Okay, so I've read almost all the way through this technical thread. Very informative but I just need someone to be real clear about fuel pressure. The truck is my cousins and she ran the battery down trying to start it. It definitely has a no run condition, not just a dead battery. I have had jumper cables on the truck for the last 25 minutes. When I turn the key to the on position the pressure jumps up to 60-62 PSI. As soon as I crank the truck it drops to around 48-50PSI. I am not cranking it long. Just long enough that the truck would normally have started, so I'm fairly confident that the battery voltage is still high enough at this point. Does this mean that the pump is no good. What should cranking pressure be?

 

So, I just went back out to check the fuel pressure again while cranking. The jumper cables were still on the truck. This time when I cranked it, it started. So I guess I was wrong about the battery voltage being high enough during cranking. So now that it's running, at idle it's running 50psi but when I jazz the throttle it jumps to 60psi and then slowly comes down as the rpm's go up. Is this normal? The reason I ask is because if I jazz the throttle real hard and hold it the engine will rev a little then start to act like it's missing or chugging slightly. If you jusy roll into the throttle it revs all the way to redline just fine. I think either the fuel pressure isn't right(maybe someone can confirm this for me) or the ignition is breaking up when this happens. What do you think?

 

So I went and drove the truck. The pressure is at 50psi at idle and rises to 60psi at the touch of the throttle consistently. So I guess the pump is okay? The only problem I sense is that if I really press th etruck hard to accelerate fast, it seems as though it gets to about 4krpm and just refuses to rev much higher. It's like hitting a brick wall. If I keep my foot in it, it will eventually just upshift. I'm going to start by ohming out the plugs and wires. Anyone know what the resistance should be, if any, in a set of stock wires? The "Service all wheel Drive" warning light comes on intermittently as well. That was a problem before.

 

The engine should rev past 4k RPM. This makes me think that it is a problem with the catalytic converter.

Concerning the service AWD light, you would have to have a Tech2 or equivalent scan tool to access the TCCM in your truck and pull the history codes. The typical off the shelf scan tool will not see beyond the OBD2 system to any of the sub-systems that communicate through the serial bus.

If you have not serviced the transfer case yet, I would start there. Sometimes old, nasty fluid can cause issues. Remember, use GM Autotrak II fluid ONLY! Any other substitutes, regardless of manufacturer claim, can damage the clutch inside the transfer case permanently. And it is VERY expensive.

 

The truck acted worse tonight. Once it was driven a couple miles, it had no power at all. It would barely get out of its own way. I thought about the cat, like you, so when I parked the truck I free revved the engine and it revved fine and the exhaust out of the tailpipe was real strong. I'm unsure if the cat is the problem unless it can cool down enough and quick enough to not seem like the culprit. I have been around some cars, mainly Audi's, that had clogged cats and when they get hot they don't flow. The problem doesn't go away until the car has been shut off and the cats lots of time to cool. I wouldn't be surprised if the cat is clogged considering all of the fueling problems these trucks have. It's just pretty inconsistent so far. Any other ideas?

 

This sounds far more like a fuel delivery issue then an exhaust issue.
There are a couple of things that I can think of that may be the problem off the top of my head. If your fuel pressure is dropping below 55psi then your lucky that the truck stays running at all. 55psi is supposed to be the minimum rated pressure for your injectors to properly open.

#1 being the fuel filter. A partially clogged fuel filter can cause many if not all of the symptoms that you described.
The second being the fuel pump relay may be going bad. If the relay is starting to go bad it may still kick in the pump but it may not be giving the fuel pump the full 12v that it needs to give the correct fuel pressure.

The third thing to check is the fuel pressure regulator. If your fuel pressure drops quickly after you turn off the key then this is the problem with your truck. The pressure should hold steady for several minutes after you turn off the key.

The last thing to check if everything else above fails is your throttle position sensor. A faulty TPS sensor can cause some of the symptoms that you described but the above items are more likely as a bad TPS usually throws a service engine light. (though not always)

 

The fuel pressure is at 50psi or so at an idle and the idle is smooth. Then at tip in the fuel pressure jumps to 60psi. Is that not normal? The fuel pressure is at 60psi while driving and when it runs poorly. The pressure never drops below 60 while throtteling the truck. I would think if the filter was causing the problem I would see that in the pressure gauge. The fuel pressure does not drop far after turning the truck off. Ijust can't believe that there hasn't been a CEL as bad as it's running.

 

Test the TPS sensor by back probing the wires. There will be a +5V reference wire, a ground, and the signal return to the PCM. You need to put the black lead from your multimeter on the ground wire and the red lead to the signal to the PCM. The output from the sensor should read ~0.5V at idle and ~4.5V at wide open throttle. As you open and close the throttle, the voltage should smoothly transition between the two values above. If there are spikes or drop outs, the sensor is likely bad. You can also perform the test again with the black lead going to the battery negative terminal. This will take the PCM ground loop out of the picture. If the results are the same, then the sensor is bad. If they are different, the problem could be a wiring fault.

Do you know anyone with a Tech2 or equivalent full function scan tool? If so, have them hook up to the truck and watch the various sensor values. These readings can be very informative.

 

Cool. Thanks I'll test the TPS. I spoke to a tech today that told me just because the fuel pressure is right doesn't mean that the quantity to the engine is correct and that there is no real way to measure it. He believes that the pressure shown onthe gauge could be air in the lines as well as fuel. He's replaced pumps on these trucks that the pressure was correct but still had driveability problems and it fixed them. I removed the fuel lines going into the spider today and ran the pump and the quantity looked pretty good to me but that doesn't mean that the pump isn't failing intermittently or the quantity is low when the demand is higher. What do these trucks do when the spiders are failing? I noticed yesterday and more so today that the power delivery isn't very smooth. While accelerating it intermittently kinda shutters or hesitates erratically. If I free rev the engine it also backfires ever so slightly through the intake. That to me tells me it's lean and that either the quantity is to low or the spider is failing, but I don't know what they do when the spiders fail.