Dreaded Multiple Misfire Code P0300

[juanni]

Background.
GF's 1999 Blazer 4.3, 245K miles.
Ran good, but apparently the check engine light has been on for a while.
I live waaaayyy out in the hills, and don't have much for diagnostic tools.
So a strict adherence to a diagnostic flow chart requiring special tools won't that helpful.

To the point, starts but stumbles and misses,,badly.
Borrowed a scan tool and checked the codes and had the following.

PO122 throttle position sensor, switch A low output.
PO144 Ox sensor circuit high voltage Bank 1 sensor 3. Not sure what that means since I think I only have 2 Ox sensors??
and PO3000 random multiple cylinder misfire.

1st, since I never have even looked under the hood of this vehicle I started with the basics.
Maintanance has been iffy.

So,,,,,
New plugs, rotor, cap, wires, fuel filter.
Next checked the FP pressure. 60psi while running dropped right down to 50 when the pump shut off.
Replaced pump, now 60 & 56 psi, so it is in specs.
Hosed the intake and related for vacuum leaks, none.
Checked the fuel spider, no visible leaks.
EGR pulled and checked for junk keeping it open, nothing.
Hmmm.....since I have a 1999 5.7 suburban that runs great, so I swapped in the subub's coil, ignition module, EGR. Not any better.
Replaced both Ox sensors with new ones, no change.

Still baffled.
Multiple misfire mean more than one so it must be more wide based component that is the problem.
Thinking dist gear wear at 245K, cat converter fouled, ECM ??? or I dunno??

I didn't swap out the TPS with the suburban maybe I should?

Any ideas of what to do next?

[juanni]

hmmmmm...... no ideas?

[Corey Young]

I had a similar problem with a 2000 Astro van AWD 4.3. I changed the plugs, wires, cap, o2 sensor, fuel pump, timing chain and crank position sensor. It runs good now. It is usually several things wrong at once.

[Odin K30]

Most likely its the Spider injector. There should be a service interval for those pieces of shit.

[Andy351]

my wifes did the same thing. it was always the cap/rotor, apparently they only last about 10k miles due to condensation and other problems. we sold my wifes blazer as it was a piece of shit at 110k mi, i can only imagine what one with twice the mileage drove like.