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Coolant Tank Mystery - Low Coolant Light Cause

New  09/30/2004, updated August 7, 2011

August 7, 2011

Here I am roughly 6 years later and today I had the "pleasure" of replacing the PS pump on that same car. Since it still had the OEM degasser at 180K and it showed the expected cracking on the bottom I splurged on the $38 part to replace it with the clear one from the local AutoZoo.

Once the old one is out I sliced and diced the factory tank up and removed the "sinker" formerly known as a float. and sure enough - it sinks. There is a lot of fine cracking in it but nothing alarming considering its environment. And when I say it sinks I mean you put it in a 12 ounce glass of water and it isn't even a second before it is resting soundly on the bottom.

I still blame the Prestone flush chemicals but without a known good Factory float and some spare Prestone flush chemicals (preferably whatever formulation they were using in 2005 as well) I guess I will never know. My suspicion is that the concentrated flush chemicals are possibly too much for the float to handle and if you added water to the car and filled it up to the cold fill to flush then added the chemicals the degasser has a very concentrated mixture until the water pump gets turning and mixing things up. Furthermore if the flush chemicals are lighter than water they will lay on the top right where the float is until they get adequately mixed in.

Leaky PS pump is now in the trash, what a PITA to get that output fitting disaster to not leak. Holding your tongue right and a few beers for patience is imperative.




As some of you know, ever since the convention the 99 has had the infamous
"low coolant" issue every time it is driven the light comes on about 5 - 10
seconds after the car starts.

I finally took advantage of the beautiful Ohio weather today and tore into
this thing. This problem started intermittently immediately after I did a
flush and fill which was right before we left for California (the day
before). The light was on about 100 miles after we left home and was
persistent, level checked good every time. I investigated the system
deeply today finally and here are my findings.

Sensor and associated wiring to it check out as good, the sensor can be
removed from the bottom of the tank by prying it out (the stem may crack
like mine did unless you can pry from opposite sides at the same time to
stay straight), the tank will still remain sealed as the sensor probe
itself goes inside of a tube that the float runs around. The sensor is a
magnetic reed switch (much like what your house alarm uses on the doors
just in a different style). The sensor itself is easily tested with a
multi meter and a refrigerator magnet.

Review the pictures below for further clarification. My end findings are
that the float is no longer floating and it is not binding or sticking to
the shaft upon which it runs (which is hollow for the sensor's probe). If
I flipped the tank upside down the switch opened up as it should, but no
matter how full of water it was the switch would not change state. With
all the water out if I pulled the sensor out about 3/4" the sensor would
change state as expected. (open is when there is no magnet present - float
is above sensor).

Only possible theory I have is that the flush chemicals I used did
something to the construction of the float to prevent it from floating
anymore. For now I have unplugged the sensor and no more light. Maybe it
is possible that the magnet is affixed to the bottom of the float and
somehow the flush chemicals broke that bond and maybe the float still
floats but the magnet would obviously sink..

Question is if anyone has one of these tanks that is cracked or something
but that is still believed to have a good float and shaft assembly that
maybe we could take that float out and determine what causes the float to
no longer live up to it's name (certain types of flush chemicals maybe???)
and save many people the whole problem of replacing that stupid tank.

I considered trying to put another type of floating substance underneath
the existing float to assist it in not being so "buoyancy challenged" but
it is almost impossible to get to (the pics make it look easy but it is
back in there a good 4 or 5 inches) and you would need something that
stands up to the temperature (250 as a rough extreme number). and you would
need to be sure that it couldn't come off and get sucked into the cooling
system and tear things up.

Without further ado - Pictures:


The Lab            The Subject            The Tank

Scott Krietemeyer
99 TG Neutered coolant tank.
96 MG


WRC to the illustration labeled "The Subject" Awww never mind!

For anybody who has not been living in a cave, you need to understand that in some instances that Low Coolant Light is not just a FUBAR Sensor - See the SOWHAT9 - The Complete Story of What Happened to Her Engine . It's not always as simple as a float.


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