1975-76 450 Models
- Remove Red/Purple vacuum hose at EGR valve. Connect a vacuum gauge to Red/Purple hose and to Red connection of EGR valve. See Figure. Start and run engine at idle. Increase engine speed to 2500 RPM. At idle, vacuum gauge should indicate no vacuum. At higher speeds, vacuum should be present.
- If no vacuum is present, check vacuum hoses for damage and proper routing. Red vacuum hose from Red port on throttle housing should be routed to center port of switchover valve (angular port of Black thermo/vacuum valve on 1976 models).
Courtesy of MERCEDES-BENZ USA
- The Red/Purple vacuum hose should be connected to outer port of switchover valve (vertical port of Black thermo/vacuum valve, then be connected to Red port of vacuum control valve and EGR valve on 1976 models). Also check Red switchover valve (Black thermo/vacuum valve on 1976 models).
- Remove Brown/Purple vacuum hose from EGR valve. Connect vacuum gauge between Brown/Purple vacuum hose and bottom of EGR valve. Start and slowly increase engine speed to 2500 RPM. At idle, vacuum gauge should indicate no vacuum. During acceleration, vacuum should be present for a brief period until engine speed stabilizes at 2500 RPM. If no vacuum is present, check vacuum hoses for damage and proper routing.
- On 1975 models, Black vacuum hose from Black port of throttle valve housing should be connected to purge valve through "Y" fitting. From "Y" fitting, a White vacuum hose should be attached to White port of vacuum control switch. The Brown/Purple hose goes to Brown port on vacuum control switch and EGR valve.
- On 1976 models, Red/Purple vacuum hose must be attached to Red port on vacuum control switch. White vacuum hose should be routed to center port and Brown/Purple hose to Brown port of vacuum control switch. If hose routing is correct and vacuum is not available, replace vacuum control switch.
- Remove Yellow/Purple vacuum hose from vacuum control diaphragm on distributor. Disconnect both vacuum hoses at EGR valve. With known good vacuum hose, connect the Yellow/Purple hose with the upper and then the lower ports of EGR valve. Engine should run rough or stall in both instances. If not, replace EGR valve.