Overly sensitive coolant level sensor?

[drj434343]

Now that winter is here in the NW, I've noticed an issue with my coolant level sensor. If I park outside, the coolant level appears to lower enough in the expansion tank to set off the level sensor during the next cold start.

I attribute the level change in the tank to the natural contraction of the coolant as it cools down to ambient temperatures.

If I fill the expansion tank all the way to the top when the system is as cold as outside temperatures, it seems to burp excess coolant into the overflow tank once hot, which I can confirm if I stop and check the level when fully hot. Then, as it cools down, it doesn't suck excess back from the overflow tank, so I get the similar drop in level of the expansion tank once cold and have to start all over.

It seems like the natural level change between hot and cold in the expansion tank is about 2 vertical inches, apparently enough to set it off.

I'm right at the edge. If I park in my 60 degree garage, it won't drop low enough to trigger.

The lever sensor seems to work fine. I can fill the tank and it stops blinking.

I am not actually losing coolant. The level in the expansion tank does not continue to drop over time. I could ignore this issue, but a blinking coolant light on the dash is pretty annoying.

Have other people experienced this?

[djkeev]

You've got a coolant system leak.

Be happy that it is working properly and heading off an engine disastor.

It should ALWAYS suck in the coolant it burps out when hot.

If it fails to suck in coolant, there is a path of lesser resistance somewhere in your system and it is choosing this easier path while cooling off.

Put two straws in your mouth.
One straw into a beverage,
One straw just in the air,
Now suck!
How much beverage did you get?

Dave
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Ghia
http://www.thesamba.com/vw/forum/viewtopic.php?t=392473

Vanagon
http://www.thesamba.com/vw/forum/viewtopic.php?p=6315537#6315537

Beetle
https://www.thesamba.com/vw/forum/viewtopic.php?t=482968&highlight=74+super+vert

[Ahwahnee]

[Wildthings]

You could have a clogged hose between the pressure cap and refill tank as well. The pressurized coolant can be forced passed the clog, but the clog stops the coolant from being sucked back into the pressure tank as it cools. Another possibility is a minor compression leak which forces coolant out of the tank as the engine warms and then effectively vents the system after shutdown.

[drj434343]

First off, the system passes an overnight pressure test without losing pressure.

Second, the system doesn't exhibit enough of this behavior when the weather is warmer to set off the sensor. I.E. the delta temp between hot engine and ambient is less.

I've recently rebuilt the entire system with silicone lines and SS pipes. The OEM pressure cap and line to the overflow tank were not replaced. I experienced this phenomenon before and after the coolant line overhaul.

Is the expansion tank meant to suck excess coolant back in from the overflow tank when the system cools and retracts?

The pressure cap passes the test posted via youtube that was linked here: I can blow through it to make a squealing noise but can't suck back through.

[90Doka_Guy]

[Wildthings]

Remove your overflow/refill tank and flush it out well, using compressed air to blow out the nipple in the bottom.

[AtlasShrugged]

I would suggest getting the blue cap for the reservoir coolant bottle directly from VW. Part number 025121321B, about $30. There are a lot of junk parts out there.

If you examine the original VW coolant bottles there is a "MIN" mark embossed on the big plastic seam. That level or below is where the coolant warning lamp should start and give warnings. If the level is above that mark..you should not be getting any warnings.

If you have any doubt about the age or reliability of your warning sensor, again get the part from VW. Part number 251919372 for a late model Vanagon. About $28. Or if you have the early style sensor plug connector part number 5B0919372 about $36.

Most euro coolant tank reservoirs have a MIN mark and a MAX mark. If you fill your Vanagon reservoir to near full cold..do leave an amount of air at the top above the big seam MIN mark. This allows the reservoir tank to "self level" back and forth to the "overflow" tank. If you persist and keep adding coolant to the tank to keep it 100% full at all times you can crack the plastic tank. That air bubble acts a cushion and lowers the coolant expansion stress on the plastic tank.

Don't be alarmed if the air bubble remains in the tank. You tank may appear to be completely full hot and the bubble reappear cold..very normal. Those tanks do not need to be 100% full all the time..that is why there is a MIN mark on the tank from VW. Anything above that line by an inch or so is okay.

If the level cold keeps dropping and activates the warning lamp..you have a leak.

In that you have pressure checked the system..You could have a leak only when very cold. The two coolant sensors in the thermostat housing..blue and black that are for the temp gauge and input for the ECU have "O" rings on them. I found some of the aftermarket "O" rings will only leak when it gets very cold outside. Check yours in the morning after a cold night. If you see a drop of coolant..replace the "O" ring(s) part number N90316802..again from VW.

Climb under the the engine and look carefully for leaks particularly around the thermostat housing when the engine is cold if your coolant level drops below in MIN mark and activates the warning lamp. See the picture from Ben on the location of the thermostat housing and one of the suspect sensor locations.

[drj434343]

If I have a compression leak, wouldn't that suggest I could sniff for exhaust gas at the coolant reservoir while running using those sniffer kits? It seems like if it vents one way when off, it should be pushing exhaust gas bubbles into the coolant system when running.

[wcdennis]

What everyone is really saying, is there is a possible vacuum leak when your system is cooling off, so that air is getting pulled in, instead of coolant from the reservoir tank.

[AtlasShrugged]

[drj434343]

I should clarify. The level in the expansion tank goes up and down between hot and cold no matter what the outside temperature is. When the temperature is below 50 F outside, the level seems to drop enough to set off the level sensor. Anything above 50 F, the level still has dropped, but not enough to set off the sensor.

[djkeev]

What does the overflow tank level do?

It is empty?
Is it always full?
No changes at all?

Dave
_________________
Stop Dead Photo Links how to post photos

Ghia
http://www.thesamba.com/vw/forum/viewtopic.php?t=392473

Vanagon
http://www.thesamba.com/vw/forum/viewtopic.php?p=6315537#6315537

Beetle
https://www.thesamba.com/vw/forum/viewtopic.php?t=482968&highlight=74+super+vert

[DanHoug]

your description of the pressure tank level going up and down vs just going down suggests a poor pressure cap or a vacuum leak between the pressure tank and the overflow tank. the overflow tank is the one that should reflux with temperatures.

just as a point of reference, my pressure tank remains 100% full with zero airspace. the overflow tank changes volume. if your cooling system is in good shape with no leaks nor a combustion leak, 'burp' the air out of the pressure tank when warm by just loosening the pressure cap, anytime you see air. when the engine cools, it draws back coolant from the overflow.

the combustion gas detectors do work, have one myself that relies on a fluid color change, but it will be messy on a vanagon because the cooling system pulls a volume out of an unsealed pressure tank when you rev the engine and the pushes it back when the throttle is released. works fine in a closed system, pukes out coolant when open. i'd be inclined to rubber band a quart sized baggy over the pressure tank and see if it puffs up.

-dan