Smell of Diesel fumes in Engine Compartment

[casey1999]

When running my Yanmar 3GMD, I smell some diesel fumes in the engine compartment. There does not appear to be any exhaust leaks on the engine or in the exhaust hose discharge. When the boat was surveyed, it had the same smell but survey did not measure high levels of CO. The exhaust does not leak cooling water (which you would think it would if it was leaking). Is this smell normal or do I have an exhaust leak? Could the fumes be coming out the engine crank case breather?
Regards

[RichH]

Could simply be fumes from the crankcase breather and the engine has begun to or has developed an excessive amount of 'blow-by' past the piston rings which then vents into the crankcase.

... or it could be that the flexible rubber fuel line connection hose to the engine lift has become permeated with fuel oil. These flex fuel lines contain 'fabric duck' material and do have a finite service life. Assay: look carefully at the flex line for 'leakage', if not visible and you still suspect the flex line take a wad of paper towels and cover the flex line covered with 'saran' and leave in place for a few days or weeks --- remove and sniff; if 'that odor' is noticeable in the toweling then suggest you consider to replace the hosing.

[erps]

Does it smell like diesel fuel or diesel exhaust? An exhaust leak should show itself with some black soot near the source of the leak.

A small amount of leaking fuel on a hot engine will put out a very strong odor and if the motor is warm enough and the leak small enough, it may be hard to locate, as the fuel may boil off.

[FishFinder]

Every time I have smelled diesel in my engine compartment, there has been a fuel leak somewhere. Check around all the fuel line connections, fuel pump, around injectors, etc.

[HPLou]

Diesel fumes could be from a fuel leak in lines, fittings and/or filters or maybe spillage from changing the filters. Diesel exhaust fumes could be from a leaky exhaust or exhaust escaping past the rings and into the crankcase.

[casey1999]

I think what I smell is exhaust fumes. I do not see any carbon deposits that might indicate and exhaust leak. I also do not get any oil or diesel fuel drips under the engine (although as posted- the diesel could be boiling off). I will do some more investigation and post if I find anything. I might have a leak on the exhaust hose where it connects to the water lock, but if it were a leak, I would think I would see a water leak as the cooling water has mixed with exhaust at this point. Note the engine seems to have good compression and does not burn oil- this would seem to indicate there may not be excessive blow by. There is a crankcase breather filter on this engine- I need to check it and clean it- not sure if this would cause fumes to escape if it were clogged.
Regards

[fryewe]

Aluminum fuel tanks are notorious for corroding and springing leaks that can't be seen. Hope it's not that. Have you had ANY evidence of fuel in your bilge?

Your crankcase breather should be piped back to the air inlet to the engine somewhere. Don't know why it needs a filter. Does the tech manual show one, or has it been installed aftermarket?

[casey1999]

Originally Posted by fryewe

Aluminum fuel tanks are notorious for corroding and springing leaks that can't be seen. Hope it's not that. Have you had ANY evidence of fuel in your bilge?

Your crankcase breather should be piped back to the air inlet to the engine somewhere. Don't know why it needs a filter. Does the tech manual show one, or has it been installed aftermarket?

I have a newer Moeller plastic fuel tank that is in good shape. No fuel in bilge or anywhere else in boat and all engine connections look good. I only seem to get the exhaust smell when engine is running hard (not at idel) You are correct, I took breather apart, all looks ok and no filter. The manual show the Yanmar 3HM did have a screen at the breather connection.

Inspection of the exhaust hose with engine running does not show any sign of leaks (exhaust or water), although the hose are bulging and looks like wire wind inside hose is cracking. I will replace all exhaust hose in next few weeks. I would think if exhasut gas were leaking from the hose that the cooling water would also leak out- but maybe not if the leak is not direct enough.

[RichH]

Originally Posted by casey1999

I only seem to get the exhaust smell when engine is running hard (not at idel) You are correct, I took breather apart, all looks ok and no filter.

Most internal combustion engines are very inefficient for burning ALL the fuel. The boat when in motion has a 'dead spot' behind it where the wind is virtually motionless ... thats where the exhaust is emitting into.

The 'cloud' of burned exhaust 'follows along' the back of the boat as the boat moves forward ... if you have no leaks, you are probably smelling the 'follow along unburned fuel in the 'exhaust cloud' following the boat. This 'follow along' of exhaust gases effect is enhanced by large dodgers, 'granny enclosures', high sterns, etc.

[casey1999]

Originally Posted by RichH

Most internal combustion engines are very inefficient for burning ALL the fuel. The boat when in motion has a 'dead spot' behind it where the wind is virtually motionless ... thats where the exhaust is emitting into.

The 'cloud' of burned exhaust 'follows along' the back of the boat as the boat moves forward ... if you have no leaks, you are probably smelling the 'follow along unburned fuel in the 'exhaust cloud' following the boat. This 'follow along' of exhaust gases effect is enhanced by large dodgers, 'granny enclosures', high sterns, etc.

I will check this, my stern is low and only have a small dodger. I get the smell when I am headed into the wind, but it could still be coming from the stern exhaust in this case as you describe (maybe a back eddy draft returning the exhaust). I will change the exhaust hoses and see if that helps, and also lower dodger to see effect.
Regards