Overflow Tank Full but Header Tank Empty

[CamMoore]

I'm having issues with my 2.1L 91 van's coolant system. Driving along coolant light on dash starts flashing, pull over and have a look the header tank in the engine bay is getting empty but the overflow tank behind the number plate is full to brim. I was only around the corner from home so I drove and left her overnight to cool down. Next day the header tank is still empty so I siphoned some coolant from the overflow because it was too full and put in back in the header.

This has happened 3 times now. I've just had the coolant system checked out from a mechanic and have checked over myself and can't find any leaks.

Could this be from a dodgy cap on the header tank?

[multisi]

Get a decent cap from brickwerks, there are a lot of dodgy new caps on the market.

[Mr Bean]

As I see it the design intent is for the overflow caused by the expansion in the system to vent through the rubber pipe into the (vented) overflow tank to be drawn back when the system cools down. Clearly the first bit is happening but not the second.
I suspect that there is either a leak in the pipe/connections or it might be something to do with the Dalek. A leak in the pipe/connections would not necessarily spill out as there is no pressure in this line but it would prevent the coolant from being drawn back.

CS

[itchyfeet]

Head compression seals possibly.
get coolant checked for combustion gasses, called sniff test.

[300CE]

As itchy says it could well be the compression seals as I'm getting the same symptoms although my header tank has strangely now filled up to the brim but so has my top up tank which is now over max. When it has cooled down, undo the cap on the header tank - is there a lot of pressure in there and does the tank start to fill up?

Can try changing the cap, but unfortunately it will probably be a head off job if not (after sniff test). I'm going to put up a thread when I start to do mine.

My engine was a recon and the head nuts seem well over-torqued (more than 70nm) apart from the left rear nut which was quite 'loose'. Over time I think this has had an effect on the compression seals causing the pressurisation.

[tencentlife]

When coolant keeps transferring to the reserve tank, two easy checks will distinguish a pressure cap not holding from a fire ring (MLS compression seal) failing:

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 completely 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). You would not have a low coolant level warning in this instance while the engine is still hot. In this case coolant has escaped due solely to expansion, the system is still completely filled with liquid.

With a bad pressure cap, it is quite common, 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.

2. If, after running 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 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, you have a high pressure fire ring leak. In this case the coolant escaped due to displacement. Combustion gases were forced past the fire ring at high cylinder pressures (large throttle openings), some of the gas bubbles were entrained with coolant flow to the radiator, where they accumulate at the top and displace liquid out the pressure cap's regulating valve.



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 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 gas at over 800psi trying to find its way out.

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 800psi will force the leak.

The hot gases burn away material as they escape, so a fire ring leak 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.

[Bubble Meister]

I have this issue occasionally and it's due to the pipe between tanks perishing. Is yours in good fettle?

[ajsimmo]

What tencent said. Beautifully described, btw!

Sent from my B1-830 using Tapatalk