P0300 - Rough cold start

[BoostFever]

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.

 

[Raider]

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?

 

[BoostFever]

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.

 

[Raider]

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

 

[BoostFever]

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.

 

[Raider]

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.

 

[BoostFever]

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?

 

[BoostFever]

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.