Tried some fixes, still code P0300

been fighting a p0300 for a while now. symptom is a rough idle on startup, sometimes but runs fine after about 10 to 20 seconds. occasionally it feels like ive lost some power. ive read a bunch of stuff on here and tried some fixes. truck is 2002 tacoma 3.4 manual supercharged w/smaller pully, 7th injector kit, 145k miles. ive replaced coils, wires, plugs, fuel filter, MAF, air filter, belts. i had the fuel inectors serviced. checked compression, all was good except cylinder 3 was just sliiightly lower than the rest. timing belt was done less than 50k ago. any leads would b appreciated as i need to get smogged and have some long drives coming up.

 

Was all that maintenance work before or after the misfire.

 

everything except timing belt was after the missfire

 

What spark plugs are you running?

 

ik22. a step down as recomended with the 7th injector kit. although now im pondering if i need different plugs with the smaller 2.2 pully? hmmm

 

That sounds correct. For Denso coding the 22s are 2 steps colder that stock. I don't think you want to ge any further, even for the 2.2" pulley.

Have you inspected them? Do they look clean?

 

well they look fine. the first time i threw the code like a year ago, i checked them and cylinder 3 plug was wet. swapped that one out and it was fine for a while. next time it happened the plug wasnt wet. seems off and on more frequently happening

 

Did you end up finding the culprit? I’m also dealing with a p0300 at the moment. Replacing the upstream O2 sensor tomorrow hoping that helps. I also replaced plugs, coils, wires, hoses, fuel filter within the past 2 years. Fuel pump got done few weeks ago. No change. Injectors have not been touched yet. Hoping it’s that and not something worse.

 

i have not solved it. light stays off mostly if i drive it every day. passed emissions test yesterday luckily so at least im good for another 2 years long as this issue isnt causing engine damage. im assuming some stupid sensor is acting up, and all the head gasket symptom checks seem to be coming back negative to that being the problem. lemme know if ur o2 sensor fixes urs, i out in new o2 sensors probly 40k miles ago...

 

Thread revival..
The upstream air/fuel sensor didn’t fix it. Also just replaced all 6 injectors along with the fuel pressure regulator two days ago. That wasn’t it either. Pulled all my plugs and inspected them. They were all dry from what I could tell. Brushed them up a little and put them back. Tightened up the valve cover bolts too. It started up fine for the rest of that day but ended up misfiring again upon start up the next day as well as the CEL. I’m running out of ideas. Guess compression/leak down tests might have to be next. Dreading the worst.
0300 0420 chase continues.

 

Question for anyone:
Is the p0300 a misfire that only occurs upon starting or is it continuous as I drive it and I just can’t tell that it is like I can when I start it?

 

hmmm, my assumption us that if it was still happening while u drive it, then it would trip the engine light again (actually it did that to me once. the engine started stuttering and the engine light was flashing on and off. pulled over restarted the truck and it was fine after that). im currently traveling but i might try another compression test to see if its changed since last time i checked over a year ago.

 

I know you've said your head gasket test come out good but I would pull the plugs after the vehicle sits overnight and stick a boroscope down in there and see if you're getting any antifreeze pooling up on the cylinders. I had a vehicle that passed a couple different head gasket tests but in a last ditch effort I used a boroscope and found that indeed I had a gasket leak. It would run rough on cold start and clear up after a few seconds after it burned the coolant up.

 

thats a good idea, i shall give it a try. although it definitely doesnt always have a rough start after sitting, it seems like it just happenes randomly although only in the first start up of the day.

 

I’m experiencing the same. I was attempting to do a compression test just now and noticed my coolant level looks lower than where I usually see it at the very top. The motor is still somewhat warm though. Is that normal? Will the coolant level come back to normal level as the motor cools off completely? I’m not sure of the mechanics of it. Or am I losing coolant and I’m possibly dealing with a head gasket issue?

 

my coolant level was slightly low too, but i also haddnt ever checked it since i bought the vehicle like 8 years ago. but i think u can loose some here and there over time if it boils over into the overflow reservoir?