Hubby's Combo is losing water but we cannot find where its going. Noticed the expansion tank was completely empty last weekend. Topped up, checked following day after his 15 miles each way trip to work, and it was empty again. Looked all over the engine, no sign of a leak. Inside van is dry so not the matrix. Its running fine, no change in normal temperature, warning lights etc. Had MOT last Wednesday, so while it was over the pit, they ran the engine and looked all over - nothing. They then parked it outside, still running for 15 minutes, with some cardboard underneath to show any drips - nothing!

Needed it for work, so he topped up the water and brought it home. Expansion tank was again empty - left the cap off when he filled it to the max mark and started the engine. Bottle immediately emptied. Left engine running and topped up again, but level stayed constant this time. Assuming perhaps some form of airlock, which had cleared, he went to work in it again today, only to find expansion bottle empty again tonight. Did the top-up, cap off, start engine routine again - bottle immediately emptied, but on filling again, once more stayed full.

Its booked in next week for a pressure test, but if we can solve it in the meantime, might save the cost of this. We are baffled. Any suggestions please.