BMC engine problem

[kalindi]

With help from a previous posting I have solved the idling problem of my old BMC some months age and everything has been working fine, until now.
The engine starts well seems to have plenty of power and general has not been a problem. Yesterday all was ok on the way out of the harbour and after a reasonable sail I started the engine to make the return. The engine started ok but after about 10 mins I noticed a trace of white smoke from the exhaust, after another ten mins there was a slight increase in the running temp and then when slowing down to manouvre the revs seemed to increase and it sounded for as though it had come out of gear with no load. The prop was still turning and the boat still moving. I slowed down to about 3 knots and the temp returned to a little below normal and after a while it again sounded normal. After tying up and without stopping the engine I ran it out of gear at full throttle. The engine responded and a hughe quantity of black crud came out of the exhaust along with the water. After a while the black stuff disappeared and lots of clear water flowed through the exhaust. It now seems ok. Has anyone got any ideas about what was going on? I intend to check out the cooling system to see if everything is ok.

 

[MoodySabre]

Are you sure it was white smoke and not just steam caused by the overheating? Sounds to me like you have caught something round the prop which had caused the overheat and lots of unburnt full in the exhaust. The prop then cleared. When you revved up it shoved all the unburnt fuel out of the exhaust.

You won't really know if all is well until you run it under load in gear.

 

[kalindi]

Thanks for the quick reply.
It could well have been steam, and I had not thought about the possibility of wrapping something arround the prop so I will check if I can this weeked. By the way the black stuff did not appearto be oily but just sort of floated on the water in smal lumps, a bit difficult to describe.

 

[Lakesailor]

It may indicate something a bit more worrying like a head gasket failure or a crack in the head. The fact the temperature went up and you saw steam when the motor was under load may be clues.
The steam may have loosened carbon deposits in the exhaust.

Worth keeping an eye on and possibly getting a compression test done on a warm engine.

 

[MoodySabre]

Another idea - perhaps the black bits were carbon deposits dislodged from the exhaust elbow by the heat effect. What caused the overheat - something blocking the water inlet that cleared when you reduced the revs? I think that you have to check the whole water intake system, strainer and impeller and check for good water flow out of the exhaust. If that's OK and the prop is clear then perhaps it was a one-off. A good run out is the only option as running in gear attached to the pontoon will take a long time to get up to working temperature. As you know, things only go wrong when you are detached from land!

Must type quicker!

 

[Stemar]

Pure speculation and I'm not familiar with your engine, but black lumps could well be soot. If the exhaust elbow is getting blocked, as the blockage reaches a critical point, back pressure would cause the engine to have to work harder and less efficiently, which could make it run warmer.

Max revs off load would build up a lot of pressure in the exhaust manifold and could blow a lump of the blockage out.

If this is what happened, I'd expect everything to be OK for a while (months, years) until the crud builds up to the point where the blockage reaches critical mass again.

However, it'd be worth having a look 'cos there's a slight possibility of water that can't get out down the exhaust finding its way back into the cylinders. You may be able to pull the rubber exhaust pipe off and have a poke around the water inlet to the manifold, which is where blockages tend to occur.

 

[maxi77]

I have a Thorneycroft, which is based on the BMC engine and it is quite sensitive to partial blockages in the inlet strainer. A slowing in raw water does cause significant temperature rises which vary with engine speed, overheat if slow, ok at medium speed and then high again on full power. Fortunately the strainer is an absolute doddle to clear.

 

[ianat182]

My suggestions1) check for impeller breakage,replace if not already done. This could be the reason for the overheating.(2) I suggest that you check the exhaust hose for delamination of the inner, causing part of the exhaust to pass between the inner laminates, and the sooty crud.
Your symptoms are the same as I had with my old Volvo diesel,leading to overheating and a blown head gasket.
You can clear any other crud from the exhaust as part of the check .Would be prudent to check the oil level and if any water present in the sump.

 

[kalindi]

Thanks for all the advice, I hope to check the cooling systems etc this weekend.