Hi I am new hereI am having problems with my 1998 blazer 130000 miles 4.3L All the sudden It started runing rough missing I got it home and read the
codes P0300 and P405 misfire and egr valve flow sensor A circrit low impute. Could the EGR valve cause it to run this rough? Or are there things I am missing

 

The EGR valve can definitely cause it to run rough. I would start by removing the EGR valve and cleaning it. Reinstall it with a new gasket. Gaskets are available with an integrated screen on the inlet side.

I have the P0405 code diagnostic all ready to post for you, but we are experiencing problems with the forum that are prohibiting me from posting it. When I get some more time, I will have a look at the diagnostic procedures and post up what I think will be the most beneficial parts.

*EDIT* - This code is particularly difficult to properly diagnose without a scan tool. Basically you can only test the continuity between the terminals in the connector and those at the PCM.