My '94 940 turbo is losing coolant and I can't figure it out. Sometimes the overflow tank will be empty after 20 miles of city driving, sometimes it will be half empty after 60+ miles of highway. The emissions place ran a multi-gas test on the coolant, but said the gas levels were normal. I need advice, please.

Overflow tank has a new green aftermarket coolant cap rated at 150 kpa. It is properly snugged down for normal operation. I replaced the overflow tank a couple of months ago because it was ballooning. All coolant hoses are soft and old but none are leaking. I have a full set of replacement hoses which I will install this weekend. Water pump is less than 2 years old and operating normally. And the thermostat is new.

I can clearly see overflow marks from the overflow tank cap after normal operation. There are no obvious leaks when running the engine to operating speed on blocks.

With the engine off and cool I pressurized the coolant system using a bicycle pump just to the point where the coolant tank cap overpressure release vents excess pressure. In this condition there is a slight coolant leak -- a few drips -- in or near the coolant drain on the lower turbo coolant line.

If I run the engine until it reaches normal operating temperature (heat gauge at halfway mark) with the cap off the overflow tank, the coolant is moving from the water pump but its level does not change. If I then rev the engine once, "vroom!", a bunch of air bursts into the coolant tank just as the engine slows down. This causes coolant to slosh out of the uncapped overflow tank, lowering the level half an inch or so. Obviously I don't drive like this -- call it a creative diagnostic procedure. This air could come from the turbo through its small coolant hose, but that small hose joins the larger return hose from the radiator just below the overflow tank so I cannot be certain of its origin.

So, here are my questions:
1. Would a turbo with coked up oil lines cause this? (I.e., overly hot turbo boiling the coolant.)
2. Could the turbo have a air-to-coolant leak on the pressurized intake side, which leak only occurs when the turbo has boosted the intake pressure past some threshold?
3. Or is the problem someplace else in my engine head or block? (But if so, wouldn't there be exhaust gases in the coolant?)

Thanks for your help.