1996 Z71 Having very frustrating problems

[DGuysk01]

I have a 1996 GMC Z71 with the 350 Vortec, and it it giving me fits and costing me lots of money. It runs great when you start it up, but once you drive it around for a while and it reaches operating temp, it begins to lose power and miss terribly, all the while pouring black smoke out the exhaust on both sides.

I have a diagnostic scanner, and it reads to me that it has stored a P0102 code (MAF Sensor Low Input) and a P0340 (Cam Position Sensor Circuit Malfunction). I replaced the MAF sensor with one from a 16000 mile truck which ran great, only to find that it changed nothing. I put brand new spark plugs and wires on it and it actually got worse, even though the gap is correct and the firing order is as well.

I just took the truck in to a shop today, and they charged me $170 dollars to tell me nothing I didnt already know. They told me that my timing is retarded 40*, and to adjust my distributor. Now, I would go for that if I didnt already know that the ignition is entirely computer controlled on this engine. They said to Change my MAF sensor, but I already did and to No avail. It says LOW INPUT, and idle speed or running condition does not change at all when the MAF sensor is unplugged. I erase the code, but it appears again immediately after it is erased.

When I am driving it under these conditions, I have to manually shift from first so that I can get my rpms high enough so it will carry through second and third. In 3rd at about 40 mph, it will surge forward with alot of power, but then it will slow back down. It blows black smoke really bad and gets somewhere in the realms of 8 mpg when running like this. But keep in mind that it runs good when first started, but after say 3 minutes or so of driving it loses power. Im Losing my mind trying to chase this thing down, so any help would be very appreciated!

[transgta]

sounds like a vaccum issue.behind ur brake fluid bowl there should b a big vaccum hose plugged into the booster.while its running pull that off and see if theres strong vaccum.as it runs the vaccum sounds like its getting some issues thats it cant pull. A buddy told me he took seafoam and filled the can lid and let it suk it into that vaccum line it pulls it into the system an cleans out debris.Im not saying do that cause i never have but u may look into it. I would pull each vaccum line off one at a time maybe u can find a weak spot.good luck maybe this can help.dave.One more thing gas cap.It should set off ur service light but for about 5 buks its worth a try.Think about it when it starts its good as it runs its pulling the system somethn not breathing right.

[DGuysk01]

Thanks transgta, I never really thought much about a vacuum issue. Now that you say that, there is one vacuum line which appears to be missing from a valve on the passenger side of the engine, it's either the purge canister or the egr valve, and there is a line fitting that is not attatched, and I have no clue as to where it comes from. Could that be causing these problems?

[transgta]

Its possible the vaccum is a big part of the system

[DGuysk01]

I've found out that this problem doesn't occur until I open the throttle half way or more. If I drive in town and baby it, it will run just fine and will not smoke