On my 96 LT4 I have noticed that the coolant overflow container level is always about the same either at HOT or COLD.

I shut off the engine after a 30 mile drive, highway and surface streets, so the engine was up to normal operating temp, checked the level and it was just above the COLD level.

Checked again two hours later and level still the same.

So is this a problem? Both temp gauges read normal temps...no overheating or anything.

Thanks

 

Here's a long scientific answer, but easy to understand

The true overflow tank is an add on item used with a conventional cooling system. The conventional cooling system uses a radiator, which has an attached filler neck to which a pressure cap is attached. The system is filled though the filler neck on the radiator and should be filled to about ½” to 1” below the bottom of the filler neck. If the system is filled any higher, it will be over full and the excess coolant will be vented from the system. This happens when the heat builds up within the coolant, it expands, increasing the system pressure above the pressure holding capability of the pressure cap (listed on the cap in pounds per square inch or PSI) causing the excess coolant to be vented out through the overflow vent which is exposed when the cap sealing plate is forced upwards. The excess coolant is then vented to either the atmosphere or into an overflow tank, which functions as a catchment basin. The overflow tank is under atmospheric pressure while the cooling system is under atmospheric pressure plus the pressure generated by the heat expansion of the cooling medium. As the cooling system begins to cool down, the internal system pressure drops until it allows the pressure cap spring to overcome the system pressure and the cap then forms a seal again on the bottom of the filler neck. This takes place when the internal system pressure drops to lower than the rated pressure of the radiator pressure cap. Thus, the cooling system pressure will always be above the atmospheric pressure of the overflow tank until the pressure cap again seals and the entire system is completely cool. At that time, both the cooling system and the overflow tank will be at atmospheric pressure. As can be easily seen, the flow of excess cooling can be in one direction only—out of the cooling system and into the overflow tank. The coolant in the overflow tank cannot flow back into the cooling system because a liquid cannot flow from a lower pressure to a higher pressure. Thus, an overflow tank serves strictly as a catchment basin, not as some for of extra coolant reservoir.

 

You know...I actually never thought of the details you brought out. I was told long ago how coolant within the overflow container would be drawn back into the cooling system as the system cools down....but as you stated, this cannot occur due to the pressure being the same and the pressure cap seals off the path.

Thanks for pointing-out the facts.

 

No problem, I'm sure the question will come up again.

 

my 91 fsm disagrees with your theory. the cap has a pressure relief and a vacuum relief, that allows vac inside the engine/radiator to draw coolant back in from the coolant recovery reservoir. it says to add coolant only to the tank. I have drained and refilled coolant 6-7 times, always simply add to the reservoir. the reservoir tank cycles from hot, back to cold, never spills out. it goes back into surge tank.

joe

 

 

Why else would your overflow bottle be marked "Hot" and "Cold"? Description might be right for a 1950 car, but not for the new hi-tech cooling systems. Sorry to disagree.

 

The cooling water creates a vacuum and pulls the water back through a second low pressure seal in the cap and self burps the radiator.

 

so so if the system is not sucking it back up, what needs to be replaced to fix that? the rad cap?

 

It could be a bad rad rad cap or the outlet on the overflow bottle on the bottom of the bottle is plugged with junk, or the hose is plugged or cracked.

 

Where is this "second low pressure seal" at?

I removed the pressure cap and I do not see how coolant could be drawn from the overflow container back into the system......please explain more.

 

my level only changes slightly too. If you are not overheating or the low coolant light is not on it is fine. My low coolant will blink for three days after I have had my radiator drained before it burps itself. A little less each day. Scary but watch the temp reading. If not hot all is ok. I check mine cold and keep it a little above the cold fill mark.

 

one of the seals, the one on the part that seals the port inside has a valve that will pull open ( towards the radiator ) letting water pull back into the radiator. When the expansion pushes, it opens the intire valve against the spring pressure. When the vacuum pulls it pulls on the smaller valve in the center of the larger valve.

 

most likely, it's impossible to plug up a vacuum totally, like with residue, whatever.

"nature abhors a vacuum," remember? except in some people's heads.

 

A "1950" radiator cap has only one rubber sealing ring, on the spring loaded center piece. A "modern" cap has three sealing rings. One on the spring loaded center piece, one in the tiny center vacuum release valve (which the "1950" style doesn't have), and one in the top of the cap, under the cap retaining tangs. If any of these three rubber seals is damaged, the fluid exchange system won't work!

Also, if everything is working properly, the radiator will be completely full at all times. This minimizes the amount of air (oxygen) that is absorbed by the radiator fluid, which minimizes corrosion in the cooling system. If your cooling system isn't staying full at all times, you have a vacuum leak somewhere.

I shouldn't have to say this, but don't remove your radiator cap when the engine is hot. The pressure cap may be preventing the coolant from boiling, but if you remove the cap, the entire amount of coolant in your system may suddenly boil, creating a dangerous guyser!

 

yep and if your low coolant light isn't coming on you are OK

 

So Ken, It's okay to ignore the warning signs that your radiator isn't staying full, and just wait 'till you are about to have a disaster? What if your low coolant light comes on when you're half way to Las Vegas?

 

Your low coolant light is your warning sign. It is designed to tell you need more coolant. If all is working properly the cap will allow the radiator to take and expel coolant as needed. If no coolant is needed it will not retrive any. If there is too much air in the radiator the low coolant light will warn you. If your low coolant light comes on 1/2 way anywhere you lost coolant in your radiator. Then check your temp and reservoir. If OK monitor your temp as engine cools it will retrieve coolant and if not (your warning light will stay on) you have a problem. WOW I'm out of breath. Hope that helps.

 

I changed my water ppm and opti. I have added 4 gallons of 50/50, the overflow tank is full and I still have a low coolant light. It goes away at around 1800 rpm's then quickly comes back.

What is going on?

Thanks in advance!

 

It seems that you have to bleed the air from your coolant cirfuit.