Gm upper engine cleaner
I work at a gm dealership and of course bought a 98 ford ranger, I also work in a gm shop.
About a month after i bought my truck the marble knock started knocking, being a perfectionist it really annoyed me so i started trying to find everything it could possibly be. Using alldata which is a site we use to pull up history of particular vehicles and info about them i came across the carbon build up that all data said it should be. And it fit perfectly especially after reading this site. To be honest i was getting scared untill i read this site because of the fact the i ran gm upper engine cleaner through it twice and it did nothing.
I thought about pulling the heads off and cleaning it myself and taking a look at if it was the problem but after reading this site i know this hast to be what it is because the description is exactly what happened to me, marble knock right after and oil change only at low rpms.
Im going to run a quart of transem through it (transmission cleaner) and take out a quart of oil and then put the quart of thransem through it and let it run for five mins then replace oil and oil filter. If transem doesnt do it then the only option is to pull off heads and clean by hand.
Ill reply with how it went soon.