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Old 02-06-2010, 09:44   #1
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Yanmar 3gm30f Questions

i've had my 1986 345 hunter for about six months now and i've been reading a lot of forums on operating temp and max rpm. my boat's max rpm is about 2100 rpm's, the operating temp is 160-170 degrees F. my manual says operating temp should be between 50-100 degrees celcius, and i've read alot of feedback indicating other engines of this type run at 3000 rpms as a max. any feedback i could get in regards to whether or not my engine is running within normal ranges would be appreciated. thanks, mark
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Old 02-06-2010, 10:13   #2
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You have a couple of problems with this engine. WOT operation of this engine should get you somewhere near the 3,600 rated rpm of the engine, so something is wrong. The cooling temp seems a bit low but that could just be the gauge.

Here is what I would suggest. In neutral push the throttles to wide open. The governor should limit the max rpm which should be about 3,800 rpm. If so then you have some sort of restriction that appears under load- fuel or exhaust, or you have a very dirty prop. Look at the prop first.

Then if the prop is clean and given the age of your engine I would suspect a fouled exhaust elbow that needs replacing. Secondarily I would suspect a fuel restriction like a fouled fuel filter- either the primary Racor or the secondary on engine filter.

David
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Old 02-06-2010, 13:35   #3
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OK,
What DJ said is correct, but before you go chasing RPM's check the tach. I had, what I thought, was the same problem. I checked the tach and it was way off.

PaulM
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Old 02-06-2010, 14:27   #4
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Yes, tachs can be off quite a bit. But I doubt that it is off 1,500 rpm. More like 200 rpm. That is why I first suggested a WOT, no load test. If you hit anywhere near 3,600 rpm then your tach is reasonably ok and your engines throttle and governor are ok too. If you don't hit at least 3,600 then check your tach, but also check that the throttle is wide open.

To check the tach borrow or buy (They are cheap, about $40. Google photo tachometer) a photo tach and check the photo tach reading against the engine tach.

David
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Old 02-06-2010, 17:52   #5
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yanmar question

thanks for the feedback, within the last week i've changed both feul filters ( racor & the one on the engine) as well i've had the bottom cleaned and inspected it myself, the prop looks good. i don't know that i've ever put the throttle wide open while in neutral i'll try that, also i'll take a look at the exhaust elbow. thanks again, mark
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Old 02-06-2010, 18:27   #6
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My Volvo MD2020 suffered a loss of power, seemingly rather abruptly.
Turns out, it probably wasn't all that abruptly until the last minute.
My exhaust manifold outlet and exhaust elbow were almost completely clogged.
I'd been meaning to check it for years...
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Old 02-06-2010, 18:53   #7
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I'm not sure running the engine at wot with no load is such a good idea. Parts have been known to go south doing that.
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Old 02-06-2010, 19:55   #8
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If this engine has never run faster than 2100 rpm, you might have a prop that is over pitched or too large a diameter. Consult some prop guys. Have your displacement weight, water line length, prop diameter and pitch info handy when you talk to them.
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Old 02-06-2010, 20:08   #9
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I'm not sure running the engine at wot with no load is such a good idea. Parts have been known to go south doing that.
diesels are built to do that, they are goverened to not exceed a set rpm.
when the parts fly off it's called a runaway
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Old 02-06-2010, 20:50   #10
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I'm not sure running the engine at wot with no load is such a good idea. Parts have been known to go south doing that.
3800 is 3800 - the forces to be concerned about are centrifugal forces in the engine. It is rated for it and supposed to hold together. I wouldn't sit there for 10 minutes but OP needs to know if the engine is being governed or limited to a lower RPM.

The first step is complete - A clean bottom and clean prop.

If it is the factory prop I wouldn't suspect that first.

I like the idea of checking a restricted inlet or exhaust. The equation is pretty simple and involves getting mass airflow though the engine. Restrictions limit mass airflow.

If the inlet and exhaust are free it starts to get complicated.

In regards to the tach - you can hear the difference between 2100 and 3800 RPM. 3800 sounds fast - LOL...

I just had a severe clogging of my exhaust mixer - another thing to advise us about is whether there is any smoke and what color. Lugging or loaded engine should produce black smoke.

Clean exhaust and 2100 PM max could indicate fuel delivery issues. i.e the engine is running normally and clean but fuel is being restricted or governed too low. A no load test at the dock may not reveal this because an unloaded engine needs a lot less fuel delivered to achieve 2800 RPM.
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Old 02-06-2010, 21:48   #11
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yanmar question continued

thanks again for all the feedback. in response to some of the suggestions; the prop is not the original so I will look at those specs and see if it is an appropriate size. Also, there is no visible exhaust while the boat is idling at various RPM's or under load at various RPM's. I will let you know how I do with the exhaust elbow, WOT test and the prop size. Thanks again, Mark.
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Old 02-06-2010, 23:28   #12
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You should be able to get over 3,000 rpm out of your engine in calm water, no wind conditions. Believe max continuous cruise rpm for the 3GM is 2800 rpm but you should be able to get close to redline rpm in flat conditions. You have a problem somewhere, either with the prop, boat, or engine.

The exhaust elbow on the 3gm30f is a notorious failure issue. They seem to clog easily and kill the performance. I may not be correct on this, but believe they restrict the water flow in the exhaust as they clog up. If you aren't getting any water out of the exhaust or very little, I'd start looking at that. Check the no load max rpm, first but then proceed from there. Check that the intake water supply isn't clogged at the thru hull, replace the water pump impellor, and clean out any broken impellor vanes and other large crud out of the heat exchanger. If you still don't have a lot of water coming out the exhaust and can't get beyond 3,000 rpm, it's time to start looking at the exhaust elbow, prop, etc.

My engine began overheating and had limited water out the exhaust. Power and rpms were still pretty good. I didn't check the water intake when my engine started to overheat. Everybody said "Exhaust Elbow" so dumb me didn't do the basics and the cause turned out to be a clogged intake. After much swearing and eventually having to call a diesel Mech to get the elbow off the exhaust pipe, we found the elbow still relatively clean. Decided to replace it since we'd gone to all the trouble. The exhaust is easy to disassemble from the engine but a bitch to unscrew the mixing elbow from exhaust riser pipe. To get it apart, you need a BIG pipe wrench; a cheater bar; a big, well anchored a vise; and a torch. The threads really get stuck together with the salt water, exhaust gases and heat. The elbow itself is one of the most expensive pieces of cast iron that you'll ever buy. Seem to remember it was close to $200. If you don't wreck the rest of the exhaust plumbing in getting it apart, the rest of the exhaust should be reusable.

After putting the exhaust back together the mechanic did what I should have done in the first place. Checked the intake and found it clogged. He reamed it out with a steel rod and then replaced the impellor as it was short a few vanes. Found most of the impellor vanes in the heat exchanger, he burped the hot water heater heat exhanger as air in that can cause overheating. After everything was back together, we had lots of water out the exhaust the engine ran like a top.

Check the easy stuff first before you get into the hard and costly items.
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Old 03-06-2010, 04:51   #13
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Let me clear up some info in this thread.

Yanmar tachs are accurate enough to call them accurate. They count flywheel teeth to establish RPM.

A GM series no load full speed is 3850 IIRC. Full load rated speed is 3600. You need to be able to achieve this number. You should only run this speed for 1 hr out of 12 hrs.
3400 is full load continuous speed. You can run this speed 24/7/365.
2800-3200 is considered fast cruise.

The major reason for decreased RPM in a GM series is blockage in the exhaust elbow. A diminished fuel supply can contribute, but it takes filters that are nearly 100% blocked.
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Old 03-06-2010, 06:07   #14
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A lot of good information but you should focus on this quote "the prop is not the original".
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Old 03-06-2010, 10:23   #15
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yanmar questions

i've disconnected the 2" exhaust host from the exhaust elbow. using a mirror and flashlight to inspect the elbow there is some build up but i would say only a 10% or less reduction in the space. if there is enough clearance through the exhaust elbow to allow the engine to operate at normal temp would it be resposible for limiting max rpm. (i tried removed the elbow with a pipe wrench, it seems fairly stubborn)

the raw water intake from outside hull sea strainer to heat exchanger has all been cleaned and or replaced.

also during testing yesterday i noticed water coming out of the top of the anti siphon / check valve which is on a u-fitting. this fitting is placed at the top of a gooseneck running from heat exchanger to exhaust elbow. i am going to replace the anti siphon / check valve but wondered if it were related to the condition of the exhaust elbow.

aside from this last bit i'm starting to lean toward prop size as my problem, although i am about to try a wot test here at the dock. i'll post how it goes. mark
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