I have recently upgraded my 1996 Blazer with a new MFI spider and got rid of the CFI one which was causing fuel pump failure and rough idel because of over pressuring the injection system. I also fitted and new fuel pump and filter.

I still have a rough Idle, although the MPG has improved somewhat from 11-12 to 14 mpg.

I had also previously cleaned the EGR, my suspicions are that this is still causing my problem. I have read that some people have blanked this off and the rough idle goes away.

My question is - regardless of whether this is a good thing to do permanently or not, surely it is a good "Test" to prove the EGR is causing the problem? I would blank the holes off and leave the EGR itself plugged in so as not to cause a problem with the computer and see what happens to the idle.

 

Over the long term, it will cause a check engine light regardless of whether the unit is plugged in or not. The PCM will do functional tests periodically during a drive cycle where it will open the EGR valve and close off on the IAC (idle air control) valve appropriately to maintain idle. If the PCM doesn't see the proper change in IAC position for the corresponding EGR position, it will trigger a CEL. If that is a problem for your local vehicle inspection requirements, then this is not a long term thing that you can do.

With that said, yes, blocking off the EGR ports is a valid way of testing the EGR valve for leakage that may be the cause of idle or other drivability concerns. You would need to fashion a block off plate that mimics the contours of the valve itself or the gasket whichever is easier. Bolt it in and off you go for your testing/diagnostic needs.