My 1999 Vanden Plas SC with 96k miles started displaying a low coolant light. After the first top off it went away for about 2 days. Then reappeared. Top off again and another 2 days it appears. There is no visible leak and the light temporarily goes away after a top off. I noticed a hissing noise after the last top off but it was coming from inside the radiator. My friend said air could be stuck in the system. I have no idea what could be causing this. Any help would be appreciated.

 

The float is sticking in the overflow bottle. Google it

 

Test the coolant cap and the heater hoses.
Feed and return under the manifold AND the 'octopus' hose above the gearbox bellhousing.

 

Your problem is obviously not in the coolant light but in the coolant loss. I don't think the hissing sound you heard is from "inside" the radiator but possibly coming from a small coolant leak somewhere in that area. Inspect carefully the main radiator, the charge cooler radiator, the water pump and the associated hoses.

 

I had the same issue for years. Ended up to replace the brand new reservoir tank and sensor it never came back again.

Used parts wont work.

 

keep driving the thing until you get a constant low coolant light

if it’s actually low you have a leak

 

Every time this happens to me that I keep losing coolant and keep adding coolant it’s from a head gasket? Slow leak I never get lucky what’s your oil to see if water is getting in it ..

 

Go to your local parts store and buy fluorescent dye and a UV flashlight. Even if small leak that evaporates quickly you’ll be able to spot it.

 

I thoroughly inspected all the hoses and even took the fan off the radiator to find no leaks. I ended up doing a heavy duty coolant flush. There was loads of gunk in the system and a lot built up around the bobber in the reservoir tank. I flushed it, put new coolant back in, drove it about 100 miles with no light so far.

My guess is that some point during previous ownership multiple types of coolant were used to top it off but it was never flushed causing the bobber to have so much gunk on it that it would stick to the bottom.