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Old 02-07-2020, 15:36   #16
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Re: White and Offensive.. Diesel Smoke Diagnosis

Id check your valve clearances as well. A little off in timing can cause the exhaust valve to open too soon letting out unburnt fuel. Once those are good check injectors. Id replace the elbow now rather than giving your engine a salt water steam bath.
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Old 02-07-2020, 15:44   #17
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Re: White and Offensive.. Diesel Smoke Diagnosis

if it's a yanmar you need to check the exhaust elbow.
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Old 03-07-2020, 16:58   #18
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Re: White and Offensive.. Diesel Smoke Diagnosis

On the hook now. Started with 20 mins of getting engine to temp under load. Nothing more than the typical 1,500-1,700 RPMs that I usually run to get out of the marina basin.

Once at temp and under load, above 2,000RPMs there was significant smoke. (Video link below.) The more throttle, the more smoke and the grayer the color became. Same when in neutral but to a lesser extent. I did full throttle with/without load and frankly got scared I might start a fire.

Returning to sub-2,000rpms the smoke reduced to relatively tolerable. Engine sounded fine, pushed like normal and seems within temp. No weird noises, no RPM or power fluctuations.

After all, it’s an old engine and the boat actually sails quite well. If I was on a mooring, I’d probably never use the engine, but need it this season for the marina. I will scope out the exhaust elbow but probably wouldn’t do anything else to the engine. In fact, I may even sell the boat. It’s otherwise in excellent condition. Do the above risks sound like catastrophic failure or just a gradual fade?

https://imgur.com/gallery/AevnkBY
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Old 03-07-2020, 17:19   #19
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Re: White and Offensive.. Diesel Smoke Diagnosis

Injectors are very easy to pull, why not pull them?
If your going to sell, I bet you’ll get more money with a properly running motor, and injectors or the elbow are just too easy to fix, to not do it.
But you need to know a couple of things first what’s rated RPM? On a warmed up motor leave it in neutral And advance the throttle and ensure it will hit full rated RPM, ideally it should hit this underway also, but missing by a couple of hundred RPM is usually considered acceptable.
Not hitting spec RPM is more indicative of a clogged elbow, but I still think you have a messed up injector because usually a clogged elbow will cause you blow black smoke when you try to run it hard, but it will run at low power and idle fine, but your not running fine at idle and low power, and that’s indicative of a bad injector.

Injectors and or an elbow will over time cause damage, but it usually accumulated over time, and it’s not that uncommon for a bad injector to cause an elbow to gum up. But clogged elbows are almost a normal operating condition over time. Exhaust elbow are not a permanent part, they are like tires on a car, they wear out and need to be replaced every now and again, but injectors are the same way
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Old 03-07-2020, 17:45   #20
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Re: White and Offensive.. Diesel Smoke Diagnosis

Hitting rated RPMs, no gaps. I should reiterate that sub 2,000 RPMs operation is flawless. Just more Smokey than it should.

I must be the 4th or 5th person to own this boat but there’s a likely chance these are still the ORIGINAL injectors. Suppose you’re right. Time to yank em.

If I were to sell, it’d have nothing to do with the boat/engine - having a blast, especially during these corona lockdown days - but suppose you’re right about the value.
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Old 03-07-2020, 18:34   #21
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Re: White and Offensive.. Diesel Smoke Diagnosis

ah yes, the Volvo MD7A...my old nemesis...I had the MD7B....same engine but with a downward sloping transmission....had it for many years on my first boat.

You do know off course, that it is a raw water cooled engine....using salt water to cool it's innards...no heat exchanger here...

Being raw water cooled means it does not run very hot at all....way cooler than an engine with a heat exchanger. Don't expect to see high engine temps, because of this. This low engine temp is likely also the reason you might see an oily sheen on the water, as the engine does not get hot enuff to burn all the fuel..and as you say, hard to detect.

My engine also developed this " white smoke"....though I could never tell if it was smoke or just steam....read on...

The problem with these engines, that being raw water cooled, the cooling passages around the cylinders tend to get gummed up with salt crystals/residue. The exhaust manifold is also raw water cooled and a common problem with these engines is that the exhaust manifold tends to " rust out" on the inside, from all the sea water, developing tiny holes inside, so what you are seeing is superheated steam that gets expunged with the exhaust gases. The problem starts to appear when the raw water cooling passages around the two cylinders gets gummed up and this build a lot of pressure.

The " white smoke" was at first alarming to me, but I noted I was not burning any oil, so it was not that.

I ran my engine routinely at around 2,200 rpm for hours without a hiccup.

I did make it a habit, to remove the cylinder head to ream out the cooling passages. Then I simply replaced the head with the original head gasket. I must have done this at least a dozen times, always replacing the original head gasket. Never had a lick of problem. Not once.

Never had a problem with the engine either, despite being nearly drowned in seawater on one occasion...just cleaned it out and it started right up.

Yes, the exhaust manifold and riser are the first place I would look for a problem, but also, don't forget to remove the cylinder head to ream out the cooling passages.

This engine is not made anymore, though I believe parts are still available. Be prepared to sell your firstborn to pay for the parts.

But, yeah, being an old cranky engine with a monster flywheel, it always started right up, never once did I have a problem starting it.

Clean up the water cooling passages and if you can find a replacement, get a new exhaust manifold and riser, and that thang will surprise you by running forever.
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Old 03-07-2020, 20:01   #22
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Re: White and Offensive.. Diesel Smoke Diagnosis

It looks like steam to me too but I already said that & OP refutes that. I'm even more suspicious that it's mostly steam looking at the 2nd vid.
Not familiar with that engine but our raw water cooled Yanmar does not blow any white smoke even cold so I don't think it's compulsory for raw water cooled engines to blow white smoke or have exhaust sheen. Maybe a little when cold will occur on a worn engine.
Wouldnt be hard for him to do an acid descale for a start.
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Old 04-07-2020, 10:52   #23
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Re: White and Offensive.. Diesel Smoke Diagnosis

well, if it was burning oil, you could check it by keeping an eye on the oil dipstick.
my MD7B ran for years with the " smoke" and never ate a lick of oil in that time and always ran just fine.

My take on the matter is that when the cooling passages around the cylinders get clogged up, and they will, and this is a certainty, that the raw water coolant now has to make it's way thru' reduced passage ways...which increases the pressure.That increased pressure will find the tiniest pinhole in the exhaust manifold and exit the boat with the exhaust fumes as " steam"

There was another thread on here about the same engine, with basically the same issue. That poster had the exhaust manifold checked and sure enuff....pin holes...lots of them..
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