Exhaust Smell of Burnt Rubber: Volvo MD22

[Cromarty]

For some time now, I have been smelling burnt rubber at the exhaust of my Volvo MD22 (on a Cromarty motorsailer) whenever I occasionally rev the engine up to +3000 rpm. Impeller is new, exhaust-collector too, all rubber hoses renewed this winter. Waterflow out of the exhaust looks normal. Together with the burnt rubber smell, I get some light grey smoke. Any suggestions how I can remedy this? Thanks!

[psneeld]

Originally Posted by Cromarty

For some time now, I have been smelling burnt rubber at the exhaust of my Volvo MD22 (on a Cromarty motorsailer) whenever I occasionally rev the engine up to +3000 rpm. Impeller is new, exhaust-collector too, all rubber hoses renewed this winter. Waterflow out of the exhaust looks normal. Together with the burnt rubber smell, I get some light grey smoke. Any suggestions how I can remedy this? Thanks!

The holes where the water exits the water manifold injection can get blocked and if there is not a good mix of exhaust and water to properly cool the rubber exhaust hose on top...it burns..same amount of water...just not evenly distributed in exhaust tubes/hoses.

My tow vessel burns through the rubber exhaust hose 4-5 times a season some years due to all the crap the engine sucks up rescuing boaters off back bay mud flats.

[Cromarty]

Thanks for the quick reaction. I'm not sure about the correct English terms, but do you mean by 'water manifold injection' part number 7 in the attached drawing? I renewed this piece completely this winter, so the wholes in there should be wide open.
I have never had the rubber hose burn through and the smell only occurs when revving over 3000 rpm (which I only do rarely, for manoeuvring) , but I'm a little worried that the hose will burn inside and cause a fire. Is that possible?

[psneeld]

yes to part no. 7...those holes could be quickly blocked of it could have been part of the manufacturing process.

at high rpms the temps soar and are directed at the rubber because of the downturn of the exhaust...usually not a problem at lower rpms....must have lot's of water mixed properly...not just flowing through the lower holes.

[Cromarty]

OK, I'll take the piece off and check the wholes. In case they appear to be open, is there another way to get a more even distribution of water in the hose? Or perhaps should I use a silicon heat-resistant hose instead of a rubber one?

[psneeld]

Originally Posted by Cromarty

OK, I'll take the piece off and check the wholes. In case they appear to be open, is there another way to get a more even distribution of water in the hose? Or perhaps should I use a silicon heat-resistant hose instead of a rubber one?

I've burned through that also with a gasser...diesel could be worse...we have tried welding up the lower holes to get better water distribution with limited results...but the right amount of water, evenly distributed is all you are looking for...whatever it takes. If you need to...a secondary water injection point may be necessary.

[four winds]

I think instead of the area labeled 7 (at the mixing elbow) a look at the heat exchanger tubes above that part could be in order.

That's the seawater path indicated by the darker grey color through the heat exchanger proper. Those tubes are a "bundle" of small diameter tubes through which seawater flows while fresh water surrounds them (area labeled 3) to transfer the heat to the seawater.

A partial blockage of some of those tubes would decrease the efficiency of the exchange which could reveal itself only at higher rpms. Until of course they become totally blocked and cooling stops completely.

Your statement that water flow looks normal contradicts my assessment, possibly.

Could it be you have serviced the mixing elbow but have not inspected or cleaned the bundle of small tubes in the exchanger?

My comments are based on my understanding of the fresh water Yanmar in by boat, might not apply your engine unless the diagram indicates the setup on your Volvo accurately.

[psneeld]

Originally Posted by four winds

I think instead of the part number 7 (at the mixing elbow) a look at the heat exchanger tubes above that part could be in order.

That's the seawater path indicated by the darker grey color through the heat exchanger proper. Those tubes are a "bundle" of small diameter tubes through which seawater flows while fresh water surrounds them (area labeled 3) to transfer the heat to the seawater.

A partial blockage of some of those tubes would decrease the deficiency of the exchange which could reveal itself only at higher rpms. Until of course they become totally blocked and cooling stops completely.

Could it be you have serviced the mixing elbow but have not inspected or cleaned the bundle of small tubes in the exchanger?

Would be a good point except that the water flow out the exhaust looks normal...the manifold is not a bad thing to check...but if the water flow out is normal and the engine temp is OK...and it's just the burning smell...than it's not the exchanger...its the injection water not doing it's job.

[four winds]

Yes, I did an edit that pointed out that contradiction as you were responding.

However if the exhaust is hot enough to burn the hose it seems logical to suggest the necessary quantity of seawater is not present to keeps things cool in the elbow.

Could be obstructed at the connection to the elbow or at the exchange tubes as well I would think. Might even be a partial kink in the hose to the elbow from the exchanger.

I would question the observation that water flow out the stern is normal. And ask the OP to note how much the flow increases at 3000 rpm compared to lesser revs. Mine has a very substantial increase at wide open revs.

I can always place my hand on the elbow at any time while running so I think I'm good to go. No gauge, just an idiot light, going to add one sometime.

[psneeld]

Originally Posted by four winds

Yes, I did an edit that pointed out that contradiction as you were responding.

However if the exhaust is hot enough to burn the hose it seems logical to suggest the necessary quantity of seawater is not present to keeps things cool in the elbow.

Could be obstructed at the connection to the elbow or at the exchange tubes as well I would think. Might even be a partial kink in the hose to the elbow from the exchanger.

I would question the observation that water flow out the stern is normal. And ask the OP to note how much the flow increases at 3000 rpm compared to lesser revs. Mine has a very substantial increase at wide open revs.

I can always place my hand on the elbow at any time while running so I think I'm good to go. No gauge, just an idiot light, going to add one sometime.

All possibly true...but so is the improper mixing from the injection elbow with nothing else wrong...most people look at the exchanger but fail to notice the mixing as a problem...there COULD be lots wrong...but I only pointed out the less obvious with the assumption the OP knew somewhat about the basics and was still puzzled.

[four winds]

Acknowledged.

I'm learning here, and considered other things to be less obvious than you.

Considering new hoses have recently been installed, I think the possibility a barely noticeable kink in the hose feeding the elbow is a likely suspect. All it would take is the hose being slightly too long yet installed anyway.

Did it do this last season, before the winter servicing? If so, then it's not this new hose.

[Cromarty]

Thanks for the suggestions, guys.
I did have the exchanger (well, the inner part with the copper tubes) serviced this winter, and the pipes are all wide open. So that's certainly not the culprit. The tubes are all new, no kinks or dents, and they're not longer than before. The smell was there before the new hoses, anyway.

That's the strange thing: the engine ran 18 years before I started having this burnt smell. Now I have it every time I push it over 3000 rpm.

The water flow at the stern (well, actually on the side in my boat :-)) is considerably higher at high revs. New impeller, too. So I'm quite puzzled here.
I'll try to get to the boat this week to try psneeld's suggestion. Other ideas welcome, of course!

[four winds]

Certainly on the face of it everything seems normal from my limited experience.

I may be learning something about "mixing" now. I would have thought if the flow increases substantially at higher revs then everything is good. Now I suspect that the mixing implies the water and gas become a more homogenous blend than when not mixing well.

So possibly the water and exhaust are passing through to the rubber hose in separate streams? The unmixed stream of exhaust being hotter at high revs and burning the hose?

I was thinking that merely turbulence in the mixing elbow is the mechanism at work here. Is there something more to this, psneeld?

Forgive me for continuing to butt into the discussion, just trying to learn.

[Cromarty]

Originally Posted by four winds

...just trying to learn.

Same here. I've already learnt a lot since I started the thread!

[Artif]

Being as you have checked the heat exchanger, you might want to check the hoses etc on the rest of the sea water cooling circuit, especially as this is a recent problem.
You might have a piece of impeller blade stuck in the pipe work or a collapsed hose somewhere or a corrosion blockage, which is reducing the flow slightly, hence only having the problem at high revs.