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I have recently fully rebuilt my engine, minus the block and pistons. All the gaskets were replaced, valves lapped, valve lashes tuned etc. I did it because I had an overheating problem and a head gasket failure. I performed straightedge and feeler gauge tests to make sure the previous overheating and head gasket failure did not cause cylinder head warpage.

When I put the coolant back, I used one of these no-spill funnels to make sure the system is fully burped by running the engine with the funnel on, which would compensate with coolant any bubbles that get burped out. I really believe I squeezed every single little bubble out, also because I turned the heat on to make sure the heater core gets its flow. I also filled the overflow tank up to about half where the FULL notch is.

What I noticed when I first drove it more than a few miles is that there was a splatter of coolant around the overflow tank, when I lifted the hood. I though the whole rebuild was in vain and overheating was back even though the truck did not overheat at the time (but I though the splatter was an indication it was going to). However, then I drove it another 3 hours in both the city and highway and there was no more splatter or overheating. In fact, the cooling worked so well that even after 3 hours the rad cap was not too hot to be taken off and the coolant level was proper.

The question of why there was initial splatter still remained. Here is how I attempted to rationalize it:

Under normal thermal expansion, the coolant, which was cold when filled 100% into the tank, spilled over into the overflow tank. But the overflow tank also had coolant in it, which I put in when I refilled the system, so there wasn't enough room left to absorb the expanded part and it splattered out the excess via the excess valve. Once the amount of coolant in both the system proper and the overflow self-regulated by splattering the excess, there was no more splattering as there was now room to absorb thermal expansion overflow in the tank.

QUESTIONS:

  1. Is the above a tenable explanation?

  2. Should you put no coolant in the overflow tank when refilling the system with cold coolant to leave enough room for thermal expansion ?

  3. In general, under healthy engine circumstances, how much coolant should be getting exchanged between the cooling system proper and the overflow tank? Is it to be expected that the overflow tank fills with thermal expansion overflow when the engine is hot (not overheated) and then that gets sucked dry (or evaporated) back into the radiator when the engine cools down?

  4. I am also an avid plumbing DIY-er. In case that you are too or at least familiar enough, does the coolant overflow tank in a vehicle perform the same function an expansion tank does for water heating systems, which is to absorb the liquid volume that expands under heated conditions? Since you don't put water in an expansion tank when installing it, by the same token should you not put coolant in your overflow tank?

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Is the above a tenable explanation?

No. While some thermal expansion happens in a cooling system, the amount which happens can be absorbed by the rubber hoses present in the system, unless the system overheats. When overheating occurs, steam is created, which causes pressure spikes, which causes more over heating, which causes more steam, etc. When a cooling system is behaving properly, it works together in harmony. The temperature is kept below the boiling point, while the pressured cap allows the boiling point to be raised. As the coolant flows through the radiator, excess heat is bled off which allows the boiling point to not be reached.

Should you put no coolant in the overflow tank when refilling the system with cold coolant to leave enough room for thermal expansion ?

You should fill the radiator up all of the way and fill the expansion (overflow) tank to the cold mark when first filling your system.

In general, under healthy engine circumstances, how much coolant should be getting exchanged between the cooling system proper and the overflow tank? Is it to be expected that the overflow tank fills with thermal expansion overflow when the engine is hot (not overheated) and then that gets sucked dry (or evaporated) back into the radiator when the engine cools down?

No, you should not expect thermal expansion. If the system is properly bled, you should have very little air in the system. This little bit of air may pass out to the expansion tank when the system is fully up to temperature, but this isn't a given. When the system cools down you'll see the system draw coolant back into itself from the over flow if it needs to. After it cycles the first couple of times, you may need to fill your overflow up a little to make up the difference. If there is a large amount of difference, you probably didn't bleed your system correctly.

I am also an avid plumbing DIY-er. In case that you are too or at least familiar enough, does the coolant overflow tank in a vehicle perform the same function an expansion tank does for water heating systems, which is to absorb the liquid volume that expands under heated conditions? Since you don't put water in an expansion tank when installing it, by the same token should you not put coolant in your overflow tank?

Completely different function. From my understanding of an expansion tank in a heating system or even in the hot water system of a house, it is primarily there to absorb sharp spikes in pressure which causes noise throughout the system. The overflow tank in the cooling system is to catch excess and provide for as I've described above. Fill the tank to the cold line and you should be golden. Recheck several times after first start to ensure there is enough to cover any contingencies.

  • the radiator cap was leaking, i found an extra gasket that closed the gap – amphibient Sep 21 '15 at 15:19
  • That's awesome. Sometimes the easy fix is all that's needed. I'll bet you'll want to put a new cap on there at some point to ensure you aren't now allowing for over pressurization. Something to think about, anyway. – Pᴀᴜʟsᴛᴇʀ2 Sep 21 '15 at 16:37
  • actually, it is not the CAP that was the problem but the neck of the rad. i ordered a new rad, $75 – amphibient Sep 21 '15 at 16:42
  • the cap is fine but the neck of the rad is all chewed up – amphibient Sep 21 '15 at 16:42
  • how can it overpressurize ? – amphibient Sep 21 '15 at 16:43

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