Where Did All My Coolant Go? Disappearing Antifreeze.

How to replace a thermostat on a Geo Metro, or where did all my coolant go?

So my 1996 Geo Metro (1.0 3 cylinder) has had a mysterious loss of coolant. I suspected it might have a leak, but could never quite find it. Of course your first worry is that it has a head gasket leak, but none of the other symptoms, like smoke, or contaminated oil were there.

It's the case of the disappearing antifreeze.

What made it even more interesting is when I filled up the coolant reserve, and radiator with fluid, I discovered the car ran 10x better and got better MPG.

So what was going on?

In addition to being the wrong size,
 the OE seal and thermostat was
 falling apart. OE temperature is 190F.
Here to find out, Geo, Suzuki or General Motors made a poorly designed thermostat seal which leaked from the factory. It ended up being corrected in a later service bulletin, and subsequently the part through the dealer, I'm told, is corrected too. However buying the part at the dealer, a $30 investment, seems silly. (They sell the thermostat and gasket as one piece.) The problem arises with auto part stores like Advance, or Autozone, Carquest and Napa (I went to all four) as their replacement thermostat seals is too small. Some don't even show the seal in the computer. They (the wrong ones) typically range from .13" -.15", but the deck height on the thermostat housing is closer to .18". The best aftermarket seal I could find would visually not leak, but if you took your finger and swiped the underbelly of the housing, you'd find it moist. It's worthy to note there were also two different housing designs. This write up concerns the deeper housing design. It should be obvious upon disassembly.

This is the seal most auto part stores show.
 As you can see (left side) the seal is lower
 than the flange.
In fact, I'd be willing to bet, many people who are driving Metros, or Swifts out there have a weeping seal and may not know it, due to the refilling of the reservoir at oil changes. 

To make matters worse, a major coolant sensor that determines when the vehicle goes into lean-burn mode is located at the top of the throttle body (or in the thermostat housing on later models). A weep, introduces air bubbles into the system on acceleration, (sucks it like a straw) and they go straight to the highest point. Guess where that is on a metro? On one occasion, I could tell the car wasn't going into lean burn mode. The moment I got the right gasket, it worked perfect. Who would have guessed a $4 part could cause such an conundrum?

So to clarify, I hunted down different gaskets and finally found one that works perfect. If you have any of the following symptoms, I highly recommend replacing the thermostat seal:

    -Weeping thermostat housing or coolant loss (obvious)
    -Low MPG / sluggish performance.
    -Mysterious loss of coolant with lack of a leak
    -Poor heater function, or vehicle doesn't want to heat up.
    -Green coolant and you suspect the thermostat has never been changed.
Here is a comparison of the Neon seal
 (white) next to a new Metro seal (black).
Huge difference.

To clarify, I've got a guess at why this problem exists. Originally in the 1990's, many of GM's vehicles were filled with Dex-Cool, an orange organic acid anti-freeze designed as a proprietary coolant which was later known to expand (it's technically a plasticizer) and eventually "eat" seals. (There's a few lawsuits out there over it.) With the demise (or de-popularization) of the orange anti-freeze, most people swapped out for the green, and by doing so, found in some instances, the seals in these vehicles would no longer seal correctly. 

That said, putting the factory seal from any auto parts store in a Metro is a mistake. It was designed based on the specs of the Metro using Dex-Cool, (or so my hypothesis goes) not the traditional green anti-freeze. Even if you don't think it leaks, it leaks. Try a paper gasket, (tried that) sealant, (and that) and you're still going to leak. I know, I tried it all.

The finally solution was to track down data on seals, and see if I could find a better solution. The answer is a thermostat seal from a SOHC 1995-1999 Dodge Neon. It's about 2x the thickness of the factory seal, slightly oversized O.D, and works perfectly. I combined this with a 180F thermostat, and haven't had a single leak yet. The vehicle warms up quickly, and stays at temperature with no seeping.

Here's how to check if you might have a crap seal.

Take your pointing finger and swipe the crease between the thermostat housing and the water neck housing.
Do this when the car is cold, hot, and after having cooled for 30-45 minutes. With the number of previous attempts I made, I found that often the car would not leak when hot (due to pressure and heat expanding the seal), and often not when cold (because it had already leaked out). Often the heat of the engine is enough to evaporate the water. 

If you find any water or moisture, go to Autozone (or similar) and ask for Fel Pro part # 35588. (Federal Mogul). [1.615" ID/1.989" OD - .25" thick ] Place thermostat in the seal. (Seal's inner diameter is slightly oversized on the thermostat, but it works just fine.) Then equally compress the two housing bolts till sealed. 

Though I have not tried it, I suspect you could simply replace the entire seal and thermostat with a Dodge Neon version, but I used a Metro thermostat (44 mm) and the Chrysler Product 4, seal.

Here you can see how well this seal works.
This is just before we compress the seal.
After tightening down, there will be no gap between
the housings, and this material is
 compressed to form a water tight seal.
This looks identical to the pictures I've seen
of the GM thermostat used at dealers.
Problem solved. 

Now, go save your Metro. (or Suzuki Swift)

Considering the widespread incorrect use of the wrong sized seal, I imagine a good number of head gaskets and burnt valves have been the result of a crappy thermostat seal.


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