If the head gasket is leaking it will usually start out by allowing the extremely high pressure of the combustion burp into the coolant system first.
It may take pressures of several hundred PSI which are present during combustion to push past the gasket at first.
This is why you get air in the cooling system long before you get coolant in the exhaust.

Putting 15 psi on the cooling system to determine a leak is nothing compared to 170 psi  for compression or the hundreds perhaps thousands of pounds of PSI when you ignite the gas.

You can use a smogg sniffer to determine if there are hydrocarbons present in the coolant, that is a sure fire way to know also.

Doug

  ----- Original Message ----- 
  From: ctonline@webtv.net 
  To: Carroll Smith 
  Cc: vanagon@yahoogroups.com 
  Sent: Monday, May 31, 2004 9:02 AM
  Subject: Re: [vanagon] bubbles in my overflow tank?

  If you use the approprite expansion tank pressure testing adaptor for a
  Vanagon, and pump the testing equipment to the reommened pressure--if
  the head gasket was leaking into the coolant water jackets--you'd see an
  immediate drop in pressure--

  Better yet---

  Pull all the plugs outa the engine as your pumping up the pressure
  --you'll be able to use another tester to diagnose the problem--not an
  expensive item either---your ear.

  You'll hear the leak into into the combustion chamber, immediatly.

  You can't test or shouldn't test the cooling system with the engine
  running--

  Right tool for the right job, at the right time---:>)

  I've seen a compression test fail to read a head gasket leak many
  times---but if you pressurize the cooling system on the same
  engine---it'll show a leak at the head gaskets--

  Why?

  Hell--I don't know--it's just the way it is--:>)

  Adios,

  TK

  <<On one occasion I modified a radiator cap (not Vanagon) to accept a 
  pressure gauge on suspicion of a head gasket leak on the notion that a 
  head gasket leak would build up pressure  on the coolant side with 
  running.>>

  To leave this list send an 'empty' message to <mailto:vanagon-unsubscribe@yahoogroups.com> 

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