2003 GMC Yukon XL 5.3 Flex Fuel P0300 Misfire Code Troubleshooting
2003 GMC Yukon XL 5.3 flex fuel. Only code is P0300. New plugs and wires, MAF cleaned, fuel pressure is ~45 psi at idle, and O2 sensors look normal. Could it be injectors?
You’ve got a random misfire (P0300) on a 2003 Yukon XL 5.3. Plugs and wires are new, MAF’s clean, fuel pressure is a little low but not terrible for idle, and O2s look normal. Injectors are a real possibility, but I wouldn’t rule out a weak coil or a vacuum leak just yet. If the misfire is steady on one or two cylinders, injectors jump up the list. If it’s all over, look for something affecting all banks.
Most likely culprits
- Leaking or clogged fuel injector(s) — Fits if misfire is cylinder-specific and you’ve already done plugs/wires.
- Weak ignition coil — Common on these, especially if the misfire moves when you swap coils.
- Vacuum leak (intake gasket, PCV, brake booster hose) — Causes lean misfire, especially at idle.
How to narrow it down
- Pull misfire counters with a scan tool — See if one or two cylinders are racking up misfires. If yes, focus on those.
- Swap coils between a misfiring and a non-misfiring cylinder — If the misfire moves, it’s the coil.
- Spray carb cleaner or brake clean around intake gaskets and vacuum lines at idle — If RPM jumps, you’ve got a vacuum leak.
- Unplug each injector (one at a time) with engine running — If unplugging one makes no difference, that injector’s suspect.
- Check for fuel in the vacuum line at the fuel pressure regulator — If wet, regulator’s leaking and causing a rich misfire.
- Compression test on suspect cylinders — Rules out mechanical issues if nothing else pans out.
Possible parts (if needed)
- Fuel injector(s)
- Ignition coil(s)
- Intake manifold gasket (if vacuum leak found)
- Fuel pressure regulator (if leaking into vacuum line)
A few questions
- Are the misfires showing up on specific cylinders, or is it random across all of them?
- Any rough idle, surging, or only under load?
- Did you notice any fuel smell, hard starting, or black smoke from the tailpipe?
Let me know what you find on the misfire counters and if any of those quick checks turn up something. That’ll pin it down fast.