2008 Suzuki Mehran Reservoir coolant decreasing

I don't know what "b/w low and full" means, but if I understand correctly, the coolant is expanding into the reservoir when it gets hot, and when it cools it contracts and gets sucked back into the engine. If that is right, that's why the reservoir has markings on the side that show a "hot" level and a "cold" level considerably lower. As long as you never have to add any coolant, the system is working as it was designed to.

Oh. Now I understand. What I would do is add a small bottle of dark purple dye, drive for a day or two, then search with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. For elusive leaks like this, don't forget to check inside the tail pipe.

Auto parts stores will have the dye for use with antifreeze, and those that rent or borrow tools should have a black light.

If there was an air pocket, it would have been under the thermostat and it would be burped out by now. The coolant level in the reservoir would have gone down perhaps an inch or two, but only one time.

We have to be sure the air is expelled when we service the system because that air under the thermostat will prevent it from opening, and the engine would overheat. A lot of thermostats have a tiny bleed hole in them to let the air circulate to the radiator where it will get pushed into the reservoir. After that, liquid will be drawn back in when the engine cools down.

A lot of engines have bleeder screws on the thermostat housing to allow for manual bleeding. On others we remove a temperature sensor or threaded plug when necessary.

The only way you would have to add a lot of coolant, as in more than a gallon, is if only the radiator was filled during the recent service and the engine block was empty. Even when the entire system is drained, there is still a lot left in the engine that doesn't drain out, so it will never be completely empty.

If the engine doesn't run hot, and the heater will blow hot air, the cooling system has to be reasonably full. After one warm-up / cool-down cycle, there won't be any more air in the system. At that point, once the reservoir is at the proper level, you should not have to add more coolant.

How are you noticing these bubbles? Are you running in front of the vehicle with the hood open? Bubbles indicate a leaking cylinder head gasket. There is a chemical test that will verify that. That involves drawing air from the radiator, with the engine running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.

Not sure how to respond to that because it isn't something we normally watch. My guess is the white stuff may be an additive in the antifreeze, like corrosion inhibitors or water pump lubricant.

The coolant has to flow back and forth between the radiator and reservoir each time it warms up and cools down. The only way it can't go into the reservoir is if there's a leak, (use the dye to find it), or there's enough air in the system to expand into the reservoir, and no liquid in the reservoir to be drawn back into the radiator when the engine cools down.

The next test is the chemical test I mentioned earlier. That involves drawing air from the radiator, with the engine running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If a cylinder head gasket is leaking, the liquid will turn bright yellow.