Yanmar 3GM30F engine RPMs fluctuate when engine warm

[dkerphunter]

Hi, I have a 1985 3GM30F installed in a Hunter 336. It's been perfect for 21 years. This year, the RPMs have started to fluctuate. When the engine's cold and for the first 20 minutes of operation, it behaves normally. After about 20 minutes at cruise speed around 2,500 RPMs, RPMs fluctuate down and up intermittently several times a minute. It gets progressively worse, and the top of the fluctuation gets progressively lower. There's a little more white vapor than usual in the exhaust when symptoms occur.

I thought maybe the head gasket was the problem, but fresh water coolant isn't being consumed and I assume the power loss would be more constant if it were that.

I checked the fuel filters and they seemed normal. I treated the fuel with biocide. Other hypotheses I've read about are fuel pump, injectors and the governor.

Has anybody experienced this? Any suggestions on pinpointing and fixing the cause? Thanks!

 

[kloudie1]

Sounds like a classic fuel supply problem, and you are looking at the correct things. Another thing to consider is a plugged fuel pick-up tube in the tank. Some pick-up tubes have a small screen in the tip inside the tank. This screen can plug with tank stuff and cause the exact symptoms you are seeing. The tube can be removed from the tank and the screen removed so that the primary filter gets the tank stuff and you can clean it out much easier.

 

[NYSail]

Instead of a biocide add an injector cleaner like "seafoam" to start. Pour the entire can into the primary fuel filter and run the engine. At least you will know the injectors are clean. When checking the pickup on my tank, I just blew into the hose leading to the tank..... found no restriction whatsoever.
Good luck!

 

[dkerphunter]

Hi NYSail, Thanks for your reply. I'll look into injector cleaner. Would dirty injectors behave differently when the engine is warm vs. cold?

 

[dkerphunter]

Hi kloudie1. Thanks for your reply. Interesting idea, but since the problem only starts when the engine is warm, it's not clear to me how the fuel pickup tube would be more or less clogged at higher engine temperatures. I'll nonetheless try to blow in the other end per NYSail's suggestion.

 

[kloudie1]

Good to eliminate that by blowing back into the tank.. also check for a clogged tank vent line.. on plugging the wire gauze on the tip of the fuel pickup, some buddies and I experienced your symptoms late one night while motoring .. it would progress until the engine actually quit.. then we'd blow back into the tank and it would run normally again for some minutes, sometimes an hour.. then start slowing and speeding up .. The stuff we found plugging the little wire screen was some kind of bacterial mess that had the consistency of damp straw.. filaments about 3/8" long and small diameter..

 

[Bill Roosa]

You will want to check your primary fuel filter. As "junk" gets dislodged from the fuel tank and makes its way to the filter it causes a momentary higher restriction as it gets sucked onto the filter element. After the moment the filter restriction passes as the junk gets firmly implanted into the filter and the new restriction is overcome by the governor opening the throttle slightly.
Time to clean the fuel tank!!

 

[Benny17441]

The problem may not necessarily be temperature related but might be due to a time lapse issue. A blocked fuel tank vent may cause the tank to become negatively pressurized over a period of time (your 20 minutes) disrupting the flow of fuel. The same could be experienced by the time it takes to collect particles and temporarily clog the mesh on the pick up tube in the tank. Taking into consideration that engine runs well for the first 20 minutes I would down play the concern for a faulty lift pump, condition of fuel, air leaks, faulty injectors and other causes that would likely render a consistent fault, but I would not dismiss any of them should the problem continue.

 

[dkerphunter]

Thanks to all of you for your ideas and clarifications. The interplay between restricted fuel supply and governor is a really interesting hypothesis.

• Checked the fuel tank vent. That's OK. And to be certain, I unscrewed the fuel fill cap while the symptoms were present. No change. So it's not a vacuum
• Tried to sample fuel from the bottom of the tank, but I don't think I got all the way own with the siphon. It looked too clean to be true.
• Today's project will be to check the primary fuel filter (more carefully), and blow into the pickup line upstream from the primary filter. Hope there are good instructions on YouTube

 

[dkerphunter]

I removed the primary fuel filter / separator. It's a Racor 110A. The space between the inside of the bowl and the filter element was crammed with sediment and the filter element was pretty clogged too. This may be the root cause! It's consistent with some of your posts above. I immediately ordered a new filter element and gasket (Racor R11T) but given the state / age of the filter, I'm tempted to replace the whole thing. I'm also tempted to replace the mechanic who told me he checked it!

So two new questions...

1. There's a lot of discussion about the Racor 110 vs. 120. Since the 110A has worked for 26 years, and I know it fits / mounts well, I don't see any reason to change.
2. Under the circumstances would you purge all the fuel and start over? I can't access the collector at the bottom of the tank so it needs to be pumped out. Would that really pull out all the gunk?

Thanks!

 

[Ron20324]

I'm not understanding how you checked the filters on Friday and I guess they were okay, but on Sunday there was a lot of stuff in there.

 

[dkerphunter]

Ron, You're right to point out the confusion between the two posts. When I "checked" the filters the first time (the one the Friday post referred to), I didn't actually remove the primary filter bowl. I opened the drain screw and sampled fuel from the bottom. It was a little dirty but not alarmingly so. Then I fully removed the secondary filter and that fuel looked OK so I asssumed they were both doing their job. Sunday, I took the Racor 110A completely apart and only then saw how bad the filter element was clogged with sediment and gunk. Hope that clarifies. Thanks. Dan

 

[dkerphunter]

Replaced the primary fuel filter / separator today with a brand new Racor 120A with 10 micron element. Replaced secondary fuel filter, o-ring and bleed screw washer. Pumped fuel from the bottom of tank. Clean and clear. The engine ran better than usual during the first 20 minutes. But at 20 minutes on the nose, it started losing power just like before, although no white exchaust. Next hypothesis is the lifting pump. Any other suggestions? Thanks!

 

[Johnb]

But at 20 minutes on the nose, it started losing power just like before, although no white exchaust. Next hypothesis is the lifting pump. !

I had intermittent episodes of loss of power that went away after replacing the on engine lift pump on my 3QM30. It was actually very easy and not an expensive item.

 

[Benny17441]

A 10 micron primary could be too restrictive. A friend purchased some replacement elements by mistake and started having similar problems, we use 20 micron elements and allow the secondary filter to capture the smaller particles. Would not think it is the lift pump as their behavior is not usually random. Make sure the engine is not overheating and loosing compression.

 

[Charles Erwin]

Guessing here but with a tank as foul as you describe - particles (sediment) choking the filter element - two things come to mind.

First - Racor 100 and 200 series heads will admit particles in the anti drain back chamber. The element might be brand new but the internal passages of the filter head fixture can be fouled.

Second - the symptoms you describe are consistent with bio waste particles overcoming and plugging the tank dip tube. These particles accumulate and strangle the pick up tube. As the motor rpm reduces the lift pump vacuum reduces and the accumulation of particles kind of falls apart. The process repeats itself with surprising regularity.

Was told of a case where the culprit was a piece of foil seal from a diesel additive container. The seal would get sucked onto the end of the dip tube, then drift away, then return and so forth with some regularity.

To find out obtain a clean gas can and fill with diesel - a portable outboard tank would be perfect. Make up a piece of fuel line from the gas can to the lift pump thus bypassing the Racor. See whether this spiffy clean tank and fuel produces the same symptoms.

Edit: To do this bleed all air from this gas can tank to the filter on the motor - that is all air must be expelled via the bleed screw at the top of the motor mounted filter. Either the test gas can is above the motor or if not, then a squeeze ball in the feed line at the tank.

Charles

 

[dkerphunter]

Thank you all for the latest suggestions.

Interesting about the fuel pickup line "breathing" debris in and out. I will definitely try running on a test tank of fuel.

I'll replace the lift pump too. It's 21 years old and I've read about cases where that was the culprit of similar symptoms, especially if my dirty Racor allowed gunk through which could foul the lift pump.

More to come!

Thanks again. The knowledge and experience on this forum is very impressive.

Dan

 

[NYSail]

I use a 30 micron filter in primary...... lift pump has a screen that may be clogged.....yes easy job as long as access is good.

 

[dkerphunter]

Yesterday, I connected the fuel line to a clean gas can. I headed out and motored at 2,900 RPM for a full hour without the slightest loss of power! Thanks to all your help and suggestions, I'm pleased to say that the root cause has finally been identified. Dirt in the fuel tank and / or a clogged dip tube.

Next step will be to pump out and clean the fuel tank and replace the dip tube, which sadly detached and fell into the tank when I was trying to pull it for inspection. It may or may not be recoverable through the limited access points on top of the tank but I don't feel there's any risk associated with leaving it in there. Alternative views on this are welcome.

And when I source a new dip tube, I will remove the screen at the tip. If there is dirt or bacteria in the future, I'd rather see it and deal with it at the Racor filter, which is much more visible and serviceable.

So thanks again for all your ideas and advice!

Dan

 

[Charles Erwin]

Here is a possible solution. In this case the tank was about 60" long and located immediately beneath the motor. I made up two plates/gaskets and installed one in each end of the tank. This tank was built in 1977 and had never been cleaned. Similar plates can be purchased from Seabuilt.

The only way you get rid of this problem is to physically clean the tank surfaces. Meanwhile store your tank empty if it sits for a long winter and use a biocide.

Charles