 RPM, in or out of gear, intermittently. Advancing the throttle has no effect, although lowering it will lower RPMs as expected. The drop was severe enough to kill the engine once. In both cases, within about a half hour, the problem disappeared without intervention after limping along for a bit.

I have pulled up the other threads on power loss but this appears to differ from those in that it is intermittent, not constant. While the power is dropped, I'm not producing smoke of any sort. The engine returns to the full rated power at some point... or it has so far! So I don't think I'm looking at a prop issue or exhaust or fouling problem.

My suspicion is fuel flow so I've changed the filters out, but they did not appear particularly crudded up. On one of the occasions, we had been in some pretty bumpy stuff immediately before starting the engine so I was thinking perhaps tank crud got stirred up, but the other time, we hadn't hardly been shaken at all. And we had been lumped around pretty good on an intervening day without experiencing the problem at all.

We're in some moderately remote territory right now (with very infrequent internet access; my apologies if I am not able to reply immediately) getting ready to get into even more remote territory, so I just want to make sure I've covered my bases with this before I get outside the realm of mechanics and engine shops. Are there any other likely causes for this sort of behavior besides fuel flow issues?

In particular, I'm wondering if it could have anything to do with not warming the engine up enough before putting it under load. Since the problem has, so far, disappeared within 30 minutes, and since it happened, on both occasions, when we had not spent very long warming up, that has crossed my mind... but so far, I haven't ever heard anyone suggest that as causing such power problems. I'm sure it hasn't happened every time we haven't warmed it up sufficiently, but I've only got these two episodes to look at for similarities so I'm grasping at straws a bit.

[Badsanta]

My 3gm30f ran for 20-30 minuits with the fuel turned off before it started acting up althow it was at a low speed. I would suspect cloging in the tank at the fuel line. Letting only a certin amount of fuel pass with increasing suction causing starvation but letting enough through at a lower RPM but no increase in rpm with increased throtal ( no spell check). Try blowing through the fuel line to the tank. It wont fix it put it may clear the fuel line enough to see a change. Sometimes there is a screen at the bottom of the pick up tube that can be removed and your regular filter will clean it. Just a thought.

[sck5]

my 3gm30f did exactly the same thing and it was fuel flow. If not the filters then somewhere else in the system. I would start by cleaning out the tank and then work through the system bit by bit. If you have a 3gm30f then your boat is likely to be old enough for the tank to need cleaning by now.

[RaymondR]

Definitely sounds like a fuel flow problem. The other thing to check is the lift pump, could be a pin hole in the diaphragm putting air into the system.

3GM 30 is a good little motor though.

[Rakuflames]

The filters didn't seem "particularly crudded up?"

Two things are a slam dunk for that engine (i have a slightly smaller version and have sailed on your "chart") to act like that -- air in the fuel line or dirt in the fuel line. If you saw *any* dirt on either of your filters (I'm assuming you have a secondary Racor-type filter), you have dirt in your fuel. It's probably coming from the tank for a variety of reasons. OR, you may have air in your line, but as someone else said, that's probably from an air leak. That's because (if you have the newer one, anyway) it's a self-bleeding engine and you often do not have to bleed the engine even after doing things like changing filters.

If there's dirt caught in the filter, it can break up and some can slip through, and it only takes the tiniest amount of dirt to make a diesel engine hiccup, stumble or even stall out. Mine stopped dead and I thought it was out of fuel but only took about 5 gallons to fill her up. That's how I figured it out.

We pumped the fuel out (a boat with a good polishing system built-in took it), and at the bottom of the tank it was like sludge. My engine was brand new but was behaving like yours because we didn't clean the tank.

[belizesailor]

Originally Posted by RaymondR

Definitely sounds like a fuel flow problem. The other thing to check is the lift pump, could be a pin hole in the diaphragm putting air into the system.

3GM 30 is a good little motor though.

+1. Sure sounds like fuel delivey issue. One thing that can cause this which has not been mentioned yet is a vaccum leak. These can be intermitent and a bitch to track down. Best way is w a vaccum gauge.

[ScuzzMonkey]

Thanks for the quick feedback, everyone. I hadn't thought to check the pickup tube, I'll try clearing that.

Cleaning the tank is a good idea, but a logistical impossibility at the moment. I'll have to manage it with filtration and/or polishing for the summer, and crack the tank when we get back to town. I'm not positive it's dirt, I should have been less elliptical about the state of the filters: I didn't see any dirt on either of them. But a good cleaning wouldn't hurt.

Anyway, you've reassured me it is most likely just a fuel flow issue of some sort (except for the dread vacuum problem; but, what's that old saying? "Never test for an error condition you don't know how to handle?" ), and I am fairly confident we can manage that with the resources aboard for the rest of the summer. I'll toodle around near civilization for another week or so to see if it becomes more frequent, and head out again if not.

Thanks!

[Badsanta]

Please post what you find. Thanks

[dochip]

Had a similar problem with my Yanmar 3GM on my Bayfiedl 32, occasional drop of rpm thatvresolved it self a bit later but eventually got more often and worse. Turned out that the seal gasket ring on the Raycor filter was seeping air. Reset it and refilled using elctric boost fuel pump and has been fine since. We are sailing (mostlly motoring) the ICW then cross Florida Canal from St Augustsine to Ft Myers FL., dodging thunderstorms the while.
Best,

[laika]

Just my two cents qualified further by me not being a mechanic, but if you didn't notice any sludge in the filters when you changed them, I'd say air. The Racor (30micron) on my 3gm30f has taken an awful lot of sludge before it started starving the engine. And even if it were a big clump getting caught in the screen on the tank, you'd think you'd see at least some sludge making it's way to the filters.

Tighten all clamps, replace primary filter gasket, check hoses... I've also had an issue with the copper (or nylon) crush washer used to seal the secondary filter vent.

All the best.

[ScuzzMonkey]

Sorry for the delayed report... been very off-line for the past month or so!

After swapping the filters out, we didn't experience the issue again, or at least, have not experienced it in the past month. We've been bashed around pretty good in that time and there have been no signs of sediment coming up out of the tank.

Whatever He-man Hercules last installed the filters on this thing (boat is new to us) had them cranked down so tight I can't imagine air was getting in that way--we had to improvise a strap-wrench out of some pliers, nylon webbing, and non-skid to get the Racor off, and I bent the hell out of it even with that--but I did notice when I was bleeding the primary that one of the banjo bolts on it was not tightened down adequately. I secured it after bleeding, and no problems since.

So, it does seem like air in the line was the most likely culprit.

Thanks everyone for the input!