I picked up a 94 dually at the beginning of summer. It had fuel issues and after researching on this forum found a bad oil pressure switch. Along about July the new OPS went bad and I replaced it. So when the symptoms returned last week I was a little mad. After checking the OPS out and finding it was good I started looking at the rest of the fuel system. Finding it all good, no leaks, clean filters I was at a loss. So back to researching the forum. Then I came across elsbland's thread (Black smoke and coughing) and decided to look at air delivery. Same symptoms as inop. lift pump but was seeing a lot more smoke. Started checking things out and pulled the lines from the vacuum pump off the engine to check them out. Here is what I found.



Replaced the lines with new. Drove it last night and it has good power under load and heavy smoke started to go away. Very little smoke after driving for few min.

 

Do yourself a favor and google OPS relay mod and install it for long term reliability-as always AC Delco ONLY for replacement electrical parts.