Sixto The way I determined it to be the checkvalve was to turn the key without starting the engine and wait for the fuel pumps to pressureize the system. When I did this the car would start in 1-2 seconds if I did not do that it would take 5-8 seconds (overnight) and 15-20 after several days of sitting. It would also run very rough for a few seconds after it started until the pressure came up. A fuel system pressure check would be the proper way to diagnose this for sure.

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Dave
88 300E 250K Sold
2000 C230K Totaled @104K
2003 ML320 156K Sold
2009 E350 4Matic Sport 46K Sold
2011 ML350 60K Sold
2014 GLK350 46k
2012 GL450 55K