This is a 99 S420, 310k miles, with 114.3 engine and an EGR problem. 2nd air injection pump failed and replaced. Before and after that event engine temp would be a normal 80c for 20/30 mins then climb to 100C. Sometimes it would drop back to 80c but rarely did/does. Every vacuum hose has been checked, every injector checked, control module checked, every spark plug checked, every coil checked, but ECM keeps throwing P0410 code. The high temp bothers me because I believe it to be related to a high CHT which could result in a burnt valve. 25-year MB tech has gone over it with a fine tooth comb. He says he's seen this before and the only solution that he found fool proof is to rebuilt heads. Seems there is an air channel in the heads that allows the recirculating exhaust gas to enter cylinders for re-burning and when those openings get clogged from varnish/sludge, the P0410 code starts showing up and rebuilding is the only solution he has every found that works. That's a 2k+ job so I'm looking for a chemical solution to remove varnish/sludge from the head's air channels but can't find any access to the chamber through which the air travels into the heads or any chemical compound that others have successfully used in the gas tank or oil. I'm aware of SeaFoam and its competitors and have poured a great deal of that stuff into the gas tank but nothing like that has had any effect. My research across the internet has not turned up anything else helpful, except "bite the bullet and rebuild the heads". Hoping someone here has seen this problem before and found a more cost effective solution. Any suggestions or advice would be appreciated (except "bite the bullet" recommendation).