Phantom Coolant Loss

Maintenance/Repairs
1

I had my SUV (2000 Ford Expedition) tuned up & it’s purring like a kitten. Thanks for the advice everyone. So that leaves me with only problem. I’m losing about 1-2 quarts of coolant every couple of days. While I was having the tune up done, I asked the mech to try to find where the coolant is going. He couldn’t. He did say that they’d pumped the system up to 18 lbs (it’s got a 16 lb cap) and it held for 45 minutes, so I’m a little (little-little) less afraid that it’s my head gasket. Questions are these:

  1. Could my 16 lb cap be at fault? It got pretty gunky when I mixed the coolants (see my post - Don’t cross the streams).
  2. Could my rear heat exchanger be leaking and where would the coolant come out? It’s got rear air and heat that’s controllable form the second seat.

I never see water on the ground or dripping. No white exhaust to detect. Doesn’t run rough at all at startup. In other words, none of the traditional head gasket indicators.

Help!

2

I’d replace the cap first. That’s cheap and easy. It could have very well been compromised with the coolant snafu. Since the pressure test your mechanic ran typically replaces the cap with a pressure adapter, the cap wasn’t part of the test.