93 K75S Radiator Expansion Tank Operation

Anyone care to take a crack at explaining to me how the radiator expansion tank works?

I get that the coolant heats up and expands and can eventually rise into the filler neck container and down to the expansion tank via a rubber water hose. But how does the coolant return into the radiator?

As the engine cools down, and the coolant cools down, the coolant in the engine/cooling system shrinks in volume, creating a vacuum in the radiator. The radiator cap has two valves in it. One that relieves pressure to the overflow tank when the pressure exceeds ## PSI (I believe around 13, but I could be wrong on that..) and the second valve that opens when there is any vacuum in the radiator, allowing it to pull coolant back out of the expansion tank and back into the radiator.

This is why it's important that the hose connecting the radiator and tank be in perfect condition. If there are even hairline cracks in it - it will allow coolant to flow down to the expansion tank, but the hairline crack will relieve the vacuum and it won't pull coolant back, leading to a low coolant condition.

Coolant overheat warning lamp/idiot light

Thanks very much for that information Deilenberger.

My Haynes manual states that the oil pressure warning lamp will light whenever the ignition is switched on. However, there is no mention of the coolant overheat warning lamp circuit illuminating a warning lamp whenever the ignition is switched on. I seem to have an opening for that warning lamp, but I do not remember every seeing it light up when the ignition is turned on.

Does your ride have a coolant warning lamp and does this one light up with the ignition on?

Here is how to test the temp light(and other functions of the system).


http://www.ibmwr.org/ktech/fan-diagnosis.shtml

roncooper said:

Here is how to test the temp light(and other functions of the system).


http://www.ibmwr.org/ktech/fan-diagnosis.shtml

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Thanks Ron for that information.

When you turn the ignition switch to your 91 K75RT, does the over-temperature bulb in your instrument cluster light-up for test purposes?

ghyber said:

Thanks Ron for that information.

When you turn the ignition switch to your 91 K75RT, does the over-temperature bulb in your instrument cluster light-up for test purposes?

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No.

roncooper said:

No.

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Thanks Ron

OK, so then the quickest way of confirming whether that bulb is serviceable is by performing the grounding A3 to 31.

Maybe it is not possible to accomplish but it would have been nice to have this bulb light upon ignition contact just to confirm it was operational.

There are several other factors to consider. First, a radiator cap has a finite life, and will probably go out the same time the hose starts to leak vacuum.
I have a tester with adapters that can put pressure onto the system and determine where the leaks are.
It will also test the cap.
The other major factor that I have seen on cars occurs when the water level has leaked to a point where even if the hose and cap is good, the vacuum is insufficient to draw water out of the overflow tank.
Thus people burn up their (car) engine by doing nothing but add water to the overflow tank.
It is always necessary to let the engine cool, and check the level in the radiator itself. To be sure that the level is not so low that it cannot draw water. If there ever is much water loss, it is time to put the tester on to see where the leaks are, rather than risk burning up the engine. Then fix the leaks.
dc

Expansion Tank Overpressure?

My expansion tank, surrounding ABS modulator and righthand bag are smeared with dried coolant spots. The coolant level in the expansion tank is halfway between the full and low marks. I suspect the coolant it is being regurgitated out of the expansion tank rubber cover. Why? I really don't know.

Don posted a very informative response above and mentioned that the radiator cap has two valves, one of which relieves coolant pressure from the radiator to the expansion tank. Could the culprit be a defective radiator cap/valve letting too much pressure build up in the radiator then onto the expansion tank causing spills? Or is it simply the rubber cover with pin-sized hole in the nipple protrusion that needs replacement?

ghyber said:

My expansion tank, surrounding ABS modulator and righthand bag are smeared with dried coolant spots. The coolant level in the expansion tank is halfway between the full and low marks. I suspect the coolant it is being regurgitated out of the expansion tank rubber cover. Why? I really don't know.

Don posted a very informative response above and mentioned that the radiator cap has two valves, one of which relieves coolant pressure from the radiator to the expansion tank. Could the culprit be a defective radiator cap/valve letting too much pressure build up in the radiator then onto the expansion tank causing spills? Or is it simply the rubber cover with pin-sized hole in the nipple protrusion that needs replacement?

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I used to see that once in a while with my K75S, and never found any specific reason for it. If your cap is original, it's worth considering replacing it - it's close to 20 years old, and they don't actually last forever.. (plus they aren't horribly expensive.)

93 K75S Expansion Tank Coolant Level

deilenberger said:

I used to see that once in a while with my K75S, and never found any specific reason for it. If your cap is original, it's worth considering replacing it - it's close to 20 years old, and they don't actually last forever.. (plus they aren't horribly expensive.)

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I am the third owner of the 93 K75S. However, the previous owner was very meticulous. Unless proven to the contrary, I am assuming everything is original from 1993.

Are you referring to my radiator cap or the rubber cover on the expansion tank?

Could this phenomenon be attributed to a K75 quirk or oddity?

Before replacing parts, I wonder if lowering the coolant level in the expansion tank could resolve the issue? Any thoughts from current or previous owners?

The radiator cap.. the other one is to keep dust out of the reservoir.