2007 Mazdaspeed3 P0302 Cylinder 2 Misfire

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Andyslapz

Greenie N00B Member
Hello everyone,
I'm new to this forum. My girlfriend has a 07 MS3 that has been giving us some problems lately. Last week, she was driving on the freeway when her car started misfiring badly. The engine shakes and the whole car vibrates. I installed new spark plugs, gapped to 0.028, tried swapping coil packs around, and it still has the p0302 code. Yesterday, I pulled the fuel injectors and replaced cylinder 2 injector with a brand new one, and it still throws the p0302 code. I even put in one of the spark plugs from the old set (from a cylinder that wasn't misfiring) and still have the issue. It misfires as soon as the car is started and misfires all the time. Where should I go from here?

80k on the odometer.
I recently (a few weeks ago) installed a good, used k04 because hers was blowing smoke and consuming a lot of oil. The car doesn't smoke anymore.
I did a compression test before the turbo install and it was 1. 150 2. 150 3. 120 4. 150
Repeated the compression test wet and the numbers didn't change at all.
Corksport Stage II Power Series SRI.
Forge BPV
 
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The compression test numbers say it all. You have 30 psi less on one cylinder than another. You could have a bent rod, bent valve, cracked piston. You wont know until you tear it down or do a cylinder leak down test.
 
L337TurboZ said:
The compression test numbers say it all. You have 30 psi less on one cylinder than another. You could have a bent rod, bent valve, cracked piston. You wont know until you tear it down or do a cylinder leak down test.
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I am curious to understand why cylinder 2 is being reported as missing but the compression is lower on cylinder 3.

Something doesn't sound right. Although its not ideal, I have seen these engines run well enough even with the low compression numbers in 1 cylinder. OP reported severe missfire felt.

Can you take a picture of the plug tip in cylinder #2?

Confirm you are getting spark in cylinder 2 by using a spark tester or by installing a plug in coil #2 while the coil is out of the plug well (connected to harness) and cranking the engine in clear flood mode (100% throttle, this disables injectors).

Something like this, but you don't need to run a wire to ground, just touch the plug threads to a piece of metal.

upload_2021-4-20_20-39-31.png

Do you see spark?
 
What if the test numbers were written down backwards? That would explain low compression on 2 instead of 3 resulting in a misfire. Either way it is a 20 percent difference in compression.
 
L337TurboZ said:
What if the test numbers were written down backwards? That would explain low compression on 2 instead of 3 resulting in a misfire. Either way it is a 20 percent difference in compression.
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It could be, but wouldn't it be in the OPs interest to confirm that prior to diagnosing it as a completely failed engine?

I understand that the engine is in a poor condition, but it doesn't necessarily indicate the engine will have a severe miss.
 
DualSpeed said:
A leak down test should also be done just to confirm.
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Leak down test will give more information as to why the cylinder is losing compression. I do not think this step is necessary to diagnose why the miss-fire is happening yet, especially if this may cost more money for the OP.

Based on the information provided as well as my experience, I am going to bet the exhaust valves on cylinder 3 are disintegrating.
 
I guess that was my point. Lol. When the compression test didn't change on the wet test. I like to err on the side of the least side of catastrophic failure, until proven otherwise.
 
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