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The Hummer Knowledge Base
Testing for Cooling System Air Leaks
- With the engine cold (off overnight)
- Remove overflow tank cap to equalize pressure
- Replace cap
- Start engine, let idle for about 30 seconds
- Turn engine off
- Remove overflow tank cap and listen for escaping pressure
If any pressure builds up during this brief time, there is a leak between the combustion part of the engine and the cooling system. There are
many possible causes, including cracked head, cracked block, and head gasket failure.
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