Loosing Antifreeze

[paulrfrancisco]

Looking for some suggeestions. My motors are 1990 7.4 Bravos with FWC. On one motor, I lose antifreeze over the course of a couple hours of running time. It not too much, but even a pint or 2 low causes the the temp gauge to climb a little... maybe to 190 degees... I cannot see any visable leakage from the hoses and or AF in the bilge, and I had the closed loop pressure tested and I'm told it was "good". It seems if I open the heat exchanger cap while cold when its low, then start and warm up the motors so the thermostat is opened, I can put in a pint or 2 when warm, cap it and then it runs at a normal 165-170 degrees. Where could this be leaking from? (also the pressure test was performed while the motors were still winterized, so they were cold). Could this be a heat related leak? I thought things generally got "tight" when hot....

 

[paulrfrancisco]

Re: Loosing Antifreeze

Bump

 

[Maclin]

Re: Loosing Antifreeze

This could be a compromised heat exchanger.

The heat exchanger has two chambers. The antifreeze is in the inner chamber and is circulated thru the engine. As the engine's temp rises this chamber is pressurized, this is normal, just like a radiator in a car. The outer chamber has the raw cooling water flowing thru it. If the inner chamber "radiator" leaks then the antifreeze will be forced into the outer chamber and exit with the cooling water out the exhaust.

 

[paulrfrancisco]

Re: Loosing Antifreeze

So should or can the pressure test be done when the engine is warm?

 

[Maclin]

Re: Loosing Antifreeze

Doesn't matter, just be careful taking the cap off if it is hot. I would do it cold, no need to warm up anything for the pressure test.

If the antifreeze chamber is leaking inside the heat exchanger then it will push antifreeze out into the raw water cooling chamber, but I am not sure how best to look for or listen for that. On a car the radiator would leak out onto the ground. On a boat it leaks into a closed chamber that has water in it anyway.

 

[arks]

Re: Loosing Antifreeze

I agree- start with a HE test (easy to do yourself- on a bench, not in the boat).
Have you noticed anything else unusual with the engine? It could be a leaking head gasket. Happened to me last year- my engine was losing coolant and occasionally it would act funny at start-up (I thought the starter was going bad). My mechanic pulled the sparkplugs and found a pair that were "suspect". I had him remove the head, which uncovered a failed gasket and also a cracked cylinder wall (hairline). Rather than put it back together knowing of the problem, I had the engine replaced.

Take a look at your plugs. Hopefully they'll be A-OK. If not, pull the head.