Hot take: I finally got the hang of reading live data on a 2004 Chevy Silverado with a weird misfire

Last week, a guy brought in his old Silverado with a rough idle that came and went. The check engine light was on, but the code was just for a random misfire. I hooked up my scanner to look at the live data stream, which I used to hate doing because it felt like too much info. I watched the fuel trim numbers for a few minutes while the truck was running. The long term fuel trim on bank 1 was sitting at +12%, which told me it was running lean. I started wiggling vacuum lines near the intake and sure enough, when I touched a small hose going to the brake booster, the number jumped. It was a tiny crack you could barely see. Fixed the hose, cleared the code, and it idled smooth. Anyone else find that starting with just the fuel trim data makes live data less of a headache?

3 comments

3 Comments

the_thea29d ago

Nice! Fuel trim is the best place to start for a random misfire, it cuts right through the noise. I always check for vacuum leaks there first before I even pull a plug. What scanner were you using?

kellys7829d ago

Yeah but I've been burned by that before, the_thea. Fuel trim can point you at a vacuum leak for sure, but on these newer direct injection engines, a dirty intake valve will give you a perfect fuel trim reading while still causing a random misfire. I've wasted hours chasing a leak that wasn't there. Now I pull the plugs first every time, it's just faster for me.

clairen8529d ago

That fuel trim trick is solid, but man, you gotta watch the misfire counter data too. Sometimes it'll point right at a single cylinder before you even start pulling stuff apart.