Completely sealed?

It’s a routine matter for an experienced automotive repair shop: replacing the coolant thermostat. Or at least you would think so. In reality, however, this standard repair poses quite a challenge. So here are two important tips right off the bat:

1. NEVER USE SEALANTS

2. ALWAYS LET THE AIR OUT OF THE COOLING CIRCUIT

PASTE MAKES WASTE

The coolant thermostat performs numerous important functions in a vehicle. That’s why it must be replaced as quickly as possible when found defective. Many people use sealing pastes in this situation. Not to put too fine a point on it, but if you’re one of them, then stop the waste and lay off the paste! At least for “repaired” vehicles—otherwise you’re just asking for leaks, failures, and other consequential damages.

THREE WATERTIGHT REASONS AGAINST PASTES

So why shouldn’t you use (un)sealing pastes with thermostats?

GASKETS AND PASTES DO NOT GET ALONG

Thermostat gaskets consist of a complex material composition and often contain compounds that are not oil-resistant. Sealants, however, contain mostly mineral or synthetic oil components. If they come into contact with the thermostat gasket, it swells and is ruined.

A PERFECT MATCH GONE WRONG

The thermostat gasket groove and the dimensions of the gasket are perfectly matched. There’s no room for additional sealant. If you do apply a sealant, the overall volume increases—and you can forget about the perfect match.

CAUTION—ROAD BLOCK IN THE COOLING CIRCUIT

Frequently, so much sealant is used that part of it ends up in the cooling circuit. This is particularly deadly when sealant particles swell up and block the coolant flow. Sometimes they even accumulate in the thermostat, preventing it from opening and closing properly. This can ultimately lead to inadequate heating and cooling capacity.

THEREFORE: WE AT MAHLE AFTERMARKET STRONGLY RECOMMEND YOU REFRAIN FROM USING SEALING PASTES.

LET THE AIR OUT!

This thermostat could no longer close completely. Why? Because sealant was used, which had deposited in the housing.
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But what can be done if temperature problems occur after the thermostat was changed? First of all: let the air out! If a problem arises with the temperature after changing the thermostat, this usually points to air in the cooling circuit. Hence, before you change out the thermostat you just installed, thoroughly bleed the cooling circuit!

To learn more, read our Technical Messenger feature “The how and why of bleeding” in the 06/2016 issue.

TECHNICAL MESSENGER

We regularly provide technical tips relating to the powertrain, thermal management, and mechatronics in automobiles.

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