RE: 1991 Mercedes Benz 500SL... when I put gas in the tank it doesnt leak until I start the car & then it leaks profusely underneath the rear passenger side.

Change gas cap.

It will be provided with the original manual/paperwork for the vehicle. If you are unable to find this, you will need to take the car to a dealer. They can provide the code with the serial number and the car's VIN.

defintly a valve,regulator or gas line leak,or injector leaking rail pressure

CHECK HYDRALIC FLUID IN TRUNK UNDER THE SPARE TIRE

Check for a vac leak. Check all small vac lines, Benz is noted for plastic lines that crack. Look closely around air filter and work your way out from there. Also run a bottle of STP gas treatment through it. This time of year with big temperature swings, condensation (water) forms in fuel tank and that wreaks havoc with Benz induction.

there could be a problem with the crank position sensor

pls locate where the leak exactly coming from.....it could be from the high pressure line O ring is not properly set or not tightened...

I'm sorry I couldn't help you ... seems to me that info should be on the cover some place.

I heard Mercedes is having a 1/2 price sale this week, so your help for just over $500,000

You Will Need to Replace The Flex Hose & Or the "O" Ring inside it.

I just had the same problem. All of the fluid leaked out and I had to get the three small engines that move the top replaced. It was about 2200. I hope this helps