Bad coolant pressure cap, or bad head gasket?

A driver of a waterboxer Vanagon had this complaint:

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Driving along coolant light on dash starts flashing, pull over and check the pressure tank in the engine bay. It is visibly down by a lot, and the reserve tank behind the license plate door is overfull. Left it overnight to cool down. Next day the pressure tank level hasn’t risen. Siphoned some coolant from the reserve tank because it was too full and put in back in the pressure tank. 

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This same sequence happens over and over whenever I drive it for awhile.  Had coolant system pressure-tested, test pressure never dropped. No external leaks can be found.

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Could this be from a bad cap on the pressure tank?

 

 

When coolant keeps transferring to the reserve tank, two easy checks will distinguish a pressure cap failing to hold pressure from an early developing fire ring* leak:
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1. If, after running at full operating temp and before cooling down, the reserve tank level has been seen to rise substantially, but the pressure tank is still full with liquid, it’s just the pressure cap failing to maintain the in-system pressure. It needs to hold pressure up to 1 bar or14.5psi. At normal temp the in-system pressure will be around 10psi, but there can be transients when high engine load causes momentary increases in coolant temp, and thereby pressure. You would not have a low coolant level warning in this instance while the engine is still hot because the liquid level in the tank is high. In this case coolant has escaped due solely to expansion, the system is still completely filled with liquid but the portion that couldn’t fit was released to the reserve tank.
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If the pressure cap’s return valve works properly and the overflow line to the reserve tank is airtight, the coolant contracting as it cools off should draw almost the full volume back into the pressure tank. In that case, if you didn’t see the reserve tank full while the engine was still hot, you might not notice the signs of the pressure cap’s failure to hold the spec pressure.
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You also might not notice the indicated temp increase because a lower system pressure means a lower boiling temp for the coolant, and it won’t get hotter than its boil point temp; adding more heat just causes it to boil more furiously, but the temp doesn’t rise. Too low or total lack of coolant system pressure can result in the situation where the indicated temp looks fine, even a bit lower than normal, while the cylinder heads can be overheating because coolant in contact is boiling. To absorb heat effectively liquid, not vapor, needs to be in direct contact with the hot metal surfaces. This puts the lie to the folks who claim they intentionally run their cooling system at zero pressure to lower stress on the system and it’s all OK because the temp gauge still reads normal. They forget that the temp gauge senses coolant temp, not cylinder head temp.
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With a bad pressure cap, it is not uncommon, while idling hot, to see bubbles rising in the pressure tank; don’t be fooled, those aren’t necessarily combustion gas bubbles. If the cap is bad they are more likely steam bubbles due to the coolant spot-boiling in the cylinder heads because boiling is not being suppressed by maintaining a high in-system pressure, which is the cap’s responsibility. Most of the bubbles recondense to liquid once they detach from hot surfaces and move into the coolant flow, but some may survive long enough to be entrained to the pressure tank where they are visible. The vapor bubbles entrained toward the radiator will soon recondense to liquid coolant, which is why a bad pressure cap alone doesn’t cause gases to accumulate there.
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The pressure cap doesn’t have to be failed wide open for this phenomenon to occur, I have seen bubbles rising in the pressure tank at hot idle with a cap that was still holding 6-7psi. Installing a cap that bench tested as holding the specified 1 bar made the bubbles stop. The same engine was leakdown tested as having excellent combustion containment. If it were a fire ring leak and it was able to produce gas bubbles at the very low in-cylinder pressures when idling, the leak would have to be a big one. Such a leak would be obvious on a leakdown test, and would be big enough to allow coolant to be forced into the cylinder after shutdown, which would cause the spark plug to look very clean, if one were to look, or could even push enough liquid in to cause hydro-lock on the next starting attempt. But such a large leak would almost never be seen in real life because long before it reached such a state the engine would have become practically unusable due to the dynamics explained in the next section.
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2. If, after driving awhile at full operating temp and before cooling down, the reserve tank level has been seen to rise substantially and the pressure tank liquid level is visibly low (this is what triggers the Vanagon low coolant warning flashing LED, and on earlier vans the temp needle will also peg high), then remove the front grille and crack open the radiator bleeder bolt while the engine is still warm. If a lot of gas escapes before it comes out as steady liquid, that’s a strong sign that there is a high pressure fire ring leak. In this case the excess coolant  was initially released from the pressure tank not only due to expansion but due to displacement as well. Combustion gases were forced past the fire ring at high cylinder pressures (large throttle openings).The overall system pressure increases due to the gases being injected at pressure, so while the pressure tank is full of  liquid it is released by the pressure cap’s regulating valve to the reserve tank. Some of the gas bubbles were entrained with coolant flow to the radiator, where they accumulate at the top and displace liquid to the pressure tank, causing more to be released. Some of the gases also get entrained to the pressure tank. Once a bunch of liquid is released and more gases are accumulating in the pressure tank and radiator top, the pressure tank’s liquid level will lower because it is being depressed by the accumulating gases on top while the trapped gas volume elsewhere is compressed to accommodate it.
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It’s at that point where the coolant level sensor detects the liquid level dropping and activates the flashing LED. The engine temp gauge would already be rising because by the time things get this bad the trapped gases in the radiator have interrupted the radiator pumping loop which must be filled with liquid for the water pump to induce flow, but it may not be noticed until the low coolant level warning LED gets the driver’s attention. At that point he’ll see that the temp is notably higher and rising fast on an ’86 and later van; on an earlier van the temp gauge would already have shot to the top of the scale along with the LED warning.
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When you stop and note the low liquid level in the pressure tank and overflowing reserve tank, if you were to open the pressure cap while the engine is still warm the liquid in the pressure tank will rise immediately and may even surge out the top. This is because of the two trapped gas volumes at opposite ends of the system being pressurised and in equilibrium, and when you release the pressure at one end the gases in the radiator are able to expand and displace the liquid back the the pressure tank. This is a sure sign you are going to find a substantial volume of gases trapped in the radiator. But it’s better not to open the pressure cap, instead use the existing system pressure to force the gas out the radiator bleeder, and if you do see plenty of gas escape before liquid comes out, you know you have a fire ring leak.
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An early-onset fire ring leak will not show up on any compression, cylinder leakdown, or cooling system pressure test for simple reasons. The fire ring only leaks, initially, when in-cylinder combustion pressures are very high. Those high pressures only occur at large throttle openings (when you vary the throttle opening with the accelerator pedal, what you are doing is regulating the mass of fuel/air charge that can enter the cylinders, which varies the end combustion pressure and thereby the amount of torque the engine produces). Full-load in-cylinder pressures are 800psi and higher, while low-load (part throttle) cylinder pressures may be only a few hundred psi. So, when the fire ring leak is just beginning to develop, gases only escape at high engine loads, when there is over 6 tons of force trying to lift the cylinder head, and very hot gas at over 800psi trying to find its way out.
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A compression test will develop 190 psi at most. A leakdown test is using usually only 100psi test pressure. A coolant system pressure test is trying to force a rather viscous liquid (coolant) at only 15psi thru a gap that probably doesn’t even exist at that moment. But hot burning gases at 800psi can force their way out.
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The hot gases burn away material as they escape, so once a fire ring leak begins to develop it will steadily worsen to where a leakdown test will eventually show it up, but by then the vehicle has become unusable because the time it can be driven before coolant flow is interrupted by the gas accumulation at the top of the radiator becomes shorter and shorter. Before you get around to testing, you’ll already know very well what the problem is. But you can diagnose it very early on by observing the simple checks I began with here.
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So, now that you understand the physics involved, in summary:
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With a bad pressure cap, after driving awhile and while the engine’s still hot you’ll see the reserve tank level rise while the pressure tank appears to still be full. You may see bubbles rising in the pressure tank. There won’t be gases trapped in the radiator.
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With a developing fire ring leak,after driving awhile and while the engine’s still hot you’ll see the liquid level in the pressure tank drop and a commensurate rise in the reserve tank. You may see bubbles rising in the pressure tank. You will find gases trapped in the radiator. It’s also common to see gooey black residue on the inside of the upper part of the pressure tank.
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*”Fire ring” is a common name for combustion seals, usually of multi-layer steel (MLS) construction, although some are copper or aluminum. These are integrated with laminated head gaskets on most engines, each cylinder will have its own fire ring integrated within the overall gasket. The rest of the gasket material seals all the other openings that go between the engine deck surface and the cylinder head, for coolant to circulate and for oil to be delivered to the cam and/or rockers and drain back to the sump. A waterboxer has individual, separate  MLS rings that seal the top of each cylinder to the cylinder head. The water jacket seal, often called the “head gasket” is a separate seal that merely seals the gap between the top of the crankcase water jackets and the bottom surface of the cylinder heads. On a wbx, either can leak without affecting the other, although if a fire ring leak becomes extreme it can increase pressure in the water jacket so fast that the jacket seal ruptures.

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