Yanmar 1GM; an RPM mystery

[Sea Hunt Video]

I have been experiencing a strange RPM issue with my 1982 Yanmar 1GM. I am hoping the expertise on this board will offer some possible causes, solutions, etc.

The symptom:

In forward gear, with the throttle level pushed all the way to the cockpit floor, she will only get up to 2,700 rpms. However, in reverse gear, she easily goes up to 3,4000 rpms.

The differential diagnosis I have come up with:

1. At first I thought the issue was the throttle cable linkage. I checked it (as did a very good Yanmar mechanic) and could not find anything wrong with the throttle cable linkage.

2. Then I thought of the bottom and prop but both are clean with no fouling. I visually reconfirmed this the other day.

The prop is the original two (2) blade prop with markings indicating a 1982 mfg date - at least according to the surveyor. 12RH10 (1982)

When I commissioned an engine survey in NJ Jul 2011, the engine was revved to 3,400 rpms in forward gear at the request of the diesel mechanic doing the survey. This caused an engine overheating issue which has now been resolved. First, a clog in a hose from the water pump to the engine block was removed. Second, the engine block was flushed with a "rydelyme" solution. No more overheating - hopefully.

I am looking for additional possible causes for this 1982 Yanmar 1GM to fail to come up to 3,400 rpms in forward gear.

Thanks in advance for all suggestions, etc.

P.S. Got out sailing the other day on a friend's sailboat. Winds were 16-18 kts with higher gusts. There were six(6) of us aboard. The five (5) other sailors were very experienced sailors with many thousands of open ocean miles under their deck shoes. I was definitely the "tadpole". I learn something every time I go out with these guys. Great weather; great sail. I am sure S/V Bali Ha'i would have enjoyed the conditions.

[Klem]

When an engine cannot get to full rpm in gear but it is fine otherwise mechancially, it means there is either too great of a load or it is not getting enough fuel.

Too large of a load is usually caused by either an improper propeller/transmission combination or fouling of the prop or bottom. In your case, it sounds like this is not a problem. Loading will be different in forwards and reverse because the propeller has a different efficiency and the transmission will also have a different ratio. In your case, I suspect that there is more load in forwards than in reverse. In my 30 with an md7a, the load is actually much higher in reverse and I can't get to full rpm.

If the load is not too high, then the engine is putting out too little power. The most likely culprits are the fuel filter and the shutdown. When your fuel filter starts to plug, the engine will run fine at lower throttle because it will be able to maintain fuel pressure at the reduced flow but it will not be able to make full power because the fuel pressure will drop too much at higher throttle. If your shutdown isn't properly adjusted, the engine will run fine but not be able to make full power similar to having a partially clogged fuel filter.

What color is the exhaust? If it is really sooty and black, you may want to check your air filter as well.

Have you double checked the throttle cable adjustment by hand throttling the engine with the cable detached?

Good luck.

[Sea Hunt Video]

If the load is not too high, then the engine is putting out too little power. The most likely culprits are the fuel filter and the shutdown.

What color is the exhaust? If it is really sooty and black, you may want to check your air filter as well.

Have you double checked the throttle cable adjustment by hand throttling the engine with the cable detached?

Good luck.

Klem:

Can you clarify what you mean by "the shutdown". Are you referring to the remote control shutdown cable and lever If so, can you explain how this would affect engine performance/rpms in forward gear.

Additional information:

1. The remote control shutdown cable broke from its bracket in back of the Yanmar control panel. I jerryrigged it in place with tie wraps. It has worked OK since. I pull it slowly when shutting down the engine and make sure it is pushed all the way in before starting the engine.

2. As part of preparations for launch in November 2011, I installed a Racor 10 micron fuel filter with a clear glass bottom container. This was in addition to the 10 micron "on board" factory installed fuel filter that is part of the Yanmar 1GM unit itself. There was a choice between a 10 micron Racor or a 30 ( ) micron Racor. The mechanic I was then working with recommended 10 micron. Several sailors said there is sort of a 50/50 split of opinion between installing filters 10 and 10 or 30 and 10. I went with the mechanic's recommendation. This is a different mechanic than did the recent work on the Yanmar 1GM. I regularly check the 10 micron Racor and so far have not observed any water or particulate in the bottom of the clear glass container.

3. The exhaust seems to be OK. As soon as the engine is started there is a small amount of soot (appears to be fine black powder) that comes out the exhaust tube. This stops after 15-20 seconds. When run at low rpms (1,200 - 2,200 rpms) there does not appear to be anything coming out the exhaust tube except water. One day with two of us on board and the engine run to 2,700 rpms, there was what appeared to be a white "vapor" at the exhaust tube. I attributed this to the fact that the stern was a little down (2 hefty guys and running at 2,700 rpms put her bow up/stern down) and that some cold sea water was getting a little ways up into the exhaust tube and mixing with expelled hot water from the engine - creating steam.

Klem and all, I very much appreciate your input and suggestions. I know almost nothing about diesel engines.

[Bob M.]

Adding 2 fuel filters in line will reduce your fuel through put if there is not sufficient capacity of the filter system to your engine burn rate at full throttle. Also make sure your primary filter is clean and check your fuel.

Bob M.
Ranger

[Klem]

1. Yes, this is the shutdown that I am referring to. Typically, older diesel engines will have a lever on the injection pump which can cutoff fuel to the engine. In some applications, this is controlled by a solenoid but in most, like yours, it is controlled by a cable. If the cable does not let the lever on the injection pump return all of the way to the on position, the engine will run but won't get to full power. If you want to see what I mean (don't do this long term), go out and go to full throttle then slowly pull this cable out and you will see that your rpm will slowly go down. I would recommend disconnecting the cable and finding the end of the travel and then reconnecting it and seeing whether it gets to the end of the travel.

2. Your fuel filter setup sounds fine to me. There is a lot of debate over micron ratings but for the fuel system on the engine that you have, you are providing adequate protection. Did you change the existing filter when you added the second one? If not, I would recommend changing this. Whenever you change a filter, I would recommend cutting it open and seeing what it looks like in there. An alternative way to monitor your filters is to monitor the downstream pressure or the pressure drop across them. I know of very few people who do this on a cruising sailboat but all of the larger vessels will do this. I do not believe that there are any racor fuel filters on the market that will be rated for a low enough fuel flow that it would cause too much pressure drop and you would not be able to make full power.

3. It sounds like you are not seeing black smoke so it is unlikely that you need to change your air filter. Anytime you see steam out of your exhaust, you should be cautious. It is usually due to insufficient cooling water through the engine passages so that it actually becomes steam before leaving the engine.

I hope that his helps.

[Jim Davis]

A common fault on the GM series is the water injection point on the mixing elbow. They have a tendency to clog just inside the brass fitting on the elbow. This is because cool water with silt and perhaps a little sand squirts directly on the hot exhaust. The rapid heating of the water causes sediment to deposit on the parts. It also corrodes the exhaust section. You can check this by removing the water hose from the elbow and feeling around inside the brass fitting with a small screw driver. You can also clean the area just inside by gently scraping with a small screw driver and a pistol bore brush.

You hinted at this when you mentioned a little steam at 2700. At low RPM there is usually plenty of water with a partial clog, but with the RPM up and under load it won't move enough water. Yes this can also cause a power reduction as well as fuel, air and fouling issues.

[Dick Kobayashi]

I am most interested in this thread. A few years ago I had the head and exhaust elbow replaced and my 1GM ran like a charm - no more black smoke, etc. However I was never able to get the rpms up over 2700 in fwd. The engine will run at its full rating 3600, I think< without a load. Why this is so remains a mystery to me. Since as far as I can tell the actual speed of the hull through the water is not affected - I have blown off this issue. My mechanic opined that 1) I might be a little over proped and that 2) I shouldn't run the engine over 2700 rpm anyway. I have lived with it, but if there is an answer in this discussion I will be most interested. I have deduced (I hope correctly) that since the engine runs at design max without a load it is getting enough fuel.

I am staying tuned to this most interesting discussion.

[Klem]

I have deduced (I hope correctly) that since the engine runs at design max without a load it is getting enough fuel.

Dick,

I would respectfully disagree with this statement. It takes relatively little fuel fuel to rev an engine in neutral and the overflow valve should be able to choke off the return line enough to keep fuel pressure up to the injection pump. When the engine is under load, a lot more fuel is needed and even if the overflow valve totally closes down, there may not be enough fuel flow which will limit the rpm and power output.

One of the best diagnostic tools in my opinion is a fuel pressure gauge. If the fuel pressure stays up even at WOT in gear, then there is no problem. However, if it falls, either the overflow has failed or there is a delivery problem such as a partially plugged filter. The easiest way to check an overflow valve is to take a pair of vice grips and clamp off the return line and see if the fuel pressure goes back up, if it does, then the overflow is bad.

I hope that this helps. It certainly could be something else but I would suspect a fuel system issue.

[Sea Hunt Video]

Klem, Jim, Bob and ALL:

Thanks very much for the information and suggestions.

I will first check the Remote Control Cable. It is possible that when I "jerryrigged" it with tie wraps that I inadvertently partially shut off (reduced) the fuel flow and thus not allowed it to be in the fully "ON" position when the cable rod is pushed "IN" at the engine control panel.

[Dick Kobayashi]

Thanks all, when I have access to Susan B and can assess the problem I'll report back on this board. But I do have a question - is there a way to check out potential sources of the problem when the boat is on the hard?

[Jim Davis]

Fuel filters are easy to check/replace. The water injection elbow may be easier on the hard than afloat. Movement of the shut off ca also be checked - disconnect the cable and see how much throw it has, then reconnect and check again. If you can provide water (intake hose in a bucket) and the yard will allow it, the engine can actually be run. However you won't be able to actually load it up sine it is in air and I wouldn't put it in gear since the packing and cutless will be dry. The yard operator would have "the big one" if he thought you intended to spin the prop while on the hard, his insurance company would also have a stroke.

Back to your original comment about only being able to get up to 2700 RPM under load ahead, this should according to Yanmar be the normal operating speed of the engine. It does indicate that you are quite over propped. You might want to check Michigan Wheel's site where they will do a prop calculate for you. It is at best an educated guess, but it is from the largest prop maker and usually pretty good. In any case you really don't want to be doing sustained operation under load up near the maximum RPM the engine will develop, it can cause premature wear and tear which can get expensive.

[Dick Kobayashi]

Jim,

Your thoughtful advice is very valuable. Would you check my logic as follows: Before I had the head replaced the engine did run at 3400-3600 rpm under load. After the repair at 2700. Seems that this indicates that the boat is not over propped, but that I must have some problem with fuel or similar - and that is the place to look while on the hard.

Seems I ought to have the mechanic check fuel flow etc while commissioning is undertaken this spring

[Jim Davis]

Dick

On the surface your latest could indicate a problem generated during the head replacement and mean the mechanic may have caused it. That said there are a few questions:

Why was the head replaced?
Before the repair, was the engine weak/anemic?
What speed did you get at 2700 before the repair, as compared to after?

If after the repair you are getting a noticeable increase in power and speed, then you may well be over propped. Other areas to consider are fuel supply, injector and exhaust back pressure, but I would tend to wonder about the head job questions. If you bring in a mechanic be sure and give him a full history and have him fully check things out.

All of this is assuming the hull and prop are clean and the stuffing bx isn't locked down too tight.

[rtbates]

Before you get all spun out of shape take her out and RUN HER HARD... The 1gm absolutely hates to be babied around putt putting.. It gets carbon build up fast...Whatever else you do with/to the 1GM please do NOT treat her like a LAND diesels and let her idle for long periods of time....