Since you've already replaced your Chevy Monte Carlo's EGR valve, and
cleaned the intake manifold passages, we can also suggest two things to
further diagnose and if need repair:
1. You should ensure there is manifold vacuum reaching your Chevy Monte
Carlos's EGR valve and that the vacuum is sufficient enough to open the
valve as needed.
2. Check that your Chevy Monte Carlo is not running lean. If you've check
the manifold vacuum and it's strong enough to open the EGR valve and reduce
NO, then the problem is a lean fuel condition. Have a smog check repair
station using the smog machine as a 5 gas analyzer fine tune your Monte
Carlo's carburetor. Also check your smog check VIR, if CO is lower then
average, you definitely have a lean fuel air mixture.
