View Full Version : Heat exchanger (radiator) cap

05-27-2010, 09:58 AM
I have fitted 2 brand new 315hp carbaretted 5.7lt 2007 marine power engines into my aussie built 26ft savage.I imported the engines over to australia myself from the USA.
We have no dealers over here and to get any imformation is very difficult,hence i am on this great website.
One of my engines (52 hrs now) is overheating 210F at idle,when i do 2800revs or there about the engine cools down to 160F,normal running temp,(tested with laser probe aswell).After shut down 30mins and removing the cap,the heat exchanger needs to be toped up with 3.5lts of coolant.I have checked all hoses and right around the engine for leaks,i have not found a leak and all is dry everywhere.I have pressure tested the heat exchanger to 15lb hold pressure and detected no leaks.I checked the impellar and it is 100% good condition.The engine,hoses and everything else seems to be all fine.
One thing i did notice was the new flooring hatches are 3 large piece sections,one of them over the problem engine was rubbing against the heat exchanger (radiator) cap and i noticed a leak from the cap.I need to replace the cap,what type should i use,the one's i have now have no imformation on them.I have fitted a 13lb type but i don't get any water going out as it heats and back in when it cools from the resevior.I do not know whether it is a "recovery" type (overflow type) or just a "standard" type,all i know it is a 13lb and i do not even know if a 13lb is correct.I saw 7lb,15lb,16lb etc,I just guessed 13lb on average.
I need help on what "lb" cap i need and if i need the "recovery" type cap,which i think i do.
If the cap i have on at the moment is a "standard" 13lb,would it harm the engine,because i have run it for 1 hr with no overheating issues "but" not water in and out of the resevior.help!!!

05-27-2010, 10:30 PM
G'day from sunny Tassie,

Sounds like you do need a Water recovery tank, but to get the benefit out of it, you need to have a Radiator Cap that is suitable for a Recovery System.

What type of Heat Exchanger are you using?

Is the Thermostat stuck?

What type of cooling are you using for the Exhaust Manifolds? The same Fresh Water cooling as the engine? or Raw water?

Have you done a Compression Check on the suspect engine? You could have a blown, or leaking Head Gasket, or a leak into the Exhaust System which is why you can't see the leaking fluid?

13 Lbs would be an ideal pressure, but remember that at 13 Lbs, the water won't reach boiling point till 238 F as for each pound of pressure rise, the Boiling Point of plain water is increased 2 F, so with an Anyi-boil additive, the temperature could be even higher. This is the reason one NEVER removed the cap on a boiling engine. Do that, and the steam instantly becomes Super-heated Steam.


05-28-2010, 09:35 AM
Hi Tassie,cold down their at the moment mate.
I think i do have a water recovery tank.does it run off the heat exchanger with a hose from under the cap and when the engine gets hot the cap set at 13lb releases the coolant into the recovery tank and then when the engine cools right down the water gets sucked back in.i think this is the way my system works.
So i need a 13lb recovery type radiator cap.do you know what is a good brand and where from to get one in sydney.
i have checked all the other things you have mentioned,and all was ok.we have come to the conclusion that my radiator cap was faulty and leaking.

05-28-2010, 11:26 AM
ideally, as the coolant warms, you will only get a dribble into the overflow bottle once the system is full.

If it is full of coolant, check the raw water flow...loose belt, loose hose clamp, strainer lid not tight....

If you still have the old cap and pressure tester, you can test the cap and see where the spring opens. Sometimes, you can find a number on the ear which is usually PSI if sourced from here (US). Most of the newer ones over here are 16#....I would think the 13# should be ok.

Also, make sure the neck fitting on the HX didn't get damaged as the new cap won't help on a distorted fill neck.

05-29-2010, 09:05 PM
I think that there isn't much difference between the brands, but for a Marine application, I would be going for a Stainless Steel one, and if you can go to 15 Lbs, then go for it.

The Header Tank should be at the highest point of the cooling system, engine included, but if your Heat Exchanger has a built-in Header Tank, then I believe that the unit should be positioned higher than the highest coolant point of the engine.

When I fitted my HX, I placed it low down, and used a Falcon EF onwards Header Tank, and mounted that up high.


05-30-2010, 12:26 AM
Test post.

05-30-2010, 12:28 AM
Being trying to post long reply but not working.Short test post went through,maybe time limit on writting a post?? I will re-post and keep things brief.

05-30-2010, 12:30 AM
I believe there is a limit on size.

Got caught once myself.


05-30-2010, 12:36 AM
Tested old HX caps and they are 7lb's.
Fixed damaged neck fitting and was very careful.

Ran the boat today with 13lb HX caps on for 2hrs,hard and slow,and everything was normal with temperatures,no water leaks.
The only difference was the recovery tank had 0.5 litre discharge with these new 13lb HX caps and the old 7lb caps discharged about 1.3 litre of coolant into the recovery tank.Is this still ok to use the 13lb caps or should i find myself a pair of 7lb caps???

05-30-2010, 12:41 AM
The 13lb caps are TVS part no:RC50-13.

05-30-2010, 12:44 AM
Personally, I would be going with the recommendations from the HX Supplier.

One thing to be aware of getting Salt Water too warm is that the Salt part of the compound starts to crud-up, and using the higher pressure cap will create a higher temperature within the HX for the Raw water to try and cool down.

The Engines will stand the higher pressure, but the HX is the concern.

If the other engine was performing satisfactorily with the 7 Lb cap, then I cannot see any reason to go a higher pressure, as in the Automotive application.

One of the reasons a car uses higher pressure, is that the car radiators have gotten smaller over the years, and need the higher pressure to control the engine temperatures which are even more-so critical with the advent of Computers and the like.


05-30-2010, 02:37 AM
Thanks Bruce for your time and good advice.
The HX is the standard on the 5.7 lt Marine Power carb engine i have.
I will chase up two 7lb compatible recovery type ones here in sydney,otherwise i would have to get 2 from the engine manufacturer "marine power" from the USA which would be a bit of a hassel for me.Anyhow all is well,i'm lucky it was just that,not head gasket,hoses,impellar,etc,etc,etc.
Cheers mate (and others) for the help.:)

05-30-2010, 08:45 PM
I cannot see there being a problem getting a pair of caps anywhere from a place like Repco, or even SuperCheap, but your local Radiator Spe******t will have the correct ones.

Can't see where a Marine one is going to be different from an Automotive one.