How do I fix misfiring

[Ericwest]

Hello everyone! I just bought a used express and could really use some advice. I have an engine code for random misfire. The van has been sitting for a year. Ran well when parked. Can anyone give me some tips on diagnosing the problem. I do have a scanner but it’s pretty low end.It’s a 2003 express 3500 with vortex 6.0. Any help would be greatly appreciated

[MountainManJoe]

Congrats on your purchase, and welcome to the forum.

There are many, many causes for misfire. More investigation and information gathering is needed to narrow it down. The easiest way is to use a diagnostic scanner to monitor live data such as individual cylinder misfire counters, fuel trims, ignition retard, MAF values, etc.
Basic maintenance can also reveal some clues, such as inspecting your spark plugs, air filter, and throttle body. Measuring fuel pressure, cylinder compression and exhaust back-pressure, and smoke testing for vacuum leaks.

Based on the fact you said the van sat for a while, I'm inclined to suspect fuel delivery problems. Old gas tends to gum things up, so run some fuel system/injector cleaner through it.

[Ericwest]

I did drain old gas. But new gas with sea foam. But hasn’t helped. Maybe even a little worse. I have a scanner. Has some live data. Unfortunately I thought it had misfire monitors but can’t get that to work. It does show fuel trims. Ignition retard. But I don’t know what the correct values should be and they are fluctuating.

[MountainManJoe]

But new gas with sea foam. But hasn’t helped. Maybe even a little worse.

But I don’t know what the correct values should be and they are fluctuating.

zero, ideally. And around 6 grams/sec of air at idle for MAF.