I got badly fooled by just such symptoms last year in one of our '95 940s. Here's my tale and the lessons learned, so perhaps something in this for you:

Coolant slowly dropping, needed topping up maybe once a month. No visible coolant leaks at any of the usual places (like water pump, hoses, rad cap, expansion tank, heater valve), no evidence/smell in the interior, no vapour out the exhaust, so yet another mystery with an older car. The coolant level wasn't dropping that quickly so I figured, heck, it will show up sooner or later.

Then it started getting worse, most notably after a highway drive. I finally spotted weeping at the top of the water pump -aha, the mushroom gasket! So I took off the water pump and put in a whole new seal kit. End of leak there, but fluid kept slowly dropping. Hmmm!

About a week later, I get a panicky call from my wife out on the highway. Steam was shooting out from under the hood. When I got on scene, I found the plug in the top right corner of the rad had blown out (in your Regina, a thermostat is in that hole). Couldn't find the plug on the road, so I went to a nearby jobber supply, of course they didn't have a plug, but I hoped they had an expansion plug that would fit. Smallest size they had was too big. So time for a tow home with my head hung in shame as the neighbours always give me a bad time when this happens. Got a plug from a wrecker, pressed it in and decided I'd better use zip straps around the side tank this time (lesson learned the hard way after years of brickboard advice).

Obviously the rad pressure was getting a tad high, so just to be safe I installed a spare rad cap I had on hand (standard green one -wished I'd had a lower pressure black one used in 240s).

Already behind on other projects in my life, I decided put the car back on the road, checking it once or twice a day, while I pondered the possibilities. A head gasket leak was top of my list, but no real hint of vapour out the exhaust, coolant was clean and the engine oil was it's usual black self.

One evening I spotted fluid running down the driveway. A quick check under the hood showed the heater valve had bust open. After all the tales on the brickboard over the years, I'd learned to keep a spare on hand, so I installed a new valve.

Okay, my old 940 friend, that's the third strike and we both know the heater core is going to be next (done that job a few times on my other Volvos and we don't need a repeat). I've got more important things to do than you and my long suffering wife needs you safely back on the road. It's now time for one of your rare visits to the doctor, my trusted and highly regarded Volvo indy. Still with no exhaust vapour, he quickly confirmed it as a head gasket leak using a simple test. Fortunately the head hadn't warped. It was also a grand opportunity to clean up the valves and 22 years of carbon deposits (what an amazing improvement that made). All has been well ever since.

So, right now, before you go any further, go out and do this simple check for a head gasket leak. From a cold start, take off the rad cap and let the car idle. Left at idle, the coolant should normally not boil over, leastwise not in the first 15-20 minutes. If it starts to bubble and rapidly rise, it's your head gasket, end of story -attend to it now and be glad you found it without having to follow my footsteps!! If that's not it, then I'd say it's a good idea to keep it down to short trips and off the highway until you sort this mystery out.

I'm sure I'll be restating the obvious here, but it never hurts to be reminded. Carefully re-check each and every coolant system connection, including the heater valve and both ends of the water pump return pipe. Wrap paper towels around all the hose joints you can, including the base of the heater core nipples (@firewall). Get that engine up to operating temp until the thermostat is wide open and keep running it to keep the coolant pressure up for a good while. Let the engine cool (the system pressure will rise further just after shutdown). Check all the hose connections. Blot around and under the water pump, inspecting the weep hole on top of the water pump (insert a rolled end of paper towel if the weep hole is underneath). Blot under the rad hoses, under the rad (drop the belly pan for closer insection), around the heater valve, especially under the lower heater valve hose, the rad cap, the carpet corners at the tunnel, the passenger floor vent outlet, everywhere! Keep the doors and windows shut overnight to better help in early detection of a heater core leak using your nose (sickly sweet smell when fresh, maybe a hint of burnt when old). Are the windows more frequently fogging up? More vapour than normal out the exahust? If nothing shows up soon, have the rad cap and coolant system presssure properly checked. Think about treating your car to a new rad cap just on spec. Keep checking, daily if necessary, until the source of the coolant loss finally shows up.

Good luck!
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

I agree whole heartedly with a pressure test.

IMMEDIATLY!

If it loses that much in 100 miles, you might be close to a total failure.

ANY repair garage should have a radiator pressure tester.

You could probably buy one for far less than a failure on the road. I found one on the web (with only a question on Google) for $15. A tester from Stant only cost about $70

Who repairs your car? Have them do a pressure test.

If you do all your own repairs, your car is a '93, and you only repair the leak, you will need to test for leaks again soon!

An auto store may have a loaner.

Dear onlyVolvo,

Hope you're well. When were the heater hoses (and heater control valve) and the radiator hoses last changed? Hoses fail from the inside, so a crack may not be obvious.

Volvo brand hoses last for about 15 years, where ambient temperatures are low (US, Canada, Russia, northern Europe, etc.) In the US southwest, the tropics, North Africa, Australia's outback, most of India, etc., I'd guess hoses' working life is shorter, because of high average temperatures.

A pinhole leak can allow coolant to spray. If it lands on a hot surface, it will evaporate. Thus, coolant is lost, but few signs remains.

If you suspect a pinhole leak, wrap a white paper towel around the hose and secure the towel with tape. Even if the coolant evaporates, it will leave a green or blue-green tinge on the paper towel.

Are the carpets in the footwell moist? If so, that's a sign that the heater core leaks.

Hope this helps.

Yours faithfully,

Spook

If you haven’t already, to cover all the bases, check your hoses and the expansion tank. Particularly the heater hoses. Maybe there is a little seepage at either the junction or perhaps at the engine end. It will probably evaporate before you see it. That might explain the smell. Check right after the thermostat opens before it gets too hot back there. Leaks at other hoses could evaporate too.

Years ago I had a slow leak at the bottom of my 90 240’s expansion tank. Took me forever to find it as it always evaporated quickly there too.

Have you checked the passenger compartment floor for dampness and coolant smell? Lift carpet.

Pressure test is a good plan to expedite your search.
--
Will I buy another Volvo??? We'll see....

You need to pressure test it. Rig up a garden sprayer w jy coolant cap. W some luck it will be an external leak vs. the heater core. At 10 - 15 oz per 100 mi. The leak needs some attention

Good luck,

Marty