There are lots of places for a car to lose its coolant. The cooling system is a network of pipes, water jackets and other plumbing that is required to keep the engine cool, provide heat for the cabin and even cool the transmission.

Obvious places for a leak to occur are any of the joints in this plumbing, the water pump itself or any of the various components that store the coolant (heater core, radiator, overflow tank). A failed welch plug will definitely cause a car to lose coolant, but it usually starts out as a small leak before developing into a bigger one as the plug deteriorates further over time.

If the water is really running out as fast as your pour it in, the best idea is to get somebody to add water to the radiator while you look in the engine bay and under the car with a torch. A leak as big as the one you describe should be very visible.

Regardless of whether you use an additive, a car’s Diesel Particulate Filter (DPF) will still need to be cleaned (or regenerated) periodically as soot builds up in it. Short trips where the engine doesn’t get a chance to warm up properly, as well as extended periods of idling in traffic and stop-start running will all hasten this requirement.

In the case of the Captiva diesel, the best way to manually force a regeneration is to put aside an hour and go for a decent drive. The advice from Holden in the day was to travel at more than 50km/h and at more than 2000rpm (which may mean locking the car out of overdrive) for a minimum of 25 minutes. During this process, you should not allow the speed or revs to fall below those two figures which suggests finding a decent strip of freeway to carry out this process. You should also not turn off the engine at any point in this procedure. The broad idea is to get the engine and exhaust hot enough for the filter to regenerate and clean itself.

If, after 100km of this type of treatment, the DPF light on the dashboard hasn’t disappeared, the solution is a trip to a workshop to have the filter investigated and, potentially, hand-cleaned. Experience suggests a couple of attempts may be needed to get this to happen as it should, and in fact, the car’s computer will give you several chances to produce the desired effect before the workshop beckons.

The used Captiva does appear to be great value, but their shaky reputation for reliability would lead me to consider other options. The Territory would be a better bet, but they are known to have electrical and front suspension issues in the long-term.  If you're after an SUV, a seven-seat 2006-12 Mitsubishi Outlander is smart buying at the moment, but if you're just after the seven seats, I highly recommend taking a look at a 2008-13 Kia Rondo. These represent excellent value at the moment. Hope that helps, and keep us posted with your progress.