I have a stumble in the engine around 2900 to 3,000 rpm. If I read the flashing light correctly I got a code 12 Cannot control RPM during ER Self-Test high RPM check.

This fits in with the stumble. I thought the stumble was from the EGR system but I made a block off plate for the EGR and test drove the truck and the stumble is still there. It does seem to take longer for the stumble to occur and it's not as frequent with the EGR blocked off and I think the truck runs stronger. That could be just me.

I have replaced the dizzy in the truck twice. The second I lost the pip signal and this stumble showed up some time after the second dizzy install.

What are the areas to look at when one gets a code 12? The truck is an 1989 351W with a C6. thanks bob