Fuel usually does not last longer than 1 year, he needs to replace the fuel not just add to it, this problem is very well known to those of us who own old cars that are stored sometimes for years. the tps sensor is leaking because the intake manifold is filling up with unburnt gas, Com'on this isn't very complicated if it runs on carb cleaner and the fuel pressure is fine than there is obviously something wrong with the fuel. Now tell him to clean out the intake manifold and completely replace the fuel before doing anything else.

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600SEL '91
300E 4Matic '88
240D '83
280SE '77
350SE '73
The most complex systems can fail in the simplest way.
Contra verbosus noli contendere verbis, sermo datur cunctis, animi sapientia paucis.

i don't believe in the lord! He's never bought me a Mercedes Benz.