1990 2GM20F issues - HELP!

Alan Gomes · Jun 13, 2020

Since you do get to full RPM in neutral, I do not suspect the governor. And besides, that would not account for the smoke. If you are certain that there are no further blockages in the exhaust (you already replaced the elbow), then I think I'd look at the prop.

Find out the diameter and pitch, which will almost certainly involve having a diver remove the prop, or doing a quick lift in the slings so you can see what is stamped on the prop hub. Then, look up the HP for that engine, and also find out the gear ratio for your transmission. It may be shown on a plate on the transmission, or perhaps the info is in the service manual.

Armed with that info, contact a prop shop and have them run the calculations to see what your prop *should* be. If for some reason the previous owner put on a prop that was way over pitched, it could do exactly what you are describing.

kloudie1 · Jun 14, 2020

As Alan says, the prop diameter and pitch would be primary suspects.. The intake air being restricted would be another.. the air filter/silencer may be choked .. The injectors might be a next logical step.. they may be dirty or plugged and need a rebuild.. doesn't sound like the head needs to come off (yet) … has the tech checked/adjusted the valve clearance?
Good luck. The little Yanmars are really robust

RoyS · Jun 14, 2020

Some people seal up the intake for the winter. Best to check.

Rick · Jun 14, 2020

Can you expand on the last part. What are the symptoms now? You said no smoke at 2200 RPM right? Engine temp is what at that RPM? Cruise RPM is 75% of WOT or wide open throttle on a diesel. Test it there. IMHO dont pull the head yet. The guy has to make a living. If you let him, by gosh, look for wrench marks on the head bolts! Give us some more info and some testing. If you can.

Alan Gomes · Jun 14, 2020

Rick said:
Can you expand on the last part. What are the symptoms now? You said no smoke at 2200 RPM right? Engine temp is what at that RPM? Cruise RPM is 75% of WOT or wide open throttle on a diesel. Test it there. IMHO dont pull the head yet. The guy has to make a living. If you let him, by gosh, look for wrench marks on the head bolts! Give us some more info and some testing. If you can.

He already indicated that he can only hit 2600 max in gear, and then it smokes considerably until he throttles back to 2200. 2600 is WAY below the RPM that engine should be able to hit in gear. He should be able to get within 150 or so RPM of max with a properly functioning engine and suitable prop. So, while I don't have the specs in front of me, I think the 2GM would be something like a target of 3600 rpm--so at least 3450, ideally a bit more, would be in the ballpark.

As for engine temp, with a properly functioning cooling system (heat exchanger, thermostat, etc.) it should be operating at the desired temp regardless of RPM. So yeah, that would be another symptom to check.

Rick · Jun 14, 2020

Thats not what I asked him Alan. 3800 rpm seems a bit hi. I would think it would be governed at around 3400 to 3600. 2600 could be a tach issue. However, you are right Alan. It should still be able to hit WOT while in gear. I assume the hull is clean since it was splashed. With a new elbow, that rules that out. Temp shouldnt matter when it comes to RPM, it will just overheat if its the cooling system. Does it smoke in neutral with WOT? I think if it hits WOT that rules out the injectors. If not then most likely its the load on the diesel, and unburned fuel. Transmission? Not likely there either. Does that shaft vibrate unusually while in gear? Those were just some thoughts.

Alan Gomes · Jun 14, 2020

Rick said:
Thats not what I asked him Alan. 3800 rpm seems a bit hi. I would think it would be governed at around 3400 to 3600. 2600 could be a tach issue. However, you are right Alan. It should still be able to hit WOT while in gear. I assume the hull is clean since it was splashed. With a new elbow, that rules that out. Temp shouldnt matter when it comes to RPM, it will just overheat if its the cooling system. Does it smoke in neutral with WOT? I think if it hits WOT that rules out the injectors. If not then most likely its the load on the diesel, and unburned fuel. Transmission? Not likely there either. Does that shaft vibrate unusually while in gear? Those were just some thoughts.

For the GM series, 3800 RPM in neutral is about right. The GM series runs at 3400 continuous, under load. Actually, I would expect it to show a bit higher than 3800 for a quick rev in neutral. But that's reasonably close.

You raise an excellent point about the tach. The Yanmar tachs can be off a few hundred rpm. It would be wise to invest in an inexpensive digital tach and confirm that. Granting that it shows 3800 in neutral, which is a very believable number, I would not expect it to be wildly off at 2600, which is the max he can hit. It's possible he's really hitting 2800 or 2850--I know my Yanmar tach (1GM) reads about 200 rpm too low in that range. But even if he's making 2850 max that is still way below what he should be getting in gear. Regardless, I would buy or borrow an optical tach and make a tach deviation table at various rpms to confirm what the engine is really turning for any given reading.

Again, my first suspicion is prop.

Rick · Jun 14, 2020

I concur Alan. If both of us could just think without any information, we would probably say, hull growth, mixing elbow, engine in general, shaft damage, or prop to just name a few. But it is narrowed down a bit. You have excellent knowledge about the two banger. Horsepower could not deliver the goods here? Tired motor? Compression? I have to agree with you on the prop. Not much else makes sense. But a two blader...? wow. Perhaps he will get back with us and give some more clues.

Be well Alan
And good luck BOS

Alan Gomes · Jun 14, 2020

BOSSailor said:
Hey guys thank you all very much for the advice. Let me work through these replies and answer some questions.

Alan: Elbow has been replaced but I cannot say for certain that there are no further obstructions in the exhaust. I suppose I should check that. The exhaust appears to take a straight path through the muffler - I can't imagine there being any obstructions in there but I'm not sure. It's grey and plastic. Maybe 8" diameter and 14" or so long. Not sure of the manufacturer.
Next time I'm at the boat (likely Tuesday or Wednesday because of work) I'll dive and inspect the prop and see if I can get any identifying info off of it or at least measure it.

Kloudie: Air filter has been inspected and was looking good. At the moment I've taken the foam filter out of the silencer housing to eliminate that as a point of failure. Did not seem to make a difference.
I don't know when the last time (if ever) the injectors were serviced. The previous owners seem very reluctant to answer any questions - I want to be like look people, I already own this project and you have your $$ so why not just maybe be a little helpful?!
Diesel tech has not checked the valve clearance. I'm leaning towards that being the next step before injectors. I've never done it myself but I'd feel comfortable tackling it.

RoyS: No obstructions in the intake that I can feel or see with a flashlight and mirror.

Rick: No smoke at 2,200 rpm and will give me about 5 kts GPS speed on a calm harbor. I don't have a temp gauge, only an idiot light. Nothing feels or smells excessively hot for what it's worth.
Per the internet I'm seeing maximum RPM as 3,600 and max sustained rpm as 3,400.
75% of max rpm would put me at 2,700 rpm which I cannot achieve in gear regardless of the smoke condition. The engine will rev to the limiter in neutral though.
Rick part 2: Hull is clean and decently smooth under the antifouling. Elbow is new. Have never experienced overheating since I've been messing with this.
No smoke in neutral at WOT. Vibrates a bit at 2,200 rpm but smooths out when I give it a little more gas, however that's solidly in smoke territory. I would not describe the vibration as excessive.

Alan again: I do not have a tach but will pick one up. Based on other forum posts from people I have no reason to think this 18hp engine shouldn't push me at hull speed without struggling. And yep, two blade. It certainly doesn't look like it belongs on the front of a Cessna either.

Thanks again for the replies. I'm going to check the valves the next time I go to the boat. My research was inconclusive earlier - would misadjusted valves actually cause this problem considering I can run through the full rev range in neutral? I'm thinking yes but not positive. Regardless I'm sure it's been a while since they've been checked.
Is it worth trying to disassemble the exhaust system downstream of the elbow to investigate any potential blockage or is that just a thing that doesn't happen? There was tape over the exhaust hole all winter so I don't think anything made a nest in there. Just not sure what the inside of these mufflers look like - is it straight through or is there a chance for monster clumps of crud to occlude the passages?

Thanks!

A couple of points:

(1) I'm not sure how you propose to measure the pitch of the prop by diving on it. You (or a diver) should remove the prop and see what is stamped on the hub--unless you are somehow able to determine that without removal. (I doubt it.) Really, what I think I'd do is remove it and just take it to a prop shop regardless of what is stamped on it, since someone could have had it repitched and perhaps the new pitch was not stamped on the hub. You never know. Do it once and do it right. If the prop turns out to be wrong, then you can have it repitched at that time before you reinstall it. If it's correct, then you'll have to look to other causes. When putting back on the prop under water (or even out of the water), you might want to find an experienced person who know what he/she is doing, because it's definitely possible to reinstall a prop incorrectly even though it looks correct. But that goes beyond what I can get into here. A prop shop can tell you the pitfalls to avoid and probably recommend someone.

(2) Note that you need to be able to get close to or actually reach MAX RPM, not just cruising RPM. Cruising RPM is where you are going to run it most of the time in everyday use, which is, as you say, about 75 to 80% of max. But if that's all you can reach at WOT, then you still have a problem.

Alan Gomes · Jun 14, 2020

BOSSailor said:
Alan: Point taken about the prop. You clearly have more experience with this than me. This isn't my first boat but it is my first inboard. I suppose I figured I'd see a big ACME MODEL 1234 or something stamped on it and the lights would shine down from the heavens (or rather my laptop monitor) illuminating the correct table I need to look it up in. I'll find someone.

And I agree about the max rpm. Right now I'm still short of cruising rpm even with the throttle all the way forward (and a thick blanket of smoke hovering above my wake).

Yeah, it really presents symptoms of being overpropped. I'm not saying it couldn't be something else but that's sure what it looks like to me. Also, make sure there is nothing wrong with your cutless bearing in the strut. When it's in neutral, does the coupling (at the transmission) rotate easily by hand? I'm doubtful this is it, but I'd check that just the same.

The number stamped on the hub will show something like, for example, "12x11." One number is the diameter (which is pretty easy to measure anyway) and the other is the pitch. So, to describe the prop fully, you might have, e.g., a "2x12x11," which would be 2 blades, 12" diameter, 11" pitch. Obviously they don't have to stamp the number of blades on it since that's pretty hard to miss!

What could have happened is that a previous owner needed a prop, bought one off of eBay or Craig's List that fit diameter and shaft-wise, but paid no attention to how it was pitched relative to the engine. That's a definite possibility.

Fortunately, this is not a huge hairy deal to determine. It does mean pulling the prop, but that's not crazy hard to do. There are worse things you could (and will!) have to do on a boat.

If a prop shop needs to repitch it, they will also make sure it is balanced while they are at it.

Let us know how it goes.

Alan Gomes · Jun 14, 2020

BOSSailor said:
Hmm.... am I correct in thinking that a few seasons of being overpropped and loading up the engine with soot because it can't rev high enough would lead to the blocked elbow?
I think I remember the shaft rotating fine in neutral on land but I will check it again. How easy are we talking about? Spinning like a car's wheel hub when you remove the wheel or more like "not difficult?"

I COMPLETELY forgot to mention that in the course of cleaning out compartments and learning my way around the boat (seriously, my next boat will have a line in the contract that says previous owners to remove all their junk, every last little item) I found a prop strut that had been snapped in half. I suspect the PO may have backed into something and caused the damage. I wonder if the prop had been damaged too and was replaced with something a little too aggressive.

It seems to me entirely plausible that this could have contributed to clogging the elbow.

I think the scenario you've mentioned about a damaged prop is also certainly possible. No matter: I'd remove the prop next thing and either confirm that it is in fact a problem (and then fix it) or eliminate it from consideration. I'd certainly do that before removing the head.

Benny17441 · Jun 14, 2020

I would think a used boat, 30 years old with a 2 blade prop has the original propeller and it should be sized accordingly. Just make sure it is clean. A speed of 5 knots under aux. engine is not too shabby. Forget about hull speed, you will need the sails for that. Check the engine valves clearance and adjust as needed. Make sure fuel is fresh. Yanmars burn the fuel air mixture by heating it by compression. At startup the engine builds compression by successive revolutions forced by the electrical starter. To reach starting compression the starter must turn freely; make sure batteries are fully charged and that connections are clean and tight. Open up the throttle fully and then crank the engine. With the engine at idle and at operating temperature remove the oil fill cap. Hover the palm of your hand over the opening; you should feel a little air pressure hitting your hand, if you feel a strong thump you have excessive compression leaking through the piston rings. That could indicate that the engine is worn or that the oil being used is too thin. Going to a 15W40 with some additives may improve compression. Diesel engines love heat, run the boat at wide open throttle for 30-40 minutes to get rid of carbon buildup. Check for fuel leaks; where fuel leaks out, when you turn the engine off air can come in. A slow air leak will be cleared by the engine running but it can delay the engine cold starting. Check all fuel system connections. What color is the smoke, white is water vapor, gray is engine oil and black is unburned fuel. Black smoke would indeed point to a fouled or incorrectly sized propeller. Do check your tachometer for accuracy as to me the boat speed seems OK . Try all of these before you consider a head rebuild. Remember the old diesel engine is an auxiliary and not a performance engine. It just needs to provide a basic function of being able to move the boat around 4 or 4.5 knots or help you dock. To me an engine that starts relatively quickly and will reliably run to move the boat at 4 knots when there is no wind is a good enough engine.

Alan Gomes · Jun 14, 2020

BOSSailor said:
Putzing around the web some more and I came across this Hunter 30 (1989-1993) 3 blade bronze propeller
Not sure if it's the original but I assume it is. If that's true it would appear the boat was originally equipped with a 3-blade propeller instead of the 2-blade that I've got.

This really tells you very little. It's possible that different engines could have been put in that boat. Some boats could have had 3 blade props, some could have had two blade props. Really, it is acting like it is over propped and the only way you were going to know is to pull the prop and check it.

Alan Gomes · Jun 14, 2020

Benny17441 said:
I would think a used boat, 30 years old with a 2 blade prop has the original propeller and it should be sized accordingly. Just make sure it is clean. A speed of 5 knots under aux. engine is not too shabby. Forget about hull speed, you will need the sails for that.

There is no reason to conclude from the age of the boat that it has the original prop, nor that it is sized correctly. Nor is it merely an issue of speed. Over propping an engine is one of the worst things you can do to it and will significantly shortened its lifespan.

KemahSailor · Jun 19, 2020

I have a 1990 Hunter 335 with a 2GM20F. In calm water, I get 6 knots at 2500 RPM.