Major Problem Trying To Pull Engine Codes.

I have an 88 lx 5.0 that I have converted to mass air. I am having issues with a backfire through the intake so I checked all of the normal causes, timing, vacuum leaks ect. I was going to pull the codes today and when I connect the code reader to the plug the engine surges to 3000 rpms and stays there until I disconnect it. I have been wrenching on these cars for over 20 yrs and never ran into this problem. Thanks

I did that. I didn't get any strange codes. Just emissions stuff that I already know because I deleted all of that. I also did not get the code 10 or pass code. I will try it again