Running Old Diesels at Low Speeds

[Guest]

I recently acquired an older motor sailer with a Volvo MD3 engine in good condition.

When motor sailing there is a temptation to run the engine at very low speed for extended periods (hours to tens of hours).

Are there any Volvo mechanics out there who can guide me as to whether or not this is harmful to the engine?

The engine runs at normal temperatures, during these extended times at low speed, (throttle open just a crack). Is there any hazard of building up deposits in the exhaust ports that will not burn away during the first half hour at fuller throttle settings? What is your experience when tearing down these engines? Have you found them heavily carboned up?

Best,

I.

 

[johnsomerhausen]

If your wallet and the space at the steren allow it, a Brunton propelleer would solve the problemm as it automatically addjusts its pitch to fully load the engine.
john

 

[Piers]

The problem with a diesel engine if run at very low RPM for long periods, is that the pots become 'glazed' by a hard deposit formed within the cylinder. A problem which requires major attention.

This is why it is good to run the engine under load.

However, regarding your specific engine, I suggest you ask Volvo themselves for guidance.

Piers du Pré
MBM Cruising Club enthusiast
www.dupre.co.uk/fsPlaydeau.htm

 

[tgalea]

So do i understand correctly that a small diesel actually prefers being run at say 90% RPM as opposed to lower revs ?

Thanks & Regards

TYrone.

 

[summerwind]

Its a bit like having sex really - you can run along on a partly opened throttle for ages, but sooner or later you have got to give it full bore to do the business.

I believe it is correct that if you don't give the engine a good work out every now and again, it will glaze up - just like your lover's eyes. Of course, you have to get it warmed up before you give it stick!

The ideal is if you have to punch the tide out of a river mouth. By the time you get there from the mooring, the engine is warmed up nicely. Then give her full throttle for five minutes or so, or as long as it takes you to get out. This will blast out all of the old carbon and other debris that has accumulated around her ports during periods of gentle running. Do that and she will be happy and have a warm feeling inside. Whilst she is flat out her life blood (diesel) will be flowing hard through pump and injectors and her cooling water will be flowing hard, again swilling out all of the deposits built up during periods of warm, but slow running.

Regardless of age, engines like a good work out, treating them too easy isn't good for them but of couse, they need TLC as well. Good clean fuel, good oil and filters changed on times will ensure a happy engine.

 

[Guest]

Thanks for the insight.

It had not occured to me that one of the benefits of a Brunton/Autoprop was keeping the engine fully loaded at low RPM, but now that you point this out, it's clear.

Actually 1,300 GBP could be spent better elsewhere on the boat than on an Autoprop, so I will not rush out and buy one. But, as you point out the Autoprop does solve the problem, and even at present prices an Autoprop is lower in cost than the cost of honing the cylinder walls, and replacing the rings.

Best,

I.

 

[scottie]

Volvo did not do a md3 as such it started with the 3b and when they changed from number of pots to displacement became 17c or 17d.
If you have to motor sail it is not a good idea to sail too long on one tack as you can get oil starvation although I think that the 2 cyl engines suffered more
In any case from the age of the engine bore glazing is well past happening but you could get sticky valves
as earlier post have said give it a reasonable run to get it it up to working temperature or you will be tempting trouble

 

[oldharry]

Diesels old or new running for extended periods at low speeds will sooner or later run into troubles of all kinds. Even more rapidly if they are run light for 'battery charging' purposes.

Well known is the glazing effect on bores reducing compression, making starting difficult and increasing oil consumption. Less well known effects include unequal warming up of the head and block - diesels run much cooler than petrol engines, and unless worked hard do not warm through properly, leading to various problems including head and head gasket failure.

Condensation may occur within the upper parts of the engine, which can cause damage to valave springs, which will then break, allowing valves to drop into the engine with terminal results! Injectors are subject to cold erosion in the vicinity of the nozzles causing early failure.

In the meantime the lubricating oil does not properly warm up so does not work efficiently at either lubricating or removing and filtering off the gunge that accumulates inside diesels....

Need I go on? Oh, and even if the coolant is up to temprature, do not suppose the engine is properly warmed through. Its not, unless its working reasonably hard.

Its a bit like the old cardio-Vascular system: it is not being exercised unless the pulse rate is raised for at least 20 minutes in each spell of exercise.