i2c bus debug session

Hello crazy engineers,
I need some crazy/not so crazy advice on fixing my i2c bus. The problem is clear, the SDA line is not being pulled low enough to register my data bits as logic zero.
I’m using an ADuC842 as the master and a PCF7584P i2c expander as a slave.
It a pain in the ass cause the circuit looks fine, the SCL is 5V and uses the full 5V pk2pk when transmitting but I can only get SDA to go to 1Vpk2pk at about 4V steady.
It could be an not so grounded ground or something like that, but in that case the 8052 would be going bananas so it can’t be that.

All replies are welcome,
Donagh