Mysterious air bubbling!

[tamarkin]

Hi
I'm new to boating and having a bad first season with a 1977 Nimbus Motor Boat with a Volvo MD 21A diesel engine.
Someone has added a header tank for the fresh water coolant at some time and I found air/gas raging up through it! My engineer said it was the cylinder head gasket had gone but he could not see any water in the engine oil.
After skimmimg and pressure testing (he could not see the usual tell-tale marks on the old gasket) the head and replacing it the air is still bubbling up into the header tank albeit not as cloudy or violent.
My engineer is puzzled and cannot work out why it is happening. He visually checked the "wet liners" and they looked OK but he reckons the head has never been off before so he is wondering if there is a hairline crack or porosity in one or more of the wet liners.
The previous owner thinks it is something to do with leaking seals in the exhaust/heat exchanger system but my engineer thinks this is very unlikely and has dismissed this theory.
Does anyone have any bright ideas or suggestions?
I would be eternally grateful!
Cheers, John

 

[vyv_cox]

What other symptoms are there? Does coolant level fall rapidly, or not at all. Does the engine overheat? Can it just be aeration/cavitation caused by normal pump action? Is this something new or has it always done it? You say no water in the oil, is there oil or fuel in the water? There are tests available that will tell you this even for small quantities.

My experience with head gasket failure or head cracks is that the first indication is that the heater stops working. On a motor boat this may be something you can check.

 

[richardm47]

Oooh, another MD21A owner! Must be getting a bit rare now. Your boat is nearly as old as mine. Sorry to hear about your season. These old boats can be a trial, but as mine say "she's worth it".

I'm just about to swap my working 21A engine and VD gearbox out of my Rasmus and put in a modern Beta instead. So feel free to contact me if you need any parts, or indeed the whole thing.

 

[RogerMayne]

Hi, if the head has been crack tested, skimmed and declared servicable then it pretty much has to be a cracked liner. If you get the liners out you will be able to get them crack tested. Hope this helps.

 

[lenseman]

. . . My engineer said it was the cylinder head gasket had gone . . . . After skimming and pressure testing (he could not see the usual tell-tale marks on the old gasket) the head and replacing it the air is still bubbling up into the header tank albeit not as cloudy or violent. . . . Does anyone have any bright ideas or suggestions? I would be eternally grateful!
Cheers, John

Hello John, welcome to the forum.

I used to be an RAC Engineer in the 1970's dealing with problems exactly as you describe.

The fact that you had the head skimmed and a new head gasket caused the fault to lessen suggests to me that the head gasket was certainly a major fault but has been only a partial cure.

Now, something that car mechanics often fail to notice or check is the state of the engine block surface. Yours is a diesel engine with a fairly high compression ratio, probably 24:1, and as such there have to have been enormous pressures inside the combustion chamber over time which caused the head gasket to start to leak in the first place.

Now, this is not all that happens!

These extreme compression pressures exert an upward 'push' on the cylinder head when the engine is running and this in turn, over time, causes the cylinder head studs or head bolts to 'pull' the cylinder block up into a cone shape and this occurs around each and every one of the threaded blind holes in the cylinder block into which the studs or bolts engage.

Now, you might be asking what effect this might have??

When the skimmed head and fresh new gasket is returned to the cylinder block, the head is tightened back down correctly to the required torque BUT it is now tightened down onto the 12 or so conical areas around each head bolt or stud and the nett effect is that the rest of the cylinder head area is not truly mating. As I stated in the beginning, the skimming and new gasket has alleviated the major compression pressure leak but not cured it.

On the matter of hairline cracks in the head, liners or block, it is often extremely difficult to notice them 'by eye'. If you can see them by eye, then the leak will be blatant and will need no further discussion.

Any hair line crack causing minimal bubbles will need some form of chemical detector.

I used to use a three aerosol 'can' Crack Detector, once the engine or block was in the workshopon teh bench. The engine was stripped down and cleaned. It was then subjected to the first can which was a sort of trichloroethane 'wash' to remove all the final traces of grease and oil and to deep penetrate any cracks.

Once this first aerosol 'can' had evaporated and it was very quick, a second 'can' was used to spray the surfaces to be inspected. This covered the entire surface with a chalky white powder but it had a secret trick in as much that it penetrated any cracks or flaws in the metal work, carried down on any un-evaporated chemical from the first can.

After 20 minutes or so, this powder was dry and the third 'can' was sprayed all over the surface being inspected. This was a similar chemical to the first can in as much as it 'washed off' the white residue white powder. and after a few moments any remaining chemical from 'can 2' was lifted out of any cracks or stresses in the metal work and showed as a very bright read dye,cochineal red in colour.

Discounting X-ray, the above method is the truly definitive bench method of finding any faults with your engine apart from the stud pulling which can be done with a steel rule or engineering straight edge. Some of the smaller cracks in head and block only open up when engine is warm and under compression pressures and sadly will not be visible to the naked eye on a visual inspection.

I have done a quick search on-line and found the following link:

http://www.ccvalves.com/cpexcat/CRACKDETECTIONREPAIR.pdf

The one I used was similar if not the same as the "KWIK-CHECK Crack Detection" (Dye, Remover and Developer) mentioned in the above link.

I hope this helps?

Just a quick addition regarding the raised conical parts on the block around each stud/bolt hole.

These can be removed using a HSS 45° countersink and a hand held power drill. The cylinder bores and all holes/oilways and water channels will need to be blocked off using copious amounts of kitchen poly-roll (suitable oiled to 'catch' swarf) and the threaded stud holes in the block face can be similarly suitably blocked (to be removed with tweezers after the raised part of the head has been 'countersunk'). Access to an high-pressure airline would help?

It always surprises me that this fault is very often missed by engineering staff at garages. The counter sinking has to be only enough to remove the raised section - and no more!

 

[Bosun Higgs]

Pal has just been through this sort of problem with a BUkh. After 2 head gasket replacements, it turned out that the head had not been properly pressure tested and had a hidden crack between port and waterway.

Look at your symptoms logically. Neither a failed gasket nor a crack need pollute the engine oil - it depends where the gasket fails and where the crack is located. The fact that the gasket didnt appear to have failed ( as inded it didnt with my friend) as well as the fact that a new gasket didnt get rid of the problem, both suggest that the failure wasnt a gasket but is either a crack in the head or a problem elsewhere.

If the problem were uneven lervel of the deck of the block there would be an obvious track. Was there? I doubt it or a competent mechanic would have spotted it.

For gas to get into the cooling system, the gas concerned has to be at a higher pressure than the pressure in the system. That rules out areas like the heat exchanger - there is no pressure there. Ditto the exhaust. In fact the only bit of the engine where the gas pressures are higher than the cooling system is the combustion spaces.

So logically, there is a crack between combustion space and the cooling system.

 

[gordmac]

Is the engine overheating or is it blowing it's coolant out?
If not then it would be tempting to leave it be and see if the problem gets worse, if it does then it may be easier to find. If the engine is working normally maybe don't worry too much at the moment unless it gets worse as maybe it is a characteristic. Is the pressure cap ok and the correct one? if the pressure is a bit low you may get some local boiling causing the bubbles.

 

[willie4646]

usually with cracked wet liners you get water in the oil sump. I would plump for a cracked head, look between the valve seats!
Willie

 

[boatmike]

I agree with Lenseman. The head is only one half of the mating pair. Very often the tracking is obvious on the head but not the block. Whenever I have had a head skimmed I have "blued" it and moved it over the mounting face with the bolts or studs removed of course. Often I have found a depression burnt in the block or as he suggests, high spots around the studs. Rather than countersink I would use a proper counterboring bit and make a 1/16" recess around each bolt but countersinking will do if you don't have one. Sometimes just going over the surface carefully with a fine flat file will remove bumps but if you have a burnt out track between the water galleries and the cylinder you will need that skimmed too.... Sorry but Lenseman is correct that often mechanics will skim a head and bolt it back without further investigation. Thats only half of a proper engineering job I am afraid. I would lay good money on that being your problem. Sorry!

 

[tamarkin]

What other symptoms are there? Does coolant level fall rapidly, or not at all. Does the engine overheat? Can it just be aeration/cavitation caused by normal pump action? Is this something new or has it always done it? You say no water in the oil, is there oil or fuel in the water? There are tests available that will tell you this even for small quantities.

My experience with head gasket failure or head cracks is that the first indication is that the heater stops working. On a motor boat this may be something you can check.

Hi, Thanks for your reply. No other symptoms really and the coolant level did go down but due to a leak where the rubber hose attaches to the header tank presumeably due to the increased pressure fro the air getting in?
The previous owner had the same problem for years and he used to loosen the cap on the header tank until the engine heated up when he said the air settled down!
Not sure if any fuel in the water. The boat has a diesel heater which I have never used!
My engineer said when he took her out at the weekend that it took a few minutes after start up for the air to appear and it did not seem to settle thereafter.
Grateful for you help.
Cheers, John

 

[tamarkin]

Hi, if the head has been crack tested, skimmed and declared servicable then it pretty much has to be a cracked liner. If you get the liners out you will be able to get them crack tested. Hope this helps.

Cheers!

 

[tamarkin]

Hello John, welcome to the forum.

I used to be an RAC Engineer in the 1970's dealing with problems exactly as you describe.

The fact that you had the head skimmed and a new head gasket caused the fault to lessen suggests to me that the head gasket was certainly a major fault but has been only a partial cure.

Now, something that car mechanics often fail to notice or check is the state of the engine block surface. Yours is a diesel engine with a fairly high compression ratio, probably 24:1, and as such there have to have been enormous pressures inside the combustion chamber over time which caused the head gasket to start to leak in the first place.

Now, this is not all that happens!

These extreme compression pressures exert an upward 'push' on the cylinder head when the engine is running and this in turn, over time, causes the cylinder head studs or head bolts to 'pull' the cylinder block up into a cone shape and this occurs around each and every one of the threaded blind holes in the cylinder block into which the studs or bolts engage.

Now, you might be asking what effect this might have??

When the skimmed head and fresh new gasket is returned to the cylinder block, the head is tightened back down correctly to the required torque BUT it is now tightened down onto the 12 or so conical areas around each head bolt or stud and the nett effect is that the rest of the cylinder head area is not truly mating. As I stated in the beginning, the skimming and new gasket has alleviated the major compression pressure leak but not cured it.

On the matter of hairline cracks in the head, liners or block, it is often extremely difficult to notice them 'by eye'. If you can see them by eye, then the leak will be blatant and will need no further discussion.

Any hair line crack causing minimal bubbles will need some form of chemical detector.

I used to use a three aerosol 'can' Crack Detector, once the engine or block was in the workshopon teh bench. The engine was stripped down and cleaned. It was then subjected to the first can which was a sort of trichloroethane 'wash' to remove all the final traces of grease and oil and to deep penetrate any cracks.

Once this first aerosol 'can' had evaporated and it was very quick, a second 'can' was used to spray the surfaces to be inspected. This covered the entire surface with a chalky white powder but it had a secret trick in as much that it penetrated any cracks or flaws in the metal work, carried down on any un-evaporated chemical from the first can.

After 20 minutes or so, this powder was dry and the third 'can' was sprayed all over the surface being inspected. This was a similar chemical to the first can in as much as it 'washed off' the white residue white powder. and after a few moments any remaining chemical from 'can 2' was lifted out of any cracks or stresses in the metal work and showed as a very bright read dye,cochineal red in colour.

Discounting X-ray, the above method is the truly definitive bench method of finding any faults with your engine apart from the stud pulling which can be done with a steel rule or engineering straight edge. Some of the smaller cracks in head and block only open up when engine is warm and under compression pressures and sadly will not be visible to the naked eye on a visual inspection.

I have done a quick search on-line and found the following link:

http://www.ccvalves.com/cpexcat/CRACKDETECTIONREPAIR.pdf

The one I used was similar if not the same as the "KWIK-CHECK Crack Detection" (Dye, Remover and Developer) mentioned in the above link.

I hope this helps?

Just a quick addition regarding the raised conical parts on the block around each stud/bolt hole.

These can be removed using a HSS 45° countersink and a hand held power drill. The cylinder bores and all holes/oilways and water channels will need to be blocked off using copious amounts of kitchen poly-roll (suitable oiled to 'catch' swarf) and the threaded stud holes in the block face can be similarly suitably blocked (to be removed with tweezers after the raised part of the head has been 'countersunk'). Access to an high-pressure airline would help?

It always surprises me that this fault is very often missed by engineering staff at garages. The counter sinking has to be only enough to remove the raised section - and no more!

Hi Lenseman,
Wow! What a helpful reply! Very grateful to you for giving me such detailed information.
I have a mate who is a marine engineer now working for a North Sea Oil related company and he has described this testing method and even showed me the photos!
I will pass this advice onto my engineer and see what he says, but thanks so much again for your interest!
Cheers, John

 

[tamarkin]

Pal has just been through this sort of problem with a BUkh. After 2 head gasket replacements, it turned out that the head had not been properly pressure tested and had a hidden crack between port and waterway.

Look at your symptoms logically. Neither a failed gasket nor a crack need pollute the engine oil - it depends where the gasket fails and where the crack is located. The fact that the gasket didnt appear to have failed ( as inded it didnt with my friend) as well as the fact that a new gasket didnt get rid of the problem, both suggest that the failure wasnt a gasket but is either a crack in the head or a problem elsewhere.

If the problem were uneven lervel of the deck of the block there would be an obvious track. Was there? I doubt it or a competent mechanic would have spotted it.

For gas to get into the cooling system, the gas concerned has to be at a higher pressure than the pressure in the system. That rules out areas like the heat exchanger - there is no pressure there. Ditto the exhaust. In fact the only bit of the engine where the gas pressures are higher than the cooling system is the combustion spaces.

So logically, there is a crack between combustion space and the cooling system.

Hi, Thanks for the reply and your logical approach!
I am no engineer but am picking up on things and take your point. My enginneer did not see any track and he says he can only take the word of the company he got to pressure test the head that they did it properly and found no crack, but what you are saying is that they may have missed one?
You agree with him that it's nothing to do with the heat exchager so that's helpful.
Grateful for your input mate!
Cheers
John

 

[tamarkin]

Is the engine overheating or is it blowing it's coolant out?
If not then it would be tempting to leave it be and see if the problem gets worse, if it does then it may be easier to find. If the engine is working normally maybe don't worry too much at the moment unless it gets worse as maybe it is a characteristic. Is the pressure cap ok and the correct one? if the pressure is a bit low you may get some local boiling causing the bubbles.

Hi, Thanks for your input.
Not sure what a pressure cap is but will run this past my engineer!
Last owner ran it for years like this!

Cheers, John

 

[tamarkin]

Thanks!

 

[tamarkin]

I agree with Lenseman. The head is only one half of the mating pair. Very often the tracking is obvious on the head but not the block. Whenever I have had a head skimmed I have "blued" it and moved it over the mounting face with the bolts or studs removed of course. Often I have found a depression burnt in the block or as he suggests, high spots around the studs. Rather than countersink I would use a proper counterboring bit and make a 1/16" recess around each bolt but countersinking will do if you don't have one. Sometimes just going over the surface carefully with a fine flat file will remove bumps but if you have a burnt out track between the water galleries and the cylinder you will need that skimmed too.... Sorry but Lenseman is correct that often mechanics will skim a head and bolt it back without further investigation. Thats only half of a proper engineering job I am afraid. I would lay good money on that being your problem. Sorry!

Thanks for your input...you may well be right!

Cheers, John

 

[EuanMcKenzie]

re MD 21A

I've got one too in another HR Rasmus but it runs fine and I don't get gas through the header tank at all. Its all very tame.

I have an MG also thats just had a head gasket done and I've had old saabs, a dolomite sprint and seem drawn to engines that blow head gaskets.

If your oil isn't getting milky you havn't got water leaking down the liner.

it may be that yours has a crack or it could be warped and it isn't level on the block.

What may be useful to know is that the block is based on the Peugeot 404 / 504 diesel engine otherwise known as the "indenor" engine.

Vetus used the same block for their engines for a while.

You may get another block from a scappy. I also know of someone who has just pulled a MD21A out of another boat and may still have the old engine / a head available.

I would live with it for while if you can, gather the spares in the meantime and see where it gets you

Good luck. PM me if you want the contact for the scrap engine

Euan

 

[rutten]

mysterious air bubbling

Pal has just been through this sort of problem with a BUkh. After 2 head gasket replacements, it turned out that the head had not been properly pressure tested and had a hidden crack between port and waterway.

Look at your symptoms logically. Neither a failed gasket nor a crack need pollute the engine oil - it depends where the gasket fails and where the crack is located. The fact that the gasket didnt appear to have failed ( as inded it didnt with my friend) as well as the fact that a new gasket didnt get rid of the problem, both suggest that the failure wasnt a gasket but is either a crack in the head or a problem elsewhere.

If the problem were uneven lervel of the deck of the block there would be an obvious track. Was there? I doubt it or a competent mechanic would have spotted it.

For gas to get into the cooling system, the gas concerned has to be at a higher pressure than the pressure in the system. That rules out areas like the heat exchanger - there is no pressure there. Ditto the exhaust. In fact the only bit of the engine where the gas pressures are higher than the cooling system is the combustion spaces.

So logically, there is a crack between combustion space and the cooling system.

If there is no pressure in the heat exchanger how does the engine push out raw water?
My Peugeot Indenor XD88 - not that far away from your Volvo - had very much the same problem: overheating, loosing cooling fluid and lots of air bubbles in the heat exchanger. Mechanics concluded very quickly that the cause must be a leaking head gasket, but this, and the head, showed no failure. Turned out to be - by testing with compressed air - a leak in the aluminum heat exchanger between a water and an exhaust gas pipe, caused by corrosion. Cooling fluid was pressed out -I had noticed the exhaust water containing soapy bubbles but ignored it. I was lucky to find a replacement heat exchanger, but could have saved the cost of repairing a not-broken head gasket.

 

[EuanMcKenzie]

re heat exchanger

You have made a fair point but I'not sure which side of the heat exchanger runs at the higher pressure

I'm guess that its the raw side though as the Raw water pump is a positive displacement pump whereas the engine re-circ pump is a centrifugal type.

The heat exchanger on the MD21A is on the front prior to the water being fed into the exhaust manifold.

The exhaust gas is injected into the raw water flow and goes out the back with it and takes the route of least resistance. It is unlikely it could build sufficient pressure against the raw water flow to get back to the heat exchanger and through a hole to the fresh water system.

For there to be lots of bubbles coming through a hole in the system I reckon it has to come from the engine. I suspect its not a dislodged liner if your oil is still clean, and you may now have a sound head gasket, but it may be a crack or pinhole in the head or exhaust manifold that allows gas into the water system.

The VP workshop manual shows the two cooling circuits quite clearly. Do you have a copy and /or do you want one, I can copy it for you if you want.

Euan