So I'm trying to chase down an intermittent misfire I've had for 3 months now.

Misfires started at a track day 3 months ago and it threw a p1399 and p0301 and p0303 and I limped it home on 3 (2?) cylinders. Tested compression and was even across all cylinders and replaced coils and moved plugs around, misfire cleared.

Car ran fine for a month or so and I even did another track day and drove 200+ street miles without issue.

Misfire came back at last track day and threw p1399 and p0301 while under load in vtec; misfire cleared by itself and I headed home early without issue. Replaced plugs, injectors, and map sensor and car seemed to run okay on a 5 min test drive. Next morning, misfire is back at startup. Seemingly cleared after I disconnected/reconnected all the injector plugs, but on a test drive, car misfires going into vtec and stays that way.

TLDR: Compression tested fine, have replaced plugs, coils, injectors, and map sensor with new and car misfires and throws a p1399 and p0301 code.

Any low hanging fruit left to try or am I at a point where the head probably needs to come off and be looked at by someone who knows what they're doing? My skills are pretty limited so I appreciate any help you guys can offer!

 

Checked the valve clearances?

 

A valve adjustment was done by the prior owner less than 10K miles ago, but to answer your question: not yet. I actually have feeler gauges and the valve adjustment tool sitting in an Amazon box at home right so will be doing that this weekend.

Also maybe relevant, I had AP2 retainers installed when I bought the car last year.

 

Did the valve adjustment; adjusted 3 valves total that were not much out of spec while all other valves were fine. Car fired up and drove with no issues.

I'm not confident that the misfire is gone for good and am leaning towards it being some kind of wiring/connection problem that I temporarily fixed when I refitted the valve cover. Will update for posterity if I ever find anything definitive.

 

Maybe, bad gas? As in fuel, not digestive issues.