I have never been to any engineering school in US but I went to a great one in UK, this is, United Kingdom just in case our honorable member do not know.

If one turns on the garden hose very gradually to back flush the heater core, there will not be any pressure build up if the outflow is greater than the inflow. If the core is 100% block then even a trickle will pressurize it as liquid is incompressible. Once the water comes out then one can increase the flow. As I have said, there is no risk to back flush the core.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.