What I have is a 91' NA, it's reading 270,xxx km at the moment.

I'm pretty sure the previous two owners did not do an engine rebuild; however the car runs quite well since last July when I bought it.

Within about one month time, the engine has had two slight symptom of flooding where I had hard time starting the car up. Today while driving when I rev pass 4,000 rpm, there were clouds of smoke coming out from the exhaust....

I parked the car and checked for leaks, overheating, or signs of broken parts but found nothing, tried revving pass 4,000 rpm again and there were no smoke coming out this time.

I'm wondering if this is a sign of major flooding issue in the future, and whether I should do an engine rebuild immediately.

 

Compression check can give somewhat of a baseline on engine health, also check coolant for any signs of oil or combustion gases/etc, and check oil for signs of coolant.

was it white smoke or blueish smoke?

 

Neither of those, coolant level and oil level are both fine, no overheating, that's why I'm not sure what is going on.

 

Like Brodie says, have a proper compression test done on the engine. If it is, as you believe, the original engine, you can almost count on low compression. My guess is that it is not the original engine, so do the compression test.

If the car runs well otherwise, drive it until it doesn't. Deflood the engine by holding the gas pedal on the floor and cranking the engine. In an S5 such as yours, that cuts the fuel to the engine.

 

Alright, thanks for the tip. I will ask for a compression check when I bring it over to my mechanic next time.