One note on the monovalve. If the plastic housing on the monovalve is not cracked you can easily buy a new "insert" to fix it as opposed to replacing the whole thing. The insert (I think it may also be referred to as a cartridge) is much cheaper than the whole monovalve. I think the $30 quoted is for the cartridge. Fastlane has them. By the way, it's located on the firewall between the back of the engine and the battery. It has two heater hoses attached to it and a small wire plug attached on the top. It's a bit of a pain to remove but not too bad. You should be able to do the whole job in less than an hour.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold