My friend with a 1980 Mercedes 450SL has convinced me to do some minor tune up work on his car in the past. My area of knowledge is pre-OBD2 Japanese cars, and so far, the Mercedes has taught me that German car design seems to be based mostly on the tomb traps in the Indiana Jones movies.

A recurring request from my friend has been that the Mercedes "does not drive the way it used to". I got more details out of the last conversation: The car is not pulling hard before it warms up. The car had this same problem "long time ago", and the problem was a part "with a rubber diaphragm". This part had already been discontinued 20 years ago, and he found a guy who figured out how to rebuild this part. That guy is, of course, long dead and gone now.

Several hours of internet searches later, and I am thinking this is sounds like the "Warm Up Regulator".

I am seeing that these things are $2,000 new, $800 rebuilt, $200 used with no guarantee, and $50+ for a rebuild kit.

I have not looked at the one on this 1980 Mercedes 450SL, and do not know if it has vacuum hoses attached to it, or an external adjustment screw/bolt. The internet searches do not seem to get specific enough to show the one on this particular car, and return the range. It looks like the ones with vacuum hoses involve some more fiber gaskets not pictured in the rebuild kits.

There are some instruction write ups that say the WUR needs to be adjusted using an expensive pressure gauge set. Even one write up describing that the ones without any adjustment screw/bolt can (should/must) be adjusted by disassembly and manipulating the position of post that attaches the bi-metal arm.

Meanwhile, the guy selling the rebuild kits says just to rebuild it and do not touch any adjustments, because they were set correctly at the factory and should not be tampered with.

I’m up for disassembly and rebuild, but I do not have the gauges to adjust this thing (if it needs to be adjusted)(and I'm not up for the $250+ investment).

Is this something that can be rebuilt and bolted back on without problems, or is this something that has to be done by a Mercedes expert?