How often should engine exhaust elbows be decoked?

M

malcp

Well-Known Member
Joined
29 May 2003
Messages
237
Location
Chichester, UK
Visit site
I have a VP MD2030B and after ~750 hours (8 years) the exhaust elbow was absolutely chocker with carbon deposits. Only about 10% of the original diameter left for the exhaust!

Is this to be expected after this time frame?

I think the engine is a little smokey (grey), and perhaps this has contributed?

Glad of any advise.

Thanks
 
I don't think a decoke is the answer, I have just changed one of the exhaust elbows on one of my engines after 1300 hours. The only reason I changed it was because it was leaking slightly from the inside bend of the elbow, which is where they always rust away. There was no coke at all in my elbow and I would suggest that perhaps your problem could be caused by the turbo unit?

Barry
 
Thanks for this. There's no turbo unit. It's normally aspirated. But maybe it's because it's a bit smokey. Elbow was not really rusted much, it was the level of carbon deposits that had built up that was the problem. If this is not normal then I need to get to the root cause of the carbon built up.
 
yep certainly not normal for an exhaust to fill with carbon. It's unburnt fuel, so injectors putting too much in or airfilters not letting in enough air or not enuf compression to explode it properly all worth a look.
 
for comparison I just pulled the elbow off my Yanmar 4LHA a couple of months ago (for access behind it) after 500h - it benefitted from a bit of a scrub (with BBQ cleaner) and the rubber flange was a little 'worn' but otherwise it showed little evidence of coke.
 
I had a similar problem with a perkins engine at around 1500 hours / 10 years of use - elbow down to about the thinkness of a pencil with gunge build up. The diagnosis from one who knows about such things was that the engine had been running consistently below the recommended revs due to an overlarge prop - thus not getting the exhaust hot enough to either burn the stuff of or prevent its deposit in the first place.
 
Ah. This might be a clue. I think the engine is slightly over proped. The engine is spec'd for max revs of between 3300 - 3600. I can get 3300 with a bit of black smoking. I usually operate at a cruising speed of 2800, with rare excursions to 3300 if the need arises. But, surely this is ok? It's not like it won't rev beyond 2000 to 2500?

If the suspected smokiness is to blame, what could cause this?
I've changed the air filter, the fuel should be clean (used Fuel set), primary filter is absolutely clean, no water, no sediment. Would the injectors need servicing with only 750 hours use? How about excessive backpressure on the fuel return line?
 
my previous engine (Volvo MD17D) had 10'000 + hour, when I replaced it last year. I had to change the elbow only once after 7000 hours due to a hole (rust), never cleaned it. I have run the engine most of the time with 1400-1800 rpms, halve of the time only charging batteries.

If you can not reach max rev, either your bottom is not clean, propeller to big, or your injectors are leaking. A real professional repair shop can check your injectors electronicly without removing them from the engine. ( I learned my lesson, I will never remove the injectors just for checking them. They tend to leak afterwards forever)

Peter
 
I'm with Mainbrace on this one. My auxillary Kubota had the same problem a few years ago. My unit has a cast iron elbow that is replaceable when this happens. Its caused in my case with a diesel generator of not having enough load on the motor. The cooling seawater exits at the elbow and the exhaust gets over cooled, causing the build up. Check carefully, as if yours is cast iron it may have rotted right through. I tried to pry the crap out with a screw driver but it went straight through the side of the elbow.
 
DON\'T BUY A NEW ELBOW

Whatever you do, don't buy a new elbow!

I had the same problem in Spain, I managed to get some Agua Fuerte - meaning strong water. It's about 25% hydrachloric acid 75% water. I soaked my elbow in it overnight, it came up like new. No need for poking with screwdrivers or anything.

Total cost was about 3 euros.

I don't know where you'd get hydrachloric acid in the UK, but that's the way to do.

Stu
 
Re: DON\'T BUY A NEW ELBOW

This sounds very interesting, but when the elbow rots and its soft enough to poke a screwdriver through it, surely the reason for this is the same problem as propellors being eaten away, i.e. electrolosis. The guts of the metal is just eaten away! so how does this mix of hydrachloric acid and water help?

Barry
 
Different kettle of ball games

Hi Barry,

My post was relating more to the original post by malcp where a mixture of dried seawater deposits and products of combustion begin to build up and cause a restriction resulting (in my case) in overheating. The acid clears away the built up crud.

Your problem, as you state is a different matter, engine anodes get eaten away at an alarming rate, once they're gone the engine starts to suffer.

imho of course!

Stu
 
Pure speculation only ....

In no way is this a pro fuel comment .... just a thought ...

You say you use Fuel Set ? Is that a regular use or just occasional doseing ?

I hear of some who are habitual users - throwing in Soltron / Fuel Set / all sorts every time they fill up even when tank is only topped up.

Now it crosses my mind whether killing - then breaking down the bugs to a fine combustible mixture in the fuel may cause soot ? I don't know - as in our biz - we test effect of Biocide / Enzymes in line with our use of ... but never look at continuous or over-use ....

Please do not take this as a pro statement ... its a comment and thought only ...
 
Yep as well and

If he is not getting the revs and seeing black smoke in the exhaust then there is also likely to be a lot of unburnt fuel sticking to the pipe at the same time.
Therefore he needs to look at his prop pitch /hull fouling as well as the basic engine checks.
Its amazing what a bit of slime/weed/shells/wormcasts do to the power requirement
 
electrolysis?

eh? The elbow would need to be full of water- permanently - for electrolysis to be the cause.

The cause of failure of an exhaust elbow is surely hot exhaust gas in a salty environment.
 
the degree to which you appear slightly overpropped, and the general running you indicate, would suggest that this isn't the primary cause unless your 3300 limit is 'clean and light' and reduces further with fouling and loading up for the summer cruise.

However your black smoke is unburnt fuel and indicates overfueling/under air - do yo have a good air supply to the engine? If you lift the cover underway when it's producing the smoke does it stop? It is also possible an injector has developed a fault of course - this would give you both unburnt fuel and a slight reduction in power (which might show up in the reduced revs under load...........
 
Hi Duncan,
The smoke at 3300 was independent of engine cover on/off. The 3300 limit seemed to me in the past to be independent of clean / light hull too. It was only recently that I have been further limited to 3000 revs, and I put that down to fouling on the prop (as seen from underwater camera on a stick). It is only some 3 months or so after this that I found the exhaust note sounding "hollow", like a "blown silencer" which I have heard when the watercooling was restricted. There was also quite a bit of smoke and diesel knock on rev acceleration in neutral. This prompted me to look further and found the clogged exhaust elbow (after checking the w/c, you understand). I think the exhaust is showing signs of increased smokiness in general. I'm getting the feeling there is something wrong with 1 or more of the injectors. Maybe I should get them serviced?

Since I have cleared out the elbow the w/c restriction, hollow exhaust note, diesel knocking and rev reduction have all gone. Now its just the smokiness, which I'm guessing caused the deposit buildup in the first place.

Perhaps if I try max revs in neutral for short duration I should be able to determine the unloaded max revs of the engine? This won't blow it up will it?
 
Re: Yep as well and

How do you check if your elbow is coked without removing it ?? I have an overpropped engine, and am having the prop repitched this winter. When new on and with a clean bottom I was getting about 2800, when it should have been about 3200/3400. Just before lift out it was down to 2200, but had a nice coating of barnacles on the keel
Never any sign of smoke from the exhaust. Do I need to check the elbow, or is their a way of decoking without dismantling?? I have a canoe stern and access is not easy!
 
no it won't blow it up - if it hasn't got a fault somewhere!
seriously, I was concerned when a mechanic just wacked mine open once but rationalised that I do the same thing with the gearbox engaged and boat flying from time to time so what the hell!

It will tell you some things but I am not sure it will mean much if it's an injector as the lack of load will probably permit it to rev fully even with a faulty injector. If it does rev fully but produces black smoke at high revs too then I suppose you have narrowed it down...........
given the further info you have given I would now suggest that you are sufficiently overpropped to make a difference - subject of course to no other problem reducing power output and thus restricting the engine speed under load.
 
I think reving it up in neutral should show whether the max speed is governor limited or load limited. And, yes it does tend to produce quite a bit of grey smoke when reved in neutral. It's also noticeable at idle.

Is 750 hours time to service the injectors?
 
You must log in or register to reply here.

Share this page

Top