Carbon Fouling

[Dave Pawloski]

I own a 1985 Hunter 31 with a Yanmar 2GM20 diesel.
Due to family circumstances, I have only used the boat a couple of times each year for the past 3 seasons.
For each of these three seasons (but NEVER within the previous 20+ seasons that I have owned the boat).... I have been experiencing problems with carbon buildup clogging the exhaust elbow causing the engine to overheat and the temperature alarm to go off.
The boatyard that does my repairs suggested running the engine at higher rpms (2500-3000),
but never in the previous 20+ years had I done that, and all worked fine.

Has anyone else experienced this problem, and if so, what was the solution ??

 

[RichH]

Several issues that cause the formation of 'coke' in the exhaust system:

1. engine is not run sufficiently so that the combustion chambers are operating at less than peak thermal efficiency. Run the engine LONGER and at at least 75-80% of max. rpm. This 'heat soak' raises the combustion chamber temperature (to normal) and effects better and more efficient combustion .... and will help 'protect' the exhaust manifold and other internal engine surfaces by converting 'red' rust to protective 'black' rust due to the 'high heat'.

2."lugging" of the engine ... engine does not develop full horsepower (max. rpm) while under full load or even 'cruising rpm' load. The governor is throttled for the higher horsepower and delivers the necessary fuel to meet the higher HP/rpm demand but since the engine is not running at that rpm ... a lot of the excess unburnt fuel simply goes out into the exhaust where it 'cokes' upon the hot surfaces.
Cause of lugging: The engine is 'overpropped' and probably should have the propeller pitch reduced or the diameter of the prop reduced.

3. Bad fuel, especially fuel fouled with microorganisms and/or their 'products of metabolism' ... such particles that so develop are soft and deformable and simply 'extrude' right through the typical fuel delivery line filters. These particles contain high amounts of asphaltic compounds that dont ignite/burn very well ... and simply deposit themselves in the exhaust system.
If your fuel is not 'crystal clear' or has a 'haze', its most probably severely infected by fungals and bacteria .... keep only the amount of fuel onboard that you 'need' plus some reserve, dont always top off the tank, drain the tank for long term haul-outs, buy your fuel only from high-turnover sources such as fuel depots or truck stops.

;-)

 

[Benny17441]

These diesel engines are designed to operate at high RPM for prolonged periods of time. The temperature inside the combustion chamber will rise with RPM and will burn fuel more efficiently with less carbon buildup. These engines do not like to idle at low RPM for long periods of time. I like my idle speed at around 900 RPM and will not allow for it to idle long. Don't believe in a warm up period before casting the lines but do believe in a cool down period before shutting the engine off after a good run.

 

[Steve Dion]

There has been several articles on this in the past. Running these engines at lower RPM (less than 2500 rpm) causes coking of the elbow & cylinder glazing. No need for prolonged warm up. Just run at lower RPM's and gradually increase engine speed when engine is at operating temp.

I also run mine at 3200-3400 for 10-15 minutes when we are on extended trips. Our normal RPM is usually about 2800-2900.