air con

Too much goes in, and this can cause problems. You can buy R134 at the auto parts store, and the necessary guages to put the stuff in. The directions are on the can.

ok but how much gas for a e39 1998 528i

If you buy the can of stuff, and the needed guages and thermometer, you will get the right amount. Frankly, the job is pretty easy, if you need to ask then you should see the professionals.

Follow the directions on the can and on the guage that you MUST have, and you will be close enough to perfect as to improve the cooling.

Have you looked at the microfilter. If this is clogged, there will be very little airflow, and even if the A/C is working well, you will be hot.