I'll second Jim's comments.
It would probably be smarter to keep the TTL level signals as short as possible — as close as possible to the ECU. In other words, the MAX232 chip circuit should be right at the ECU plug.
Even if you leave your adapter box as-is, shielded TTL and RS-232 cables may help (if they're not already).
You may be getting additional noise if your laptop is connected to a different power and ground than the ECU (are you using the cigarette lighter jack, perhaps?).
So that USB-to-TTL adaptor (where the chip is inside the USB plug and the long wire is TTL) may not be such a good idea. However, one of the companies has
this version that is a USB socket on a little PC board with the tiny chip right underneath it...
(img)
That could be attached to the ECU plug, with a USB A-to-B cable to connect to the laptop.