P0300 - Rough cold start

Year/Make/Model: 2011/Mazda/Mazdaspeed3
Mileage: 105k
Location: Washington, US
Concern: Rough cold start and P0300
DTC's: (please include descriptions): P0300
Modifications: Stock
ECU/Tuning Software: None
Tuner: None
Is the concern intermittent? No
Can you duplicate the concern? Yes
Recent Repairs: N/A
Correction: N/A

Hey guys, I've a 2011 Gen2 Speed 3 with 105k miles. I spun a bearing last year, due to low oil, and had my engine replaced. A very painful lesson very well learned. I supposedly got a "Grade A" engine with <45k miles.

Since it has been replaced I've had a P0300 error on cold starts but only when it's warmer than 19C / 68F. The start up is very choppy and cuts in and out until everything is fine and smooth. I've taken the car to AutoX and also a track day with a handful of 20 minute sessions at both. Engine ran perfectly fine.

So far I've replaced the coils with OEM coils and replaced the spark plugs with pre-gapped NGK plugs. But no luck.

I've scavenged old threads and have come up with the follow things to do next:

• Check cylinder compression
• Clean MAF sensor
• Check for vacuum leaks (smoke test)
  
• Check / clean injectors (most expensive option for last)

I've attached a FORScan datalog during cold start, I can easily add more PIDs and re-run if helpful.

What else should I be doing? Is there something super obvious in that datalog?

Any help is really appreciated, I just want my old reliable speed 3 back.

 

It's a JDM motor, isn't it? Those are 99.69% junk.

Compression test is needed. Then leak down. Also if using aftermarket coilpacks, those suck too. Pics of motor?

 

I'm not sure where they got the motor honestly. I swapped out the coil packs with an OEM set and that didn't solve it.

I've a compression tool coming in the next couple of days. Will post results and photos of the motor.

 

Also check the plug gap. .026-.028. Sometimes the pregapped ones are not right.

 

I rechecked the pre-gapped plugs, all at 0.028.

Did the compression test, motor warm. I ran the tests four times per cylinder, changing cylinder after each single test.

I realized I didn't have the throttle fully open for these, but it does look like cylinder #1 is the outlier. Should I be concerned with that difference?

• Cylinder 1
  • Test#1 - 180
  • Test#2 - 180
  • Test#3 - 185
  • Test#4 - 185
• Cylinder 2
  • Test#1 - 190
  • Test#2 - 190
  • Test#3 - 200
    
  • Test#4 - 200
• Cylinder 3
  • Test#1 - 190
  • Test#2 - 190
  • Test#3 - 195
    
  • Test#4 - 195
• Cylinder 4
  • Test#1 - 180
  • Test#2 - 190
  • Test#3 - 195
    
  • Test#4 - 195

Motor photos attached.

 

Next step would be to get injectors pulled. Look for leaks. Then get em tested and cleaned and good seals like CorkSport, and new o rings.

 

I guess a leaking injector lines up with the constant long-term negative fuel trip in the datalog?

Thanks for this. I have the CS fuel injector removal tool coming. I'll take them out and get them cleaned. Seems SP63 is where everyone recommends?

 

Pulled those suckers out. Looks like they're pretty grimy. Sending them off to get cleaned. Not seeing any leaks around the injectors, though injector #2 came out in my hand without any effort oddly.

Valve carbon buildup seems pretty bad on some. I might try to clean those while I have it all apart.

Also going to replace PCV valve while I'm here.