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FAQ: Engine Air Lock, Engine Heating Problem


Engine Air Lock, Engine Heating Problem?


Original Message:
Engine Air Lock, Engine Heating Problem,
Hi --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. This is the first e-mail I received on this topic. Could you "fill" me in on the details of what happened and what the cure is? I'd appreciate it.
Pete Lyssa, #393, PSmullen@connstep.org

Subj: Re: Engine Air Lock, Engine Heating Problem,
After you drain the internal coolant (50/50 Prestone and water), 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 Operators 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, Ronphylhill@erols.com

Subj: Re: Engine Air Lock, Engine Heating Problem,
Dear Ron:
Thank you for your reply and the "CURE" I didn't have a small hand pump so I used a 50 CC (ml) BD hypodermic syringe without the needle to suck out the coolant from the small hose to the thermostat assembly. After a few attempts, it worked. Our water here in Monterey Bay runs about 53 to 58 F and my normal engine temperature is about 160F Again.
Many thanks, Howard Robinson

Subj: Re: Engine Air Lock, Engine Heating Problem,
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 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, RPDIII@aol.com

Subj: Re: Engine Air Lock, Engine Heating Problem,
I fully agree on the PAR pump. I have had one for years, and use it to change engine fluids. With a long pickup hose, removing transmission oil is a piece of cake, and a funnel with a long hose makes refilling just as easy.
Pete Lyssa, #393, PSmullen@connstep.org

Subj: Engine Air Lock, Engine Heating Problem,
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.
Hope this helps.
Chap Hodges, Kemosabe #344, kemosabe1@home.com

Subj: Engine Air Lock, Engine Heating Problem,
Well, I put this off long enough but it is time to tackle my engine overheating problem. I have an '85 c-30. Starting last summer I noticed the higher the rpm (above ~2000) the hotter my m-25 would run. It could keeps a steady temp of 180 at 2000 rpm

  • The raw water pump impeller is good.
  • I have never lost any impeller blades in the engine
  • The raw water intake hose doesn't collapse at any rpm
  • The raw water strainer is clean.
  • I have a good stream of water from my exhaust.
  • I get the same temps with the thermostat in and out.
  • No other noticeable changes in engine performance


Subj: Engine Air Lock, Engine Heating Problem,
My first thought is the heat exchanger (it has the older smaller one). Can anyone suggest some other things to look at if it checks out OK?
I have an 86 Cat 30 with M-25 engine. I fought overheating problems for 5 years (bought boat a neglected repossession in 90).
Do you have the hot water heater mounted in the port cockpit lazerette?
Do you have the fresh water (Prestone) overflow tank in the cockpit lazerette?
Do you have the Rube Goldburg plumbing/bypass valve mounted under the heat exchanger that controls the amount of engine cooling water going to the hot water heater?
You are on the right track with the heat exchanger. Lime and salt deposits lower its efficiency. Take it to a radiator shop and have it boiled out.
Another major problem is zinc tips. The small pencil zinc eats through in the middle. The tips drop off and will plug the water passages lowering the efficiency of the heat exchanger. Take it off and shake it hard. If it rattles you have zinc tips. They are difficult to get out because the old heat exchanger only has an access cap on one end. (The wrong end for zincís) If you pound and shake long enough you can get them out through the zinc-mounting hole.
Another major source of problem is air locks on the fresh water (Prestone) 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 goldburg plumbing. Plumb the heat exchanger direct. Plumb the hot water heater to the little tiny hose that bypasses the thermostat (itís 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 lazerette, 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 Prestone and is probably not working properly. (Itís 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.

You said the impeller "is good". Do you mean it "Is good" or do you mean it "looks good". The blades can get stiff and not seal well even though it looks ok. If the impeller is more than 2 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 canít quite hack it. Catalina recognized this and went to the bigger heat exchanger. I recommend that too.

Subj: Engine Air Lock, Engine Heating Problem,
Finally plucked up enough courage to attack my heat exchanger. Pulled it out of the boat and flushed it internally. After reinstalling it, I was unable to put back all the green (antifreeze) water. I have read everything in tech talk and discovered that each boat is set up differently. The boat would run for hours at about 165' now I can get it up to 180' at the dock in about ten minutes. It would seem to me that there is air in the system, and I'm also assuming that it is at the pump, as I'm not feeling any heat at the heat exchanger or the lines going to it. The only bleed point I can find is on top of the thermostat housing.
What temp should the thermostat open? I was wondering if I needed to run the temp up higher to open it, and maybe the air will vent that way. I do have a small water heater but can only find something that looks like a pressure relief, no vent plug. This is a 25XP engine.
Ian "Repose", #5125, SW FL, igfowler@gateway.net

Subj: Engine Air Lock, Engine Heating Problem,
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, 83 C-30 #3221 TRBS, Houston, TX, danlives@swbell.net

Important: The opinions expressed here are those of the individual contributors to this page, and not those of the Catalina 34 National Association or Catalina Yachts, Inc. Additionally, this material has not been reviewed by Catalina Yachts, Inc. for technical accuracy. This page's maintainer cannot guarantee the accuracy of this information or the desirability of suggested modifications or upgrades. Please obtain assistance from a competent marine mechanic or boatyard prior to making any significant modifications to your vessel.   


Last modified by Phil Imhof, Wednesday, August 11, 2004 . Copyright © 2001 by Catalina 34 International Association.  All rights reserved.