Chasing codes on my 99 GMC SIERRA Z71 4x4
 

I am at a complete loss and need input from others. I bought this truck 4 months ago and I have been chasing codes constantly and still having issues. I am beyond frustrated and need it up and going and dependable before weather sets in I have disabled kids and this is our only transportation to hospitals and everything. Ok so I have already done oil change, plugs wires, all 3 O2 sensors, rear breakpads and rotors, light, fuel cap, cv axle, tie rod, crankshaft position sensor, 2 ujoints, rear seal on transfer case, tps, one of the coil packs. It's still cutting out losing power keeps saying random misfire and once in a while it'll read gas cap and catalytic converter. I have put over $6000 total in it so far in buying for $4200 and getting everything fixed. And when you start it you get a thud noise when you put it from P into R and there's also rattling noise I have had numerous people say they'll come look at it and never do. I need it fixed please help me stop chasing codes. :confused:

Is this the 5.3L engine in the new body style, or the old body style 5.7L?

It has the 5.3

I'm thinking a broken engine mount for the thud and rattle noise, as far as the random missfires, first place I'd check is for leaking intake gaskets. Your engine, unlike the infamous GM V6 intake gaskets, can only leak vacuum. That in mind, a really easy trick, is on a cold start, get a helper to start the vehicle for you and at the same time the truck is starting use a garden hose and soak down the area where the intake manifold joins the cylinder heads. You can't hurt a darn thing by doing this - don't worry! If the engine starts to missfire and chug, and if you see white steam coming out of the tail pipe - BINGO! You've diagnosed yourself a bad set of intake gaskets :) It's a pretty easy job to do. I'll give you a pointer too - when you have the intake off, pop the rubber grommets out of the intake valley cover, these are your knock sensors. I'll bet ya a box of doughnuts that the rear most knock sensor cavity is filled with yucky rusty water. Get an air gun and blow all that crap out, then use RTV silicone to seal that rubber grommet back to the intake valley cover - it will save you some heartache later down the road.

If you're unsuccessful with diagnosing your intake gaskets, you might need too use a scan tool to monitor the missfire counter used by the engine computer, it is far more sensitive than it gets credit for. You might not notice the engine missfiring, but it might pick up the water being sucked into the intake, and register a missfire. If the computer notices missfires when you soak the intake gaskets, again BINGO!

Also a bad engine mount just rattling away can cause a false reading to be sent to those knock sensors in the valley cover. If you find you have a blown out engine mount, might want to take care of that first and re-evaluate the situation before you dive in and start replacing parts.

Hope this helps and is coherent.