Hey all! I have a 96 GMC Sierra K1500 5.7L 4x4 Z71, 165k on the clock. I was having a boatload of trouble a few months ago, replaced plugs, wires, cap, rotor to no avail. Took it to a mechanic who replaced the throttle position sensor, map sensor, a few bad grounds, and the left bank O2 sensor (which didn't need replacing as it didn't fix the problem-no 6 misfire).

It's been running fine (aside from the no 6 miss) but last week started to kick/miss at highway speeds when i put light-medium pressure on the pedal. Full throttle or near clears it up. Gas mileage appears to be about the same, 13mpg on last tank but I was hauling six or seven hundred lbs for half of the tank, in the throttle around town for the other half. I have four codes coming up, no 6 misfire (nothing new but perhaps related), transmission control system malfunction, P1192, which my scanner does not have but online found "Post-catalyst Fuel Trim System Bank 1". Also should note the mechanic said I had high vacuum pressure, like 10 or 15 lbs or something; possibly a cracked plenum, but it was running alright even with this condition. Thoughts?

My initial thought is the fuel pressure regulator. Checked the fuel pressure and it reads 52lbs at idle, ~60 in the higher rpms. I can't really find out if that's normal or not, but it seems okay.

Any thoughts? Any help would be much appreciated!

 

Do you remember how the plugs looked?
When is the last time you replaced the fuel filter?
Are you losing any coolant - Vortecs are infamous for blown intake gaskets.
Coolant temp sensor

 

Still have the plugs I'll have to take another look at them. One of them was fouled for certain. The fuel filter looked new when I bought the truck about 4k miles ago so i didn't replace it. I think I am losing coolant, but very slowly.

Edit: four of the plugs were fouled. Don't remember where they came from specifically.

 

I think I would start with the coolant temp sensor first then - not the one for the gauge, but the one for the computer. It can cause a rich condition if it isn't reading properly (ECM thinks the truck is always cold). If you are losing coolant at all I would definitely check for an intake gasket leak. If the leak is external it should be evident. If you don't see any sign of a leak then it could be leaking internally which isn't good for bearings. Keep an eye on your dipstick for signs of water (milky looking) although a very slight leak won't be real obvious. That could also be your vacuum issue that the mechanic found.