I have a 2003 GMC Sierra with a 4.3 vortec. It’s my sons first truck. We are having code 0300 random misfire, and a Evap leak. The truck runs pretty good when it don’t misfire. It don’t smoke or get hot or anything. I’ve done plugs wires cap rotor, didn’t help so I replaced the distributer, still no help. New coil. New map sensor, new mas air flow sensor. There are no catalytic converters. Fuel pressure reads, 60 when you turn the key on, 50 running. It will idle all day without a misfire. But I’d you hold the gas at a stead rpm. Like 2500 it will start to misfire and bog down. Then pick back up then bog back down. I can’t figure it out. I live in a small town in the middle of nowhere. Only shop around has screwed me badly before.
mileage please.
Truck has 200k miles, motor has 140
140 miles, or 140,000 miles?
Replace the intake gasket. Machine the manifold if it's badly pitted.
I forgot to add that. There appears to be no leaks at the intake used a vacuum test, and smoke.
they can still leak coolant without external symptoms. Are you losing any coolant, or getting oil contamination?
None, I’ve been watching for both. Just the misfire. I can’t find anything. I don’t know how to check spider injectors. If that could even be it. I’m assuming the evap leak wouldn’t have anything to do with it.
Yes, the spider injector system is also a weak spot.
https://www.motormanfuelinjection.com/Vortec_spider_fuel_injection_conversion.html
EVAP system draws off engine vacuum, so it's worth looking into.
https://www.youtube.com/c/ScottyKilmermechanic/search?query=evap
By that I don’t have the long injectors, mine is a 2003. Is there a way to test them. With the scanner. I have a pretty good scanner. But don’t know crap about reading the live data
Record your fuel trims