Hopefully you never find yourself in a position where you need to replace your heat exchanger (like I did!). But if you do, firstly, I’m sorry! And secondly, hopefully this article will help ease your pain.
On this page, “front” refers to the end of the engine with the crankshaft pulley and belt, while “rear” or “back” refers to the end with the exhaust elbow and transmission.
Critical note before you begin! The heat exchanger rests on two studs which screw into the engine block. If you want to preserve these studs, you’ll need at least 4 inches of clearance on the right (when facing the front) side of the engine to slide the heat exchanger off the studs. Otherwise, you’ll need to remove the studs, which may damage or destroy them. I did not have enough clearance, and had to use vise grips to remove the studs, which gouged the studs and removed a bunch of their delicate plating. So I had to replace the two studs as well when replacing the heat exchanger.
DISCLAIMER. I am not a professional mechanic. This is just a record of what I did. It could be entirely wrong. You work on your engine at your own risk. If in any doubt, engage a professional.
1. Close raw water seacock and disconnect or switch off engine/starter battery.
2. Drain both cooling sea water and fresh water from engine.
3. Remove alternator.
4. Remove exhaust elbow.
5. Remove two sea water hoses from rear heat exchanger end cap.
6. Remove the overflow hose from the coolant cap.
7. Remove thermostat cap (2x M8 bolts, 12 mm hex head). This aids in the removal of the upper fresh water hose (below).
8. Remove the two sensor leads from the thermostat housing. The one on the right is a Phillips head screw, the other is a nut with an 8 mm hex head. This is necessary so that removal of the heat exchanger isn’t hampered by these wires.
9. Remove upper fresh water hose from front of heat exchanger (5/16″ hex nut on hose clamp).
10. Remove bolt securing cooling fresh water pipe (1x M8 x 55 mm bolt, 12 mm hex head). This aids in the removal of the lower fresh water hose (next step).
11. Remove lower cooling fresh water hose from front of heat exchanger (5/16″ hex nut on hose clamp). Note, if you didn’t fully drain the heat exchanger in Step 2, coolant will gush out when this hose is removed.
12. Remove nut securing impeller pipe to bottom of heat exchanger (1x nut, 12 mm hex head).
13. Loosen three upper bolts on side of heat exchanger (3x M8 x 160 mm bolts, 12 mm hex head).
14. Remove two lower nuts on side of heat exchanger (2x nuts, 12 mm hex head).
15. Remove lower bolt on side of heat exchanger (1x M8 x 100 mm bolt, 12 mm hex head).
16. Remove three upper bolts on side of heat exchanger (3x M8 x 160 mm bolts, 12 mm hex head), taking care to not misplace the washers.
17. While prying down on the raw water pipe below the heat exchanger—to get the pipe clamp clear of the stud there—slide the heat exchanger over, taking care to not lose the spacer that sits between the heat exchanger body and the pipe clamp.
18. If you have a minimum of 4 inches clearance to the right, you’ll be able to slide the heat exchanger right off. Otherwise, you’ll need to unscrew the two studs (see the caution at the beginning of the article if you do this). If you unscrew the studs, pay attention to the thin metal cylinder head gasket, which may drop off once the studs are out. Note that the end with fewer threads is the end that goes into the engine block.
To install a new heat exchanger, see Yanmar 3JH2E Heat Exchanger Installation, Step-by-Step with Photos.
Good evening Chuck… I have to ask. What do you think caused your heat exchanger to “go bad” in the first place
Cheers, DK
I’m no expert in these things so I’m not positive. But the o-ring on that side was not in the best shape. Perhaps the o-ring allowed some minute amount of sea water through, which, over time, slowly ate away at the metal on both the housing and the cover plate. I don’t know if the o-ring was like that when it was installed, or if it deteriorated over time. If the latter, I don’t know what would cause the rubber to degrade. I’ve since learned that the heat exchanger should be opened and inspected (and o-rings replaced, I figure) every three years, which seems like a reasonable interval.
Replacing the entire heat exchanger might seem extreme. One of the bolts holding the cover plate had corroded so badly that the head snapped off when I tried to remove it. I was able to drill it out and tap a new hole. The surrounding surface on the housing, though, was badly corroded and rough. It wasn’t clear it could maintain a water-tight seal any longer with that texture. I did consider the possibility of silicone sealant. But the most prudent course of action seemed to be outright replacement. I try to lean towards prudent, even when it’s painful.
Whether or not replacing the heat exchanger on a 27 year old engine makes economic sense versus replacing the engine altogether, I won’t know for a while.
Lesson learned… open and inspect that heat exchanger on schedule!!