strange coolant loss

[butch]

my car is losing coolant. i"ve checked all hoses and they are a tight, no leak under the car and on the water pump. heater core already bypassed.
No visual smoke in the exhaust, no oil in coolant or coolant in oil. could it be a faulty headgasket? If it so, then how to diagnose a faulty headgasket?

[pnd]

It depends how much coolant. i have had m20's that use a little coolant say1/2 an inch over two weeks and there was no apparent reason one of them had a new head and continued like this for years still is in fact. heavy coolant loss is another matter. try changing the cap on the expansion tank as these can leak.

[Brianmoooore]

What colour is the bottom of your coolant cap?
If it's black, get down to your dealer with your VIN, and ask nicely for a new one with a yellow disc on the bottom.
My wife's touring has used about a third of a litre of coolant a week for the two years that it's had that particular engine in it, and I'd never been able to find out where it was going untill a couple of weeks ago I was under it with the engine running, and caught sight of one drop of water rolling down the front from the water pump.
Pump is now changed, and the coolant level is staying where it should be.

[Brianmoooore]

Pull your coolant tank overflow pipe in from it's hole in the inner wing and fix a polythene bag over the end of it with a rubber band.
Drive the car, and see if any coolant ends up in the bag.

[butch]

Pull your coolant tank overflow pipe in from it's hole in the inner wing and fix a polythene bag over the end of it with a rubber band.
Drive the car, and see if any coolant ends up in the bag.

My coolant cap is still the original w/ yellow at the bottom, water pump is new. just did what you have said, tie a plastic bag at the end of the overflow hose and after 15 miles drive it is still dry. I will try check under the car while it is idling, hopes to see drops of coolant in water pump.
Thanks.