Honda Accord 03-07 CDTi leaking coolant, no drips, no steam, no mayo

Question

My car is a UK Diesel Honda Accord 2.2 CDTi

So I've had a problem with my car for about a week, it started overheating and not accelerating when I put my foot down on the accelerator (losing power)

I realised after the first time it happened that it's because the coolant level was below the minimum in the reserve tank

So I topped it up with about 0.75 litres of tap water and the engine temperature was ok after that, drove it a couple hundred miles over the next few days and then had to top up the reserve tank again because it was low, but the car did not have any problems with overheating or losing power

A mechanic friend brought over a pressure testing kit which he used to increase the pressure on the coolant system to 1.5 bar (the system is rated to 1.3 bar) and we ran the engine for about 20 minutes but couldn't see any water leaking from anywhere in the engine compartment, water pump, any of the hoses, radiator or exhaust

There's no white gunk in the oil filler cap, or in the dipstick so I don't think it's a gasket

I can't see and haven't seen at any time water dripping from anywhere underneath the car or in the driver or passenger foot well

Any ideas where the water could be going? It must be about 3-4 litres now over a week, the coolant system holds 6.3 litres according to owners manual

EDIT: I've ordered a UV dye and trace kit to see if that helps trace the problem, will update once I've had a chance to use it

UPDATE 28-07-2019: Have tried the UV dye for a few weeks now but still haven't been able to find the source of the leak, tried another liquid called FIXT engine coolant stop leak but I think it's still losing coolant, perhaps more slowly than before. The car itself runs fine otherwise

Answer 1

If its not in the oil, not in the cabin, and you are really really sure its not on the ground, then it must be in the cylinders. This is consistent with the loss of power earlier this week. A leak down test could help to confirm this.

Here is a bit of maybe-false hope for you though: You don't really know for sure how long its been low on coolant. An empty overflow tank and a partially full radiator may still keep you out of trouble longer than you would expect. Considering your heater has been intermittent for months, I'd ponder that the problem has been slower coming than it seems.

The first thing to do is figure out, when cold, if the radiator is full, and how much water is in the expansion tank. From there, you can figure out exactly how much its losing over a week. This tells you either "a bunch (more than a liter)" or "just a bit(1/4 of a liter)"

"a bunch" leads you to what you already know: the cylinders

"just a bit" could lead to a re-examination of the "on the ground" possibility. Assuming the diesel looks like other Hondas from that era, there is plenty of places a small amount of fluid could pool and evaporate without your knowledge. A drip could travel along the bottom of the car for quite a ways. It might not even make it to the ground if its losing, say, 30ml a trip for the last 6 months.

Make sure it isn't gonna freeze anytime soon if you are running tap water. And whatever you do, keep a close eye on the coolant level until you get it resolved.