/diy/ - Do It Yourself

I apologize in advance if the question that I'm about to pose is stupid, profane or downright retarded. I didn't pay attention in my HS physics class and the only reason I'm posting here is that I'm in a bit of a pinch and I'm hoping that some kind anon will point me in the right direction.
Here goes: I'm battling a solenoid valve that is causing electrical noise to be introduced into my car's electrical system. The noise manifests itself as a false RPM signal. The solenoid is controlled via PWM (around 300kHz). Through the power of the internet, I've found out that I probably need to install a flyback diode, so I did just that. I wired in a 1N4001 diode as shown in pic related. However, while it did help reduce the noise (it now appears intermittently and less frequently), it did not make the problem go away altogether. My main question is this - is there a diode with different characteristics that would somehow filter the noise better? I also wonder if there are other things I can do to reduce the noise, or even if the solenoid valve has gone bad and a new one would be far less noisy.
Thanks in advance to anyone who bothers to read or reply.