Volvo Penta MD7a Severe Engine Blowby Path Forward

[SV_Shadowfax]

Looking for advice/thoughts:

Yesterday when we took our 32’ sailboat out I noticed we could only get to 1800 RPM on our Volvo Penta MD7a. Typically we can get to 2500 RPM but cruise around 2000 RPM. Additionally, when we ran at the max 1800 RPM we could get, there was some black sludge coming out of our exhaust. I believe this is from engine blow-by. I knew it was starting to happen since we were getting a small amount of oil dripping from the air-intake and our mechanic suggested removing the air filter, so the engine didn’t have to work as hard. However, yesterday was the first time we seriously power limited.

I see a couple options here:
A) Remove and rebuild engine
B) Remove and purchase replacement MD7a engine
C) Remove and purchase replacement modern diesel engine
D) Remove and purchase replacement electric engine

Am I missing any other options? Any chance I misdiagnosed? I’m leaning towards option A, as it seems like it would be the most affordable and I would get to learn the most. It would obviously be a lot of work. Variants of B & C would be paying a mechanic to do the swap. But I think this would end up in the $20k-$30k range which is too much. D) is somewhat appealing but besides being more expensive seems like it would also get pretty complicated.

If we went with option A, the next question is whether we

1) Pull the engine while in the water
2) Get the boat in a boat yard and pull the engine

#2 seems like it would probably be easier to work on and could let us take on additional projects like painting. But this could also get too expensive if we want to maintain our slip in Anacortes (they apparently can be hard to come by). Or just give up the slip, take the time we need in the boatyard to fix things, and then find a new slip.

Obviously this really sucks, but just trying to move past that on focus on the words of Gandalf:

“I wish it need not have happened in my time," said Frodo.
"So do I," said Gandalf, "and so do all who live to see such times. But that is not for them to decide. All we have to decide is what to do with the time that is given us.”

Looking for any thoughts/advice here. What would y’all do?

[SY Harmony]

This just started out of nowhere?
How dirty is the bottom and prop?
Is the exhaust elbow carboned up?

[pembquist]

I have been avoiding doing it myself so take it for what it is worth but before digging in you should try to diagnose the problem by doing a leakdown test. Maybe the problem is with the head and you could get that rebuilt on its own. As a caution I once talked to a mechanic who said they didn't like to do that as it inevitably seemed like once they did the top end the bottom would give problems leading to recriminations from the customer. I personally wouldn't recommend A or B in less you are really interested in doing the work yourself and are willing to go on a snipe hunt for parts and have a good machine shop available, my understanding is that at some point the water passages on one of these get plugged up and the block is scrap so I don't think having a rebuilt is comparable to a new MD7A that somebody put in a time machine and sent forward 40- 50 years; a new to you used MD7A doesn't seem like it would be worth the effort. Getting a Beta Marine seems to be the popular thing but probably costs more than the boat is worth and won't increase its value by a concomitant amount, obviously in strict economic terms the thing to do is buy an identical boat where someone else has already repowered it but that's probably a unicorn and a hassle.

Electric seems like it would be great for short distances, (getting out of the Marina,) but it seems like it puts the Sail in Sailboat which while perfectly fine is really down to what you are going to be doing with boat. I don't have any idea what it would cost to have the range of a 12 gallon diesel tank and how you recharge the batteries when away from shore.

My MD7A still runs OK but I confess I don't feel super confident about it. Like I said I haven't done the leakdown test but I think it isn't developing full power and it is a bit of a tooth rattler. I think that Ultimately I am going to become one of those suckers that repowers the boat for somebody else but maybe I will get a couple years low anxiety and less vibration.

[DougR]

As mentioned by Harmony, pull the exhaust elbow and clean it and the inside of the exhaust manifold.

Then re-evaluate ABC&D.

[Ballsnall]

I'm sure you already know this

A) Cheapest option
B) Fastest option
C) Expensive & time consuming option
D) Most expensive option

[Don C L]

Rebuilding is not necessarily so cheap. My friend had his MD7A rebuilt by a mechanic friend and there was a lot of complaining about the cost of parts! You could look into what is probably most common, to replace with a Beta. https://www.nauticexpo.com/prod/beta...96-258692.html
As far as pulling the engine, our mechanic friend disconnected the transmission instead of the shaft! It makes a bit of a mess, but it seemed to be easier. Two of us pulled the engine out on 2x4s and then hoisted it with the crane at the boatyard. I think they charged us $100. I gotta say though, now that it is rebuilt and back in, that is the smoothest running 2 cylinder diesel I've ever heard!

[Don C L]

You can also do a search for "MD7A" here for tips and there may be a thread in this social group to help:
https://www.cruisersforum.com/forums...o+engines.html

[Searles]

The MD 7A is an excellent small diesel ,well worth the rebuild ,mainly because it fits ,everything is there ,lift her out in the water is not a biggy ,use the boom and main hlyd plus toppin lift ,swing onto a pontoon or into the dingy ,replace the same way .⛵️⚓️

[SV_Shadowfax]

Thank you for all the responses.

I noticed the oil coming out of the air-intake October of 2022. That's when I removed the air filter. This weekend was the first time I noticed I was power limited and had black gunk coming out of the exhaust at full power.

Bottom of the hull seemed pretty clean, we sail the boat at least monthly and that keeps a decent amount of the crud off. Haven't checked the prop, can do that next time I'm at the marina.

Stupid question, what's the point of doing a leak down test if I know I'm getting leakage into my crank case? When the engine is running I'm getting a constant flow/mist out of the crank case ventilation tube. Doesn't that mean I'm leaking pressure past my piston rings and therefore that's the issue? Sorry if this is a silly question - I'm new to working on diesel engines.

Video of engine idling: https://photos.app.goo.gl/sntcZhCdvjFnHShE9

[Ballsnall]

Originally Posted by SV_Shadowfax

Stupid question, what's the point of doing a leak down test if I know I'm getting leakage into my crank case? When the engine is running I'm getting a constant flow/mist out of the crank case ventilation tube. Doesn't that mean I'm leaking pressure past my piston rings and therefore that's the issue?

All engines produce some blowby, a leak down test will tell if it is excessive or not and from which cylinders. Also there are several causes not just rings, so for a simple test you get the cause and extent of the engine condition which will guide you on the repairs required.

[SV_Shadowfax]

Originally Posted by Ballsnall

All engines produce some blowby, a leak down test will tell if it is excessive or not and from which cylinders. Also there are several causes not just rings, so for a simple test you get the cause and extent of the engine condition which will guide you on the repairs required.

How will I tell if its excessive or not? Just if its audible or not? Or would you use one of the leak down testers that actually says low/moderate/severe? If so, any advice on what brand to buy? I hear there's a bunch of crappy ones where the low/moderate/severe gauges are bogus.

Also, the MD7A's don't have glow plugs, so where do you apply the leak pressure?

And finally is it worth doing a compression test first? Verify there is an actual compression issue? If so anyone know what the nominal compression PSI is?

Thanks in advance everyone... figuring this out one step at a time

[Lepke]

Whether the blowby is excessive or not it doesn't cause power to suddenly go away. You may have a burnt injector tip that allows the diesel to enter in a large stream instead of a mist. All the diesel doesn't burn correctly so you get heavy black smoke. And because the engine isn't making required rpm the injector pump is supplying more diesel to all the cylinders including the bad one, making even more poorly burnt diesel.
And it also could be a failure within the injector pump.

My point is, you can probably fix your current problem and put off the engine rebuild/replace decision. All engines have some blowby. The pressure in the crankcase is vented, sometimes to the intake. That could explain the oil in the air cleaner.

[SaylorMade]

I had an MD7B, and went with option D this year.

Best thing I ever did.

https://www.cruisersforum.com/forums...ed-275309.html

[Ballsnall]

Originally Posted by SV_Shadowfax

How will I tell if its excessive or not? Just if its audible or not? Or would you use one of the leak down testers that actually says low/moderate/severe? If so, any advice on what brand to buy? I hear there's a bunch of crappy ones where the low/moderate/severe gauges are bogus.

It's rather subjective but any decent mechanic could just remove the oil cap and have a reasonable idea.

And finally is it worth doing a compression test first? Verify there is an actual compression issue? If so anyone know what the nominal compression PSI is?

The first thing I would do is a compression test.
I'm sure you can find the actual values but most importantly is the variation between cylinders which can tell you a lot about your engine condition.

You also need a wet compression test which will tell you if the issue is due to valves or piston rings. This is just a 10s spray of lubricant into the cylinder and if the piston rings are worn the compression test pressure will be higher.

If you have a low cylinder pressure and it doesn't improve with a lubricant then it's a valve problem.

[skipperpete]

From watching the video, crankcase pressure is the least of your problems, the hold down plate on the saildrive diaphragm looks terribly corroded. The point of doing a leakage test is to define the reason for the crankcase pressure, it could just be a blown headgasket across to the pushrod gallery, not the piston rings. Its nice to know whats going on before pulling the engine apart.