Intermittent random misfire (P0300)

[JJ93]

Hi everyone!

I am having a somewhat strange issue with my Suburban. It's a 2001 with 113k miles. The other day I was cruising on the interstate (coasting down a hill) and the check engine light started flashing. I pulled over at the next exit expecting the engine to be noticeably misfiring, but it was running smooth. So I continued on.

The next time I encountered a downhill stretch, the CEL started flashing again! Once I got back on the gas, the light went out. The weird thing is that even when the light is flashing, it runs smooth with plenty of power. I don't think it is actually misfiring. I used the Torque Pro app on my phone to try to diagnose the issue. It throws a P0300 code, and it looks to be cylinders 1 and 6 that are the main culprits. Long term fuel trims look good I think, sitting around 4 or 5. I will double check on that tomorrow. I swapped could around and nothing changed.

I can replicate the problem in my driveway by holding the rpms at about 2,000 or so. The CEL will flash until I let off. Even with the light flashing, the engine feels very smooth with no vibrations or lopsided-ness. Driving normally (even under WOT) it doesn't act up at all. It feels and sounds healthy, and will get up and go when I want it to. It just flashes the CEL when coasting down hills at speed or when I raise the rpms while parked.

Since I have no idea when the fuel filter was last replaced, I replaced it with no change. I am thinking about changing the plugs and wires. I have read a few forum posts that say my particular problem could require a crank sensor relearn, but I am not 100% sure if that is the route I should take.

What do you guys think? Has anybody ever had this sort of issue?

Thanks in advance, I greatly appreciate any ideas!

 

[SnowDrifter]

First pass id go around and check for vaccuum leaks, clean MAF. Clean throttle body if you feel comfortable doing so. I've cleaned mine and customer vehicles without issue, including going so far as do disassemble, clean, and relube the gear box. but I'm aware it's a common thread that someone cleans their TB and something gets irreparably ****** up. YMMV on that one

 

[JJ93]

Update! The crank relearn procedure fixed my issue. Only cost me $40 and a half hour's time at the local shop.

Apparently this was a rare issue but common enough that the mechanic I talked to knew exactly what was up.

Basically if you have a random misfire code p0300 and the misfire data shows misfires mainly on cylinders 1 and 6, but you don't actually feel any misfires going on, a crankshaft relearn should fix it.

She runs like new

 

[SnowDrifter]

Curious

I wasn't aware such a procedure existed

Glad to hear it's solved!