Never codes running on the road, only if idling 5 minutes or longer (like at a smog test!)

Question

It popped a code once (p0301) maybe 1000 miles before taking in for smog. I reset it when it happened and didn't see it again until I took it for smog test. At the start of the test it popped code p0301. Test immediately fails, but the sniff numbers all looked great. Replaced cap & rotor, plugs & wires. Plugs all looked normal, no clues there. Drove it 10 days (~500 miles) and it never popped a code, until I tried to retest smog. P0301 again and this time I noticed it was at the beginning of the test where it just idles a few minutes. Replaced a vac line and a 1-way valve that blew both ways, that's all I could find. Idled for 5 minutes and it popped p0301 again. Grr
It's not running rough, can't find anything else bad. Any ideas or do I have to get a diag.?

Answer 1

P0301 Misfire number 1 cylinder

When you clear the CK ENG LT, the fuel adaptation reset. and the comprehensive monitor needs to run, So a certain drive criteria needs to be met, when you meet this criteria this can be days or even hours,once its met, The ECM looks opens grounds short, and even misfires, This is when you code up P0301, Try swapping the number sparkplug to a different cylinder,and if you can do also the plug wire of number , You have CSFI injectors nicked name spider injector, these tend to plug up , so you should try a fuel injector cleaner  added to the fuel tank