proper method for bleeding coolant system

Xplicitlnck

Well-Known Member
Jan 2, 2012
4,030
Long island ny
Boat Info
330ec
Engines
Twin 7.4 straight drive
Its time to change my thermostats and heat exchangers. Now I know how to bleed an automotive system, simply drain change your paRts fill your radiator, start the car, wait for operating temp, raise rpms to about 1500 (all with rad cap off) wait for air bubbles to stop bubbling, top off rad, install cap, drop down to idle, fillreserve bottle to hot line.

What's your marine process? I have 7.4 mpi closed cooling.
 
Someone can correct me if I am wrong, but IIRC the process is similar.
Pull the two plugs and drain. Replace plugs. Repair the system with new parts. Fill system. Tilt boat so front of engine is higher.
Leave cap off. Start engine, but only run at idle speed - never faster. Add coolant if needed after it comes up to temp.
Replace cap.
 
Last edited:
There is a bleeder valve on top of the thermostat housing that should be open while you fill the heat exchanger. Once coolant starts to appear at the bleeder valve close it and fill the heat exchanger the rest of the way. This will get rid of most of the trapped air.
 
From the manual on my 2008 8.1:

Filling Closed Cooling System
Alcohol or Methanol base antifreeze or plain water are not recommended for use in
coolant section of Closed Cooling System at any time.
It is recommended that the coolant section of Closed Cooling System be filled with a 50/50
mixture of Extended Life 5/100 Coolant and pure, soft water. This coolant MUST BE used
regardless of whether freezing temperatures are expected to provide adequate corrosion
protection. In areas where Extended Life 5/100 Coolant is not available and the possibility
of freezing DOES NOT exist, it is permissible to use a solution of rust inhibitor and pure, soft
water (mixed to manufacturer’s recommendations).
NOTE: Coolant section capacity is approximately 19 U.S. Quarts (18 l).
1. Remove pressure cap from coolant reservoir.
DO NOT operate engine without water flowing through seawater pickup pump, as
pump impeller may be damaged and subsequent overheating damage to engine or
sterndrive unit may result.
Front of engine should be higher than rear to purge trapped air out of the system
during initial filling. This will minimize the possibility of air being trapped in the
closed cooling section which can cause engine to overheat.
IMPORTANT: This closed cooling system flows coolant at a high rate. Higher idle
speeds increase dispersion of trapped air into system making it more difficult to
purge trapped air. Operate at idle during filling and air purging when specified.
2. Fill reservoir to the FULL mark with recommended coolant solution.
3. Replace pressure cap.
4. With engine running, check hose connections, fittings and gaskets for leaks. Also
observe engine temperature gauge to make sure that engine operating temperature is
normal. If gauge indicates excessive temperature, stop engine immediately and
determine cause.
Allow engine to cool down before removing pressure cap. Sudden loss of pressure
could cause hot coolant to boil and discharge violently. After engine has cooled
down, turn cap 1/4-turn to allow any pressure to escape slowly, then push down and
turn cap all the way off.
5. Recheck coolant level after first boat test and add coolant, if necessary.
6. Maintain coolant level in reservoir at or near the FULL mark with engine at normal oper-
ating temperature.
 
thank you all for the input,but I have another problem,exhaust manifold not getting any coolant until thermostat opens, any thoughts ? It seems mercruiser has fixed this problem by moving the thermostat, I have found a inline thermostat housing but I wanted to do some research before I tried it. Also a marine mechanic I know said it doesn't matter what temp thermostat you use they all run around 180, is this true ? Anybody agree with that ?
 
I seem to hear from you a lot, is that malice or concern ? I see you are a migrator, probably not this year thou, ah ? well, thanks for keeping me on my toes, but info is info,it seems this air problem is difficult to get rid of. I took off one of the 3/4" plugs from my exhaust manifolds and it was not full. I think I might install a clear hose so I can see coolant flow.
 

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