Join Date: Nov 2009
Thanked 0 Times in 0 Posts
Rep Power: 0
2gm20F leak into heat exchanger unlikely
Don, I've had my boat and it's 2gm20f for over 20 years, and once I thought I had the problem you describe. Note that raw water goes through the core tubes, and fresh water circulates around the tubes--inside and outside of the cylinder. I think it unlikely that there is a leak in these components which could result in fresh water loss. The leak would have to be in either a tube itself or one of the two end plates that the tubes go through. As already said, the tubes are solid copper, and they are pretty thick. There could be a leak where a tube goes through one of the plates, or in a plate itself, but you would probably be able to see this.
A simple test of the core seems possible since it is out. Drain and dry the core completely. Add water where the fresh water would go (inside the outer cylinder, not the tubes), tape up the big holes where freshwater enters and exits the cylinder, and let the core stand in various positions for a week or so. If you see any water coming out of a tube, you have a leak. If there is no water coming out of any tube, your problem is elsewhere. (I don't think a pressure test is necessary for the kind of problem you describe).
A thought on what might be your problem: If you had drained, cleaned, and refilled your freshwater system (which is a good idea every couple of years or so), you might not have removed all the air out before you restarted the engine. Air is a particular problem when the fresh water system has a long run to a water tank heat exchanger. The freshwater moves with very little pressure, and there are lots of bends in the whole system, so it can be hard to force all the air out the top. So, after the engine has run a bit the air may only have moved around. You can even have an "airlock" which causes the system to overheat. The reservoir is empty and no water is visible when the cap is opened--it is easy to mistakenly think water has leaked out.
Note that if there is a freshwater leak that is not in the heat exchanger there should be evidence under the engine or elsewhere in the bilge. (If water was leaking inside the engine it probably would have caused a fatal event by now--and you said the oil and exhaust looked free of moisture). So if you don't see anything outside the engine, it's pretty much has got to be an air problem.
A final suggestion. Scale (particularly if your boat sits in salt water) will accumulate whether you use your engine or not. The accumulation may actually be worse if raw water just sits in the tubes for long periods of time without moving. The scale can be hard to see since the inner diameter of the tubes is small. A cleaning with acid and a checkup at a radiator shop is a good idea after about ten years. I think I paid $125 the last time. I have also poured muriatic acid through the raw water side a few times, and you don't have to remove the heat exchanger to do this (disconnect both engine connections, use two pieces of an old cut garden hose long enough so you can stand at the dock, a funnel, gloves, and eye protection; use one of the hose ends to force tap water through as a rinse after 15 minutes or so). Either way, it does make a big difference.