1) Over time, I eventually lose some coolant, but most of this loss (I
assume) is through the overflow tank overflowing...

2)No steam in my exhaust (no smells or external leaks either). Had my
girlfriend follow me. When the overflow overflows, my tailgate gets a
light film of coolant on it.

3)Compression on all four cylinders is equal (I can't recall the
reading, but my mech told me it was at 90% of spec, which he says is
exceptional for a 12 year old engine).

Any other input ?

-----Original Message-----
From: William Dummitt [mailto:williamd@compuserve.com]
Sent: 19 mai, 1998 15:16
To: INTERNET:Felix.Martel@CUM.QC.CA
Subject: Re:Air in my coolant - part deux

Hmmmm-am I reading correctly that coolant is disappearing, as evidenced
by
low level in the main coolant tank?  In other words, do you have to add
coolant after each highway trip to replenish the main coolant tank?  Or
can
you merely bleed air and go on your way without adding coolant?  I'm
assuming you've looked for external leaks and found none.

If coolant is disappearing, I would suspect you are losing coolant via a
cracked head (fairly common) into the combusion chambers, that would
explain most of  the symptoms.  Easy to check out-if it looks like you
have
some steam in the exhaust, that's the answer.

 If you are losing no coolant, a cracked head is still a possibility,
allowing exhaust gases into the coolant.  I don't understand how this
would
only happen at highway speeds and not at idle, though.  What about a
leaking washer on the radiator bleed screw?  If, as you say, the bubble
forms in the radiator, that's where it would probably come from.
Coolant
flow to the radiator is much higher at highway speeds (which is why you
have to rev the engine to do the Bentley bleed procedure) and that might
explain the symptoms.  But then I'd expect you could see the leak when
you
did the bleed.

This is a toughie.  I'd suggest a compression test- if you have a
substantially cracked head you should be able to spot which cylinder it
is.
 Most of the time, cracks are between the exhaust and intake valves and
you'll see low compression on the guilty cylinder.

Good luck, Bill