Diesel Heat Exchanger Service: Zincs, End Caps, Tube Cleaning and Pressure Testing

Before you take anything apart, with the engine warmed up at cruising rpm, use an infrared thermometer on the hoses where coolant enters and leaves the heat exchanger. A mechanic quoted by PassageMaker expects roughly a 25°F drop across a healthy exchanger and says only about 10°F means it is not moving enough heat (or raw-water flow is low). Write your numbers down; they are useful for comparison after cleaning. This is a rule of thumb, not a factory spec.

Shut down and let the engine cool completely. Turn off the battery switch. Close the raw-water seacock. Put pads and a bucket under the exchanger.

Find the zinc plug or plugs on the exchanger (and on other coolers, if fitted). Unscrew each one. BoatUS says if the zinc is only lightly corroded you can scrape it bright and reuse it, but if it is heavily eaten away or gone, replace it, and check at least once a year. Cruising World suggests checking monthly at first to learn how fast your zincs wear in your water, then setting your interval. Never paint zincs or plugs.

If the plug comes out with little or no zinc left on it, the rest broke off inside. Plan to open the end cap and fish out the pieces.

Remove the end cap on the raw-water inlet side first (where the hose from the raw-water pump connects), then the other end if your exchanger has two removable caps. Expect some seawater to drain. Note how each cap and gasket sits so it goes back the same way. Westerbeke points out the end caps are made to come off for exactly this cleaning.

Remove everything you find: zinc chunks, eelgrass, shell, scale flakes and rubber. Look closely at the rubber. If you find impeller pieces, inspect the raw-water pump impeller (see the diesel impeller guide), even if the pieces look old. Count the missing blades and keep looking until you have found all of them, including in the hose between the pump and the exchanger.

Shine a bright light through from one end and look from the other. You should see light through every tube. Look for:

• blocked tubes (no light)
• white or brown scale lining the tubes
• green corrosion, pitting or thin spots on the tube ends and the plate they pass through
• cracks, erosion or heavy corrosion inside the end caps themselves

Try a strong shot of water from a garden hose first. If a tube is still blocked, push a soft copper wire or wooden dowel through it gently. Cruising World stresses caution: never use steel wire or anything that could scratch or puncture the thin tubes, and never force anything. A punctured tube mixes coolant and seawater and can ruin the exchanger.

If the tubes are lined with scale, rodding will not fix it. You have two options:

• Chemical descaling on the boat: circulate a marine descaler through the raw-water side following the product's and your engine maker's instructions (see the cooling system descaling guide).
• Off-engine service: remove the exchanger and take it to a radiator or heat-exchanger shop for cleaning and pressure testing, which mechanics quoted by Power & Motoryacht and Yachting recommend on a regular schedule.

Pulling the tube bundle out of the shell, cleaning the coolant side and pressure-testing the core is usually a shop job. Have it done:

• on your maker's or mechanic's schedule (often every two to three years in salt water)
• if temperatures stay high after you have cleaned the end chambers and checked the impeller and strainer
• if the coolant level drops with no outside leak, or the coolant looks cloudy or contaminated

PassageMaker's mechanic says small leaks can often be soldered, but about one in ten coolers he sends out fails testing, especially when service has been put off. A shop pressure test is the only reliable way to know a core is sound.

Clean the mating surfaces. Fit new gaskets or O-rings, line up the caps the way they came off, and tighten the bolts evenly a little at a time. Cruising World warns not to overtighten: many end caps are cone-shaped and crack if the bolts are pulled too tight. Use the torque in your manual if it lists one. Reinstall the zincs, using thread sealant only if your manual says to.

If you lost coolant, top up with the type in your manual and bleed air from the system the way your manual describes (see the closed cooling coolant guide). Air pockets can cause overheating.

Open the raw-water seacock. Start the engine and confirm cooling water comes out with the exhaust right away. Check every end cap, zinc plug and hose for drips. Run it up to temperature and repeat your infrared readings from step 1. Check the coolant level again after it cools.

Record the date, engine hours, zinc condition and what you removed from the exchanger in BoatMend. Set your next zinc check and the next off-engine service date.