97 1500 z71 random/multiple misfires, runs horrible

a misfire can be mechanical as well as electrical
code p 0300 is random/ multiple misfires detected
low fuel pressure is a probable cause from a problem fuel pressure regulator or problem EVAC system not venting properly causing a vacuum in the tank and starving the pump
erratic could be from valve problems on heating
loose valve seat insert/s, incorrect valve clearance from faulty hydraulic followers, cam shaft lobe failures are all part of the mechanical reason for random /multiple misfires

code p0300 refers to random/multiple cylinder/s ---misfire detected=== causes spark plug/s--HT lead/s--injector/s---ignition coil/s--low compression --wiring
Seeing as you have covered all the electrics, now concentrate on the injectors and fuel pressures
If you will do all injectors and the fuel pressure is where it is supposed to be ( pressure test) then you will have covered all bases
Low compression will be in the one cylinder only ans will remain in that cylinder regardless of injector or plug changes
I would leave that to the last resort as if it is random, it may be a loose valve seat insert that moves randomly causing a compression problem

The code 300 is a Random/Multiple Cylinder Misfire....a 302 is Cylinder # 2 misfire detected. ..and the same for 304 and 306..being 304 is cylinder 4 misfire and 306 is cylinder 6 misfire...All these at the same time normally is a faulty coil pack..,and or faulty coils...spark plugs.,plug wires.....It should be running horribly or barely ....However it could be internal losing compression.....It could just need a good tune up.....Disconnect battery before changing coils,wires..,or plugs in order to clear the fault codes.....

Could be bad crank and cam sensor,carbon build up,vacuum leak,poor compression.

You are misfiring. If you have Autozone (or similar) research your check engine light, they will come up with one or more of the following codes P0300, P0301, P0302, P0303, P0304 etc. etc.P0308. The P0300 code refers to "Random/Multiple misfires" whereas the other codes refer to that particular cylinder (last digit) misfire. The P0300 code sets when 2 or more cylinders misfire (can be the same 2 over and over, but it takes at least 2 cylinders to misifre to set that code. The others only take one misfire to set the code. Now that you know what is happening, here is how to fix....
Takes all to fire a cylinder (Fuel, Spark, Air & Timing) properly. If you have random or multiple misfires, it is likely something that is COMMON to all cylinders - like the air intake/throttle body area - throttle body can get carbon buildup and not open/close properly. Check the fuel pressure to the fuel rail(s), fuel filter clogged? When you turn your key to the "ON" position, do you hear the fuel pump turn on (whine) and then shut off? No whine, means no fuel pump. Check all fuses and fuse relays related to the fuel delivery system. Check the distributor cap/rotor for any signs of carbon tracking/arcing. Do you have good spark from coil? Check wiring for the Camshaft Position Sensor/Crankshaft position Sensor for timing issues. Finally, check compression on all cylinders. Need good compression for combustion to take place.

For a single cylinder (example Cylinder #2) - Easiest to check is spark. Pull the plug and look at it. Is it wet and smell like fuel? If so, then swap it with cylinder next to it. If the problem moved, then its the plug. If the misfire is still on Cylinder #2, then swap the plug wire (both ends). If the misfire is still on Cylinder #2, then swap the coil. (see where this is going?) If the plug is dry, then check the fuel injector for operation. Noid light tester or you can get a long screwdriver and stick one end on your ear and the other on the injector - it should make a steady, rhythmic clicking sound if it is functional when car is running. No sound? Replace. Is the plug oily? Check cylinder #2 for compression. No compression or little compression means either head gasket blown or bad/sticking valve (intake or exhaust) = big money to get fixed. Timing on one cylinder is hard to test without correct test tools. Timing is likely not a problem on a single cylinder misfire. Good luck.

bad upstream O2 sensor on driver side. Low voltage to sensor sends lean code, which in turn sends more fuel to bank 1, resulting in multiple misfires. That being said, lets me say this as well, take it to a shop and have them look. So many other things can cause this problem from vacuum leaks to looking at it cross eyed. Hope this helps

i can recommend you to RUN the scanner/computer for easy diagnostic and fix it. The electronic vehicle is complicated you have faulty the PCM or BCM.etc.
see the diagram attached.
God bless you

I would take it back and demand a refund. Could be the crank or cam sensors, an intake leak, faulty coil, or a list of other components.

Use some electrical spray cleaner or tuner cleaner and give mass air flow sensor a good cleaning. then try it. If that doesn't help, a fuel system pressure test may be smart to confirm proper pressure required is there. Low fuel pressure will cause multiple random misfire as well.

Hello your po135 is o2 sensor bank 1 your po141 is O2 sensor bank 1 sensor 2 and your pO300 is random multiple cylinder misfire these problems are your oxygen sensors and engine misfire pO300 is you engine missing o2 sensors are switches in the exhaust system the engine misfires are the engines cylinders missing.