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PostPosted: Sun May 26, 2013 9:55 am 
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Joined: Wed Apr 09, 2008 5:53 pm
Posts: 14779
Location: CT
Zorro ~
I watched your YT video about your 260 regurgitating after shut-down. You were asking why it did that. Here's why:

FIRST SCENARIO: your radiator was improperly filled. Whoever filled it did so with the engine cold and OFF. That means the thermostat was CLOSED when the radiator was filled. A closed thermo is a 99% stop-valve in the cooling system. As you fill the radiator with a cold engine, the coolant forces air up against the bottom of the CLOSED thermostat, where it forms a big (grapefruit-to-football-sized) air pocket. The area above the thermo is filled with cold coolant (in the upper radiator hose), and the area below it is filled with compressed air. No coolant can get past the big air bubble to reach the thermo, so the thermo doesn't know how hot the coolant is. It thinks everything is 'cool'. Therefore, it doesn't open when the engine gets hot.

When you run the engine, the coolant becomes super-heated, because the thermo doesn't get touched by hot coolant (the coolant isn't exchanging btwn your radiator and engine, it's recirculating thru the block/head getting hotter, blocked from reaching the radiator by the closed thermostat). The thermostat is only in contact with the air bubble below it, and the colder coolant in the hose above it. It doesn't recognize the desperate heat which is building up in the cylinder head and in the block. Your temperature gauge may read cool, since air is not as good a conductor of temperature as liquid is, and your temp sensor is designed to work in liquid. For the same reason, the air appears cooler to the thermostat than the liquid would if it could reach the thermostat.

Eventually the super hot coolant transfers enough heat to the air bubble to make it transfer enough heat to the pellet on the underside of the thermo, to allow it to open. But by that time the engine is FAR overheated. You may hear a 'bang' while driving, followed by a surge up and a plunge down on your temperature gauge back to hi-normal ranges. That's the thermo finally opening and allowing all that superheated coolant to surge forward into the cooler radiator. You might smell steam as the pressurized surge overcomes the 8lb radiator cap spring and allows some pressure to blow-off. You might even blow a gasket instead. The force can sometimes blow upper radiator hoses and the sudden temperature changes have been known to actually crack radiators.The radiator is now "low" the amount of coolant which should have filled the air pocket below the thermostat. It's not a good thing, generally;

SECOND SCENARIO: After your air bubble is bled, while the engine is running and air is coming thru the radiator, normal cooling is taking place. When you shut the engine down, no air is moving thru the radiator, and the water pump stops pushing coolant thru the hot block and head. At that time the residual engine heat transfers into the stagnant coolant and it super-heats (because it has no way to transfer heat to the radiator). It's called afterboil. Opening the radiator cap at that point allows the superheated, expanding coolant to surge and overflow until the thermostat cools and closes (thereby creating ANOTHER air pocket under the thermostat and that problem starts again). It's very dangerous to open the cap at that point, due to the explosive force of super-heated coolant scalding anyone nearby (and completely spraying your car);

3) You are using water instead of antifreeZe, which has better cooling capabilities than H2O.

The proper procedure for filling a radiator is this:
1. Engine cold, remove radiator cap and start engine;
2. As engine idles, slowly pour coolant (not water) into it until it appears full (it really isn't, because the closed thermostat is still trapping that air bubble beneath it. You still "owe" the engine a 'football' of coolant, but you have to get that air bubble out of the way before you can pour more coolant in);
3. When the engine heats up and finally opens the thermo, you will see spurts and splashes and hear gurgles and surges. Stay back; that stuff is hot and will burn you;
4. When the spurts and surges stop (engine still idling slowly), you will see the radiator looks empty again. Carefully and slowly pour the remainder of the coolant into the radiator until it again fills up. At this point you should see a stream of coolant pouring into the radiator from the upper (hot) radiator hose. Now you know your system is completely filled with coolant, not with big air pockets. An occasional burp will assure you that all the air is being purged from your system. Top off the radiator as needed;

At that point you can carefully replace your radiator cap (I wear gloves) and hose down the splashed coolant. Radiator cap should hold 8-12lbs pressure, which raises the coolant boiling point.

Everybody has an experience (or a horror story) about this.

I fail to find a simple illustration of how the closed thermostat traps the air beneath it, but the thermostat is at the top of the engine and air rises in liquid, so that's where it goes.

Frank


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File comment: Some Nissans now offer air pocket bleed valves under the thermo to relieve this problem, but our 40y/o Z cars don't have that.
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nissan-2.gif [ 11.38 KiB | Viewed 2496 times ]

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