Extreme overheating problem -- Penta MD5A

[Ship's Cat 73]

While the engine runs fine for about 20 minutes, after that, steam begins to emit from the exhaust and the waterlock is too hot to touch . . . and I really mean TOO HOT. At that point the engine has to be shut down before it shuts itself down. With the engine running water only ever comes out of the exhaust in very weak spurts every 12 - 15 seconds and sometimes longer periods. (Other yachts I see are like a fireman's hose by comparison!)
Have already done all the usual checks - umpteen times.
Today I removed the 'final' pipe which delivers the 'spent' cooling water to the exhaust elbow.
The attached pic shows the port on the elbow and the 'hole' is around 12mm.
However, thinking it might be 'crudded up' I tried poking a small screwdriver in the hole, then tried poking a 2.5mm sheathed electrical wire -- none of these went in.
I stripped he sheathing off and twisted the remaining wires into a 1mm spike and only then was I able to penetrate the minuscule hole.
I can't believe that size of hole is seriously meant to cope with the amount of cooling water. When I see the size of that hole I can now begin to understand the very weak delivery of water out of the boat.
I'm tempted to dril a 'sensible sized' hole into the elbow but wouldn't want to go ahead with that until I know it would be safe???

As you look at the pic you won't be able to see the actual hole -- what looks like a 12mm hole is solid metal . . . is it possible that this elbow hasn't been cast properly?

 

[michael_w]

That looks full of crud. A mild acid ought to help.

 

[Stemar]

The crud that builds up in an exhaust elbow it pretty tough stuff. A wire won't touch it. I'd give it a good poke with a screwdriver; if whatever it is isn't supposed to be there, you can't do any harm, and if it is part of the casting, and you go through, it's knackered anyway. The one on my old 2003 was pretty bad, so much so that an overnight soak in brick cleaner didn't clear it, but did weaken it enough for a bit of brute force and ignorance to restore it.

 

[Dellquay13]

I don’t know this particular engine, but is it possible that while this elbow is bunged up, what you see through that port is an internal sleeve to stop cooling water running back into the cylinders? Maybe the blockage is all around the sides of the port, laterally between the outer pipe and the inner sleeve, not directly through the port?
The aftermarket stainless exhaust elbow on my Yanmar has a short internal sleeve to stop water back flow

Very roughly sketched;

 

[pete]

I guess you have checked that you have sufficient water coming from your engine trying to enter the exhaust and it's not your water pump that's the issue?

 

[VicS]

I don’t know this particular engine, but is it possible that while this elbow is bunged up, what you see through that port is an internal sleeve to stop cooling water running back into the cylinders? Maybe the blockage is all around the sides of the port, laterally between the outer pipe and the inner sleeve, not directly through the port?
The aftermarket stainless exhaust elbow on my Yanmar has a short internal sleeve to stop water back flow

I think you are close to the OP's real problem.

I don't believe there should be a hole where he suggests and making one or enlarging the very small hole he has found may be a mistake.

I can only find one picture of the elbow and that is on Keypart's website. I have enlarged it and attached it to this post. It looks like there is a water channel along most of the length of the elbow with the water finally entering the gas stream much further along the elbow. (The parts diagrams also indicate a core plug part way along this channel.)

 

[Dellquay13]

I think you are close to the OP's real problem.

I don't believe there should be a hole where he suggests and making one or enlarging the very small hole he has found may be a mistake.

I can only find one picture of the elbow and that is on Keypart's website. I have enlarged it and attached it to this post. It looks like there is a water channel along most of the length of the elbow with the water finally entering the gas stream much further along the elbow. (The parts diagrams also indicate a core plug part way along this channel.)

View attachment 176940

It’s pretty clear from that image how the cooling water running down that top channel only mixes with the gas below the level of the cylinder exhaust, the only cooling for most of the length is by conduction

 

[Ship's Cat 73]

Thanks for all the comments so far.

'pete' -- the whole system from a new seacock, strainer and pipework all the way to the pump is new. A new impeller also fitted a month ago and the pump functions perfectly.

'VicS' -- I've inserted the best pic I currently have of our elbow. I'm not a metal expert but it looks like cast iron(?) to me. Maybe not iron but it definitely looks cast. > > >

 

[scottie]

2 basis points
1 the overall hight of this engine is kept to a minimum to suit the flat bottom shape of boats of that era introducing water above exhaust port’s would be catastrophic
2 if it’s been as hot as you say the exhaust hose will most likely need replacing before it collapses which in itself could be a reason for lack of water flow

 

[scottie]

Whilst you have the exhaust bend off to inspect the hose you can check it to see where the water flow actually goes
You should have a good flow through the elbow
Now might be time to open a hardship fund to buy another elbow
It is a specific version of the normal bend used on many Volvos of that era

 

[VicS]

Thanks for all the comments so far.

'pete' -- the whole system from a new seacock, strainer and pipework all the way to the pump is new. A new impeller also fitted a month ago and the pump functions perfectly.

'VicS' -- I've inserted the best pic I currently have of our elbow. I'm not a metal expert but it looks like cast iron(?) to me. Maybe not iron but it definitely looks cast. > > >

I reckon your next step is to remove the elbow and examine it. Look to see if Dellquay and I are right about its construction and decide if it is feasible to unblock it. I think you will have to plug the small hole you have found if it goes directly into the gas flow.

Exact replacement is available from Keypart obviously but fabricated alternatives may also be available elsewhere. Expensive though whichever you choose

 

[Thresher]

The crud that builds up in an exhaust elbow it pretty tough stuff. A wire won't touch it. I'd give it a good poke with a screwdriver; if whatever it is isn't supposed to be there, you can't do any harm, and if it is part of the casting, and you go through, it's knackered anyway. The one on my old 2003 was pretty bad, so much so that an overnight soak in brick cleaner didn't clear it, but did weaken it enough for a bit of brute force and ignorance to restore it.

Exactly the same on my old mdb5, the gap went down to the size of a pinhole. Brick cleaner won't clear it but I remember it did something to give temporary relief. Brute force is required, what the OP thinks is solid metal is rock hard crud.

 

[Ship's Cat 73]

Just so that I properly understand what should be going on inside that elbow -- I've drawn on the following pic the route I "think" the cooling water takes after entry into the elbow. Am I correct? >


** That elbow above is different to ours in that ours is fairly flat on top and doesn't have the ridge along the top as shown above.

Also, VicS -- the location of the small hole (pinhole) I found is shown, arrowed yellow on the following pic, and it appears to head off in the direction of the red arrow >