Misfire guidance needed - new to me car

[Scum]

Just picked up the 2007 Mazdaspeed 3. I am totally new to the car and am trying to debug a misfire issue. It seems to misfire while accelerating and in boost. It hesitates, stumbles and backfires. So long as I keep it out of boost it runs great. Obd code is just p0300.

Doing some reading, it sounded like bad plugs or poor gap or intake leak could be the culprit. So we pulled it apart, changed the plugs (old ones were bad and all over the place - as bad as .60 gap). The bpv return hose had a crack in it so we replaced that. We ran seafoam through it. Then tightened everything up. We used ngks gapped at 0.3

After all that, it ran strong. No misfire, no hesitation. Smooth all through the gears accelerating strong.

It ran great until later that night after about 70miles. Check engine light came back and started acting up I'm boost again.

Some things I noticed while working on it. It has a custom intake with a "Cobb's Tuning" sticker on it. It has a bov that says Forge. Ive come across these names when reading so thought I'd add that detail. Also, when running the seafoam, we of course got the smoke out the tailpipe, but it looked like we had some coming from the back of the engine as well. Maybe the turbo? So I'm wondering if there's an exhaust leak.

I'm looking for any ideas to go after here. I really thought that we fixed it and the fact that it went away for a day tells me we are pushing the right buttons but I'm at a loss of what to go after next.

I know my way around cars but turbo is new to me. So any guidance is appreciated.

[Scum]

Ok. So pulled the code again. I assumed it would be the same misfire code. Turns out it's P0126. We noticed that the coolant tank has a crack so we are ordering a new one (stupid expensive by the way - a little too proud of them).

Is it possible that's the reason for the code and the hesitation we were feeling (car going into protect mode for some reason)?

[MSMS3]

Originally Posted by Scum Just picked up the 2007 Mazdaspeed 3. I am totally new to the car and am trying to debug a misfire issue. It seems to misfire while accelerating and in boost. It hesitates, stumbles and backfires. So long as I keep it out of boost it runs great. Obd code is just p0300. Doing some reading, it sounded like bad plugs or poor gap or intake leak could be the culprit. So we pulled it apart, changed the plugs (old ones were bad and all over the place - as bad as .60 gap). The bpv return hose had a crack in it so we replaced that. We ran seafoam through it. Then tightened everything up. We used ngks gapped at 0.3 After all that, it ran strong. No misfire, no hesitation. Smooth all through the gears accelerating strong. It ran great until later that night after about 70miles. Check engine light came back and started acting up I'm boost again. Some things I noticed while working on it. It has a custom intake with a "Cobb's Tuning" sticker on it. It has a bov that says Forge. Ive come across these names when reading so thought I'd add that detail. Also, when running the seafoam, we of course got the smoke out the tailpipe, but it looked like we had some coming from the back of the engine as well. Maybe the turbo? So I'm wondering if there's an exhaust leak. I'm looking for any ideas to go after here. I really thought that we fixed it and the fact that it went away for a day tells me we are pushing the right buttons but I'm at a loss of what to go after next. I know my way around cars but turbo is new to me. So any guidance is appreciated.

I hope you meant .030”, not .3”. But, they need to be a bit tighter, .026-.028” would be better. Anything over .035” will misfire under hard acceleration with an abrupt cut and recovery cycle. It’s boost blowing out the spark if the gap is too wide.

Seafoam is a waste on our engines. Doesn’t do anything useful.

P0126 is usually a faulty coolant temp sensor or the thermostat being stuck open and needing to be replaced.

[Scum]

Yes, sorry. I've gapped them 0.03". The bad plugs were easily in the. 04 to .06 range. I took it for a test drive today. Seems to be running strong. A little extra poppy when shifting under hard acceleration. Not as bad as I originally thought when that cel came back. I may regap down as you suggest to see if it clears up the last of the popping.

I've ordered the overflow tank and may just replace the tstat and sensor at the same time to be thorough.

Thanks for the response and suggestions.