Runs poor when at Normal Op Temp

[ken_man_1]

1999 GMC 3500 5.7L Auto trans, 4x4

Previously thought there was an exhaust restriction, but there isn't any.

When the engine is cold, it will idle find and accelerate ok, but if you floor it it will misfire at high rpm only and hold the top end of the gear too long. To get it to shift I have to let off the gas a little then it will shift and then the same thing all over again for the next gear.

After driving the truck for about 30 minutes the conditions get worse. At approximately 2000-3000 rpm's the truck misfires horribly to the point where I can't get over 30mph. I'm going to check the cap rotor wires and plugs, but it seems to be a temperature issue. I've heard of the ignition modules going out on these trucks, but I want some input before I jump head first.

The only cold is the P1347 code (i believe) - the cam/crank correlation code. That code has been there forever - this is a new problem.

[maxwedge]

How about fuel pressure, what do you have under these conditons?

[ken_man_1]

Well, my quick response is that after the truck sits for more than 10 minutes there is no pressure at the test point.... I'll start there lol!! My temperature thoughts were overpowering my fuel thoughts.

[ken_man_1]

Alright, I tested the fuel pressure - primed it goes about 66 or so, and running down the road it's at about 59 to 62 - so I'm going back to the idea that it's a temperature issue. I ran the truck for about 20 minutes and then shut it down - tested the pressure again - after 30 minutes it was at 38psi. It leaks down, but pretty slowly.

Wondering what else might be the correct point. I've changed cap, rotor, wires, and plugs (as they were all GM original and we're at 70,000 now) with exact OEM parts only. Still having the same trouble. Thought that maybe the wires were getting worn or maybe the plugs were out of spec, but I have the exact same problem.

[ken_man_1]

Max, am I getting the right pressures? I just bought a new guage as the other one I used was a friend's - and I'm not sure that it was right. I'm going to retest the pressure and see what I get. I just want to know for sure what I should be seeing. Thanks a ton!

[ken_man_1]

Ok, my new pressure guage tells a different story than what I had before. When the truck is priming (from sitting over night) it takes 2 priming cycles to get it up to 60psi. It primes up that high when the pump is running, and when the pump stops it falls to about 55-56. It holds pressure for a period of time - more than 10 minutes it stays over 50. When the truck is running at idle, the guage reads about 46-48, and sometimes the guage needle jumps between 46-48 (it wiggles back and forth). When flooring it from a dead stop the about 50mph, the guage reads about 56 and slowly declines to about 48 - that is when the engine stumbles. RPMs about 4000 and the pressure is at 48-50. Now, since I've been driving the truck all morning, it hasn't stumbled like before where I could only get to about 35mph or so.

I just checked the guage, and the pressure after shutting off the truck is at 20psi - that is 45 minutes later.