Mixer elbows and heat exchangersSep 27, 2012
Tucked away out of sight behind your inboard engine lies the exhaust mixer elbow. Its function is to cool the engine exhaust gases by mixing them with the cooling water discharged from the engine heat exchanger. It requires no regular maintenance, but neither should it be completely forgotten. Over time, the exhaust mixer elbow can become restricted with accumulated soot from the engine exhaust, resulting in increased exhaust gas back pressure and loss of engine power. Also, the combination of exhaust gas products and seawater can corrode the relatively thin metal of the exhaust mixer elbow, allowing exhaust gases and cooling water to escape into your vessel.
Top: rust has formed on the corroded and leaking exhaust mixer elbow. Bottom: the interior of the exhaust elbow coated with soot.
We replaced the engine in Cormorant, our Corbin 39 cutter in 1997, and by late 1999 the stainless steel exhaust mixer elbow on our new Yanmar engine had developed pinhole leaks around the weld at the mounting flange. The telltale spots of rust around the weld signaled trouble, and I was fortunate to discover it before any external damage occurred. We were preparing to depart Trinidad on our way to the Panama Canal so had no time to order a replacement elbow. An effective field repair was the only solution. First, I closed the seawater valve, and then I pried off the exhaust hose and water hose, removed the four bolts from the flange, and then removed the exhaust mixer elbow to a convenient work area. First, I used a hacksaw blade to chip the accumulated exhaust soot out of the inside of the exhaust mixer elbow, and the inside and outside of the corroded area was given a thorough polishing with a small wire wheel. A Dremel tool makes this almost joyful!
After wiping the polished area with acetone, about a tablespoon of J-B Weld was mixed up and applied around the neck of the exhaust mixer elbow, covering the pinhole area. J-B Weld takes a while to set up during which it will flow out of position. (Thoroughly mix up the J-B Weld and let it sit for about 15 minutes before applying it to the exhaust mixer elbow.) By carefully and slowly rotating the exhaust mixer elbow, I was able to keep the J-B Weld in position until it hardened sufficiently to stay in place. The result appeared to be nearly perfect.
After the damaged area has been thoroughly polished, Harry Hungate applies J-B Weld repair material.
A year later when we arrived in Opua, New Zealand, I purchased a replacement exhaust mixer elbow, and took the original one to a welding shop for a more permanent repair. In retrospect, I needn’t have bothered, as the J-B Weld repair was more than satisfactory. In fact, the welder complained bitterly about the hard time that he had in cleaning off the J-B Weld so he could get a clean surface for a weld.
Every two years I treat the engine cooling and exhaust system to a thorough service. I remove the exhaust mixer elbow, inspect it, clean out the accumulated soot, and reinstall it.
The very expensive metal gasket can be reused several times, as long as the flange bolts are not over-tightened. A thin coating of high temperature silicone gasket cement on both sides of the gasket will help ensure a leak-free joint.
While servicing the exhaust mixer elbow, I also remove and clean the heat exchanger tube bundle. On this Yanmar engine, I simply remove the coolant hose from the oil cooler and leave the two hoses connected to the heat exchanger flange, which can then be easily moved aside to gain access to the tube bundle. Use a rubber mallet to tap the side of the heat exchanger flanges to loosen them. On the other end, loosen the alternator belt adjustment arm and swing it up to gain clear access to the front end of the heat exchanger. Cover the alternator with a plastic bag or towel to prevent any salt water from dripping onto the alternator. Gently tap the tube bundle with a rubber mallet to remove it from the heat exchanger. Clean out the tubes with a .22 caliber rifle cleaning brush and hot soapy water. This is the seawater side and likely to be fouled with whatever comes in with the seawater.
Top: removing the heat exchanger flange. Bottom: prying off the aluminum sheath on the heat exchanger core.
Inspect the outside of the tubes for evidence of fouling. If your old coolant is rusty colored, then the outside of the tubes is likely to be coated with rust. Cleaning the outside of the heat exchanger tubes is impossible without removing the aluminum sheath that surrounds the tubes. It isn’t designed to be removed, however, so some delicate surgery is required. Again, the Dremel tool comes to the rescue: a cutoff wheel and a lot of patience can create a slit from end-to-end of the aluminum sheath. Small wood wedges and a couple of large, straight-blade screwdrivers will provide enough leverage to spread the slit wide enough to allow the sheath to be slipped off of the heat exchanger tube bundle. Use a stiff toothbrush and toothpicks and soapy water to scrub the outside of the tubes until clean. Severe fouling will require the services of a steam cleaner or pressure washer. Replace the aluminum sheath after cleaning the tubes. The O-rings on either end of the tube bundle can be re-used a couple of times, but always keep a spare pair aboard (70 mm x 3 mm). Wipe out the inside of the heat exchanger housing with a wet soapy cloth to remove any rust before reinstalling the tube bundle.
Cleaning heat exchanger tubes with a rifle bore brush.
Carefully inspect the rubber coolant hoses for signs of failure such as swelling or cracking. A bad hose will feel very soft compared to a good one. Pay special attention to the hose on the outlet of the thermostat. This hose handles the hottest water and is therefore most likely to fail before the others.
The cooling system is then treated with an automotive cooling system cleaning solution and flushed several times with clean water. If your engine coolant is routed through your water heater, be sure to flush it, too. To complete the job, the cooling system is filled with new anti-freeze/coolant. Don’t forget to clean and refill the coolant overflow reservoir. Run the engine until it is up to operating temperature and then shut it down. After it has cooled, check the coolant level in the reservoir and top off as necessary. Finally, inspect the coolant fill cap for corrosion and for deterioration of the gasket. Clean or replace as necessary.
Harry Hungate and his wife Jane have been cruising since 1997 aboard their Corbin 39 cutter, Cormorant. They completed a west-about circumnavigation of the world in December 2011, and now plan to cruise the U.S. East Coast for the next couple of years. Follow their adventures on http://harryjane.weebly.com/blog.html.