Engine Air Lock - Engine Heating Problem

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I changed my coolant just before lay-up last season. The engine wanted to overheat, but I shut it down before that happened, and decided to deal with it during spring commissioning. Pete Mullen, Lyssa #393


How To BURP Your Engine

Replacing Antifreeze and Burping Your Engine - Stu Aug. 2009

Please go to this link on the Main Message Board see Reply #6 - the text is here, the picture is there!

http://c34.org/bbs/index.php/topic,5147.0.html

A fellow skipper recently asked: After five seasons I am thinking of changing the antifreeze in my M25. When I did it 5 years ago I recall having trouble removing air after the change. If I slowly drain from the petcock on the block and fill thru the plastic expansion tank simultaneously with the pressure cap off on the engine would this work so I don't introduce air?

I responded: We just did this the other day when repairing the exhaust riser connection to the flange. Don't bother with the petcock on the manifold, it's hard to catch the water.

Do this, it's a lot easier. Loosen the screw on the hose clamp of the big black hose underneath the freshwater pump and use a thin screwdriver to open up the space between the hose and the elbow. Do it right from the front of the engine. You're right, take off the manifold cap when doing this.

As for the air block, it's called “BURPING” and all you need to do is take the 3/8 inch hose off the bottom of the thermostat. Pour distilled water or antifreeze or the 50/50 mix into that hose with a funnel until water comes out of the bottom of the thermostat. Loosen the hose clamp and pull the hose off. Hold that hose up above the thermostat and pour the stuff into it until the water comes out of the bottom of the thermostat - the nipple that you just removed the hose from. Once that happens, just put the hose back on the nipple under the thermostat housing and tighten up the hose clamp. Put some Lanocote on the nipple to make it easier to remove the hose the next time. That gets the air out of the loop of hose that goes to the hot water heater and avoids overheating at startup and does not require that you rev the engine while the thermostat petcock is open. You won't need to open the thermostat petcock at all.

Added: Ron uses a Par hand pump - does the same thing.


The air lock could be in the lines that run from the engine to the hot water heater and back. What I have done in the past is start the engine and, one at a time, remove the lines from the top of the engine (not from the hot water heater) until antifreeze comes out of the top of the engine (beside the thermostat) and then replace them. After this is done, open the petcock on top of the thermostat until antifreeze comes out. This should take care of the problem. Chap Hodges, Kemosabe #344


[NOTE: This was written for a C36 - same engine, different HW heater location.]

A major source of the problem in older models is air locks on the fresh water side. Air locks are a problem because the filler cap is NOT the highest point in the system. (If you have the hot water heater in the cockpit lazerette). The hot water heater is. Catalina recognized this as a design problem and moved the heater to in front of the fuel tank in about 87-88. However, moving the hot water heater is a major project. (You have to move the fuel tank back and the new design is slightly smaller I think). There are several things you can do.

  1. Take out all of the bypass water heater rube-goldberg plumbing. Plumb the heat exchanger direct. Plumb the hot water heater to the little tiny hose that bypasses the thermostat (it’s on the thermostat housing on the front of the engine-top). Simply remove the little hose and hook up the input and output hoses of the water heater to the connections. You will have to get some hose size reduction couplers. Catalina also made this change in newer boats.
  2. If you have the overflow tank back in the lazarette, there will be a little check valve between the overflow tank and the hose leading to the hot water heater. This check valve is black plastic about 2 inches cube. This check valve is NOT designed for anti-freeze and is probably not working properly. (It’s supposed to help get the air bubbles out of the fresh waterside.) West marine carries them in the catalog. You can try cleaning if you want but the rubber is probably shot and swollen from the antifreeze. Both of these actions will help prevent air locks and poor circulation on the fresh waterside.
  • Check the impeller. The blades can get stiff and not seal well even though it looks ok. If the impeller is more than two years old, I would recommend you replace it even if it looks ok.

Bottom line. The older heat exchanger system is marginal and has very little reserve capacity. You could have one major problem or a number of little things all lowering the efficiency enough so that at hi power; the system can’t quite hack it. Catalina recognized this and went to the bigger heat exchanger. I recommend that too.

George G.


After you drain the internal coolant, you will probably experience an air block in the hose that goes through the water heater (which is a heat exchanger) causing the engine to overheat. Although the Universal Operator’s Manual tells you how to purge the system, I found an easier, faster method.

  • Remove the "radiator cap" and fill the reservoir with the coolant mixture.
  • Then remove the hose from the water heater return to the bottom of the thermostat housing.
  • Connect a small hand pump, like the PAR Pump ("Handy Boy", Boat U. S. Item # 162003), and pump the coolant from that hose into the open reservoir until you get all of the air out of the system.
  • Then reconnect the hose (with your thumb capping the top of the hose until reconnected) to the thermostat housing.
  • After reinstalling the radiator cap, start the engine.
  • If the temperature starts to go up to fast periodically open the butterfly valve on the top of the thermostat until you get the juice squirting out and then close it.

This should have eliminated the air block and the coolant should flow freely.

I also use this PAR pump to bring up water to the raw water-cooling pump (Oberdorfer) after changing an impeller. It's much easier on the engine pump impeller to have the water "right there." If you don't have a coolant recovery system, you may want to install one. See MAINSHEET article Aug 1990. The Par pump is one item that I consider a must item on any sailboat. I suck out over filled transmission fluid or engine oil with it.

Your engine temperature should be running at about 160 Degrees Fahrenheit. The range of the error for the Teleflax engine temperature gauge at 180 degrees Fahrenheit is + or - 3 degrees. FYI 71 degrees Centigrade or (159.8F) is stamped on the thermostat (MAINSHEET Aug 1992).

Ron Hill, Apache #788


I have found the following way is the most effective way to get air out of the entire fresh water system. I fill the water tank with 50/50 anti-freeze/water (distilled), then open the petcock on the top of the thermostat, which is on the top of the engine.

The next step is to crack open the water heater, heat exchanger, line that is located at the engine compartment access door in the aft cabin. You will see the two water lines with a connecting fitting. I open the return line fitting and start the engine. When the air is purged, fluid will start coming out of this line. Tighten the fitting and go to the petcock. When the fluid comes out of the petcock tighten that up. Refill the water tank with 50/50 and let the engine run for a while. Make sure you run the engine above idle.

If you can, run the engine while the transmission is in gear. This should bring the operating temp to a more realistic level. Before you go on that long trip make sure you take the boat out for a test run, and be ready to solve an overheat during the test run.

Rich Dwyer, Rebellious #328


An alternate problem solution

I don't think that my story relates directly to your problem, but just in case, I had the same problem some time ago and I tried, with no luck, almost everything listed in the previous letters: changed impeller, removed the water heater from the circuit, cleaned the (small) exchanger, etc.

Finally, I called a good local mechanic to request his services fearing the worst (valves, injectors, compression, etc.) but he suggested to first check the bottom and propeller for rope, growth, etc. A bottom-cleaning job performed by a diver solved the problem. The anti-fouling paint had protected the hull but the prop and shaft were heavily coated product of a warm winter and not enough use. He also changed the zincs. Overall, a bargain. Enrique, Argentum, C30 #3221 (1983), Houston, TX