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Blue smoke


Midnight

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A friend just bought a boat with a BMC 1.5 which runs fine except it smokes quite badly (blue) when under way. He got it for a good price so is relaxed about the repair costs.

 

The previous owner, who I also know, had the shaft re-ground, new shells, the bores glaze-busted and new rings about 2 years ago. We are suspecting the valve seals are worn as the previous owner said he didn't overhaul the cylinder head at all.

 

If that were the case would it cause such a lot of smoke - it also uses a fair bit of oil.

Edited by Midnight
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A friend just bought a boat with a BMC 1.5 which runs fine except it smokes quite badly (blue) when under way. He got it for a good price so is relaxed about the repair costs.

 

The previous owner, who I also know, had the shaft re-ground, new shells, the bores glaze-busted and new rings about 2 years ago. We are suspecting the valve seals are worn as the previous owner said he didn't overhaul the cylinder head at all.

 

If that were the case would it cause such a lot of smoke - it also uses a fair bit of oil.

 

 

I think probably yes, if the valve guides are badly worn and/or the little seals are missing from the top of the exhaust valve stems.

 

The million dollar question is did it smoke like a chimney before the glaze busting and new rings?

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Look at the topic immediately below this one and then see if you can find that contributor's previous one. It is a perfect illustration of what may happen if you only hone bores without ,measuring them.

 

It certainly is not going to cost much to change the valve stem oil seals so its a good first step. You will probably find that if you put a cylinder to TDC and use a "Ford" type all work from the top valve spring compressor that you can change them withtout taking the head off. I did no my Bukh. If you did get it wrong and ro[p a valve into the cylinder you would have lost nothing unless you tried to start the engine because you would just take the head ff.

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Thank you for the responses chaps

 

 

 

I think probably yes, if the valve guides are badly worn and/or the little seals are missing from the top of the exhaust valve stems.

 

The million dollar question is did it smoke like a chimney before the glaze busting and new rings?

 

I can ask the previous owner Mike I know for sure he overhauled the bottom end and fitted new rings

 

Look at the topic immediately below this one and then see if you can find that contributor's previous one. It is a perfect illustration of what may happen if you only hone bores without ,measuring them.

 

It certainly is not going to cost much to change the valve stem oil seals so its a good first step. You will probably find that if you put a cylinder to TDC and use a "Ford" type all work from the top valve spring compressor that you can change them withtout taking the head off. I did no my Bukh. If you did get it wrong and ro[p a valve into the cylinder you would have lost nothing unless you tried to start the engine because you would just take the head ff.

 

A broken ring was my first thought until I heard the valve seals hadn't been changed. With the engine running there's a bit of blow back (?) when you remove the oil filler and put your hand over the hole. Not sure if this is normal.

 

I followed the boat up to Brighouse recently and there was a fair amount of blue smoke.

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There will always be some pulsing gasses coming from the oil filler, its the volume and how fumy it is that is used for diagnostics. That is not a lot of help unless you check that sort of thing all the time so know what is acceptability for a articular engine type. You can not really extrapolate from a petrol engine to a diesel although that would give a guide.

 

As I said you will lose little by changing the valve stem seals so that would be what I would do first in this case. You might get lucky. If its anything more you are faced with the head off a least. Once the valve spring is off you can check the valve for wobble in the guide but we are back to experience to know what is acceptable and what is not.

Edited by Tony Brooks
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There will always be some pulsing gasses coming from the oil filler, its the volume and how fumy it is that is used for diagnostics. That is not a lot of help unless you check that sort of thing all the time so know what is acceptability for a articular engine type. You can not really extrapolate from a petrol engine to a diesel although that would give a guide.

 

As I said you will lose little by changing the valve stem seals so that would be what I would do first in this case. You might get lucky. If its anything more you are faced with the head off a least. Once the valve spring is off you can check the valve for wobble in the guide but we are back to experience to know what is acceptable and what is not.

 

Thanks Tony that's exactly what my colleague intends to do now. Even if the head comes off for new valve guides the cost will be very reasonable.

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If he does take the head off please get him to measure the bores while he is in there.

 

He may find there is a significant ridge at the top the thrust side of the cylinders. If he has see if he can look down the side of the pistons so he can inspect the top ring. If that ridge was not removed when the bores were honed and new rings were fitted one or more top rings may be broken.

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the little seals are missing from the top of the exhaust valve stems.

 

 

I thought the general consensus was not to fit seals to exhaust valve stems as oil starvation in the guide was more serious; and only fit seals to inlet valves?

 

Even then, unless the air filter is seriously blocked I wouldn't have though much vacuum would be pulled on the inlet side?

 

Given the oil breather pipe seems to go into the air filter, once it becomes clogged everything can only get worse!

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