For your 88 300CE? It is behind the battery, under the plastic water shield. It is silver with a red top with a hinged cover on it.

It would not affect the car's starting preformance when warmed up however. Only affects cold start...

__________________
-Marty

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
http://www.peachparts.com/shopforum/showpost.php?p=831799&postcount=13
http://www.peachparts.com/shopforum/showpost.php?p=831807&postcount=14