MD22 smells of burnt rubber when running over 3000 rpm

I have an MD22 in my Cromarty 36 motorsailer. It ran perfectly for 20 years, until recently I started smelling burnt rubber whenever I rev the engine over 3000 rpm. The smell comes out of the exhaust pipe. This winter, I have put a new water manifold (not sure if this is the correct term in English, but it's part nr. '7' on the drawing) before the exhaust, I have changed the impeller and I've renewed the distribution belt.

I presume the smell comes from the rubber exhaust hose heating up at high revs. But this was not the case before. What can be the cause? And is there a risk of fire?

I would remove the rubber exhaust hose and take a look at the inside to see if it is chard. If it is I would say you are having a water flow problem.

There is nothing good about the smell of burning rubber on your boat!

I've had the rubber smell intermittently last season (when revving up to stop the boat off, for instance), so I removed the old hose before winter and I noticed it was charred, but not heavily. I replaced it anyway with a new one. I also, at the same time, replaced the elbow where the waterjets mix the water with the exhaust fumes as the old one was quite clogged with oxidised material. So both the hose and the elbow are brand new, but the smell is still there when running over +3000 rpm.

It is extra puzzling that I ran the engine for 18 years without this problem, and it came up at once. As I suspected a bad water-exhaust mixture to be the problem, I changed the mixing elbow, but that hasn't helped at all. As I already stated, I also had the heat exchanger serviced and all it's pipes are clean now. Engine is never running hot either, not even at max rpm. And water flow from the external exhaust looks pretty normal. What else can I do?

Sure you have not changed diesel quality?
Old diesel mixed with bio-fuel smells rather 'harsh' when burned!!!
Compared to old 'pure' diesel that has a 'pot-life' of more than a year, the new bio mixed diesel is max 6 months.

Well, I must admit that the smell came up some two seasons ago, but I didn't make much of it at that time, because it only occurred when I was manoeuvring and had to rev up to over 3000 rpm to stop the boat off.

But after the rather intensive rehaul that I carried out this winter, I took it for a ride at different regimes, and when I drove it some 3 to 4 minutes above 3000 rpm, the burnt smell really was overwhelming. But perhaps you have a point: it was around two years ago that I noticed a completely different colour of the diesel I tanked. But could it be that bad? It really smells burnt rubber.

If I don't find anything wrong with the water flow, It might be the diesel quality. It's the only thing that has changed, in fact.

Well, I've been to the boat today and did a few more checks. I let the engine run warm to normal operating temperature (85° C). Then revved it up to 3000 rpm for a few minutes so I got the smoke and the smell.

A few remarks:

- My wife, who sees colours better than I do, says it bluish grey smoke.
- the exhaust water (outside the boat ) is very cool and abundant
- the elbow stays cool to the touch, approx. 25°C
- the rubber hose stays cool to the touch, approx. 20°C
- engine stays at normal operating temperature
- no leaks anywhere
- some traces of diesel oil (or oil?) in the dock

My temporary conclusion: the rubber smell is certainly not the exhaust hose. Everything in the exhaust line is nice and cool. Where is that smell coming form. The mystery stays...

Would adding some additive to clean the injectors help?

Where is the smell the strongest?
(inside the engine room or from the exhaust)You may try 'injector cleaner for diesel engines, but might be better to have the injectors checked by a diesel shop.

There is no smell at all in the engine room, only outside, coming out of the exhaust tube.

Then you may 'relax'.
Just have your injectors serviced, check engine/ignition and timing be sure you have clean fuel.

Guess I'll do that. Thanks for the input guys!