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Code 34 means the signal output from the EVP is above 0.67V when the EGR is closed. That can be caused by a faulty EGR valve (yes I know you stated you swapped in another, but it could be bad) or the EVP sensor is bad. If the EGR valve is used then you could have a build up of carbon on the pintle seat. That will keep it slight open which in turn triggers Code 34. Once again, it's a process of elimination and diagnosing the root cause.
When you replaced the EGR valve, did you use your old EVP sensor or one that came on on the"new" one?
There are different EVP sensors between years. I did have that code 34 but when I replaced the EVP for that year truck it went away.
Another issue that can cause code 34 after EGR valve replacement is the O ring for the EVP sensor to seal onto the EGR valve is missing or doubled up O rings installed. I forgot which post that someone found out about that here.
I left the original sensor top on. I can try the other one and see if it makes a difference. Unlesss new egr valves dont come with new sensors and this could be the original from my 87. It was a mechanic that replaced the valve. The o ring i know is there