Like the title says, I have an old polo 3 - maybe 20 some years old. It runs on unleaded fuel.The problem is that I keep having to refill it with cooling liquid once every two days, sometimes even every day. Many mecanics looked at the problem, but no one could pinpoint the leak. Does any have a clue about the cause ? or has anyone encountered such a case ?
"Many mechanics"?? Really? Clearly the coolant is going somewhere and if it's not running out onto the ground, and a few drips are not going to empty gallons of fluid in a day or two.
I'm going to suggest that your coolant is going into the engine and either collecting there or being burned and going out the tailpipe. If it's collecting you will find it in the oil and unfortunately that means at the BOTTOM of the oil pan where it will be used instead of oil to "lubricate" your engine. It's easy to check for this, pull your dipstick and if the level is way high and it's covered with brownish foam you might as well plan on getting the engine replaced.
More likely is that your head gasket is failed and the coolant is being sucked into the cylinder during the intake stroke and then being burned along with the gasoline. In this case you will only have telltale signs like a coolant smell in the exhaust, a lot of moisture in the exhaust (can sometimes be seen) and possibly poor performance.
Any competent mechanic should be able to figure this out and a compression test is helpful to diagnose as well as an exhaust gas analyzer.
Bottom line is that I think you need to find better mechanics.