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Misfire on startup in '01 B3000

Hey everyone, so I’m having an issue with a misfire whenever I start my truck. It happens regardless of whether the engine is hot or cold, and lasts for about 10 minutes. I had a similar issue not too long ago, except last time it just stopped starting all together until I had the injectors cleaned. I have new plugs, wires, new coil pack, and a semi recently replaced fuel filter. I’m hesitant about it being the injectors stuck open again as its only been about 1000 miles since I had them all cleaned. Any ideas?

Have you checked for codes? Is the misfire random or a single cylinder?

Right after starting the engine the engine computer uses various sensors other than the O2 sensor to decide how much fuel to inject. After the O2 sensors start working then it becomes the primary decider for how much gas the computer to inject. So one of two possibilities are likely

  • One of those sensors used prior to the O2 sensor coming online is faulty. MAF/MAP, IAT, TPS, ECT good place to start. Usually a faulty one would throw a diagnostic code, but might not turn on the CEL.

  • Nothing wrong w/those sensors above, instead there’s a problem with the basic fuel/air mixture which is then corrected when the O2 sensor comes online. Could be a fuel delivery problem or an air or exhaust leak. That would usually show up with a fuel trim test.

I had the codes ran, I got one for EVAP control, and one for a misfire on cylinder 4, no signs of buildup, and compression test was good.

Fix whatever’s causing the EVAP code first. Could be an overly rich operation due to a faulty EVAP purge valve for example. EVAP systems tend to be complicated and have numerous failure modes. But they’re usually easy for a shop to diagnose & not overly expensive to fix.

If the purge valve is leaking it will cause a missfire on cylinder #4 at idle.