I had to have my old van e-tested last fall, did a tune up, oil change, etc. It failed miserably.
Decided to try the often lauded Seafoam for the first time. Put in 30 litres of premium fuel into my near empty tank with a full can of Seafoam, ran the van up and down the local highway for a few days and then took it in for another e-test. It passed, my carbon emmisions fell over 50 points.