P0302, Full Compression, Dirty Plugs, Rough Idle

Okay, I have done a lot of research and posted on multiple forums but I am stuck.

Preface: 2007 Stock Mazdaspeed3 with short ram intake. Bought at 108,000 miles from the original owner, currently has 120,000 on the clock. The Head gasket blew and burnt a valve at 112,000 miles. Replaced the head gasket and valve set (burnt intake and corresponding exhaust), turbo, timing chain, timing chain guide, tensioner, and VVT actuator. Also replaced the water pump, oil pump, and most gaskets as preventitive maitinewnce. Once assembled, the car ran great until this issue I need help with. I did not replace the head studs, but they have been rock solid so far. Anyways, on to the current issue.

Check engine light came on after boosting hard on a cold engine (dumb, yes I know).

Threw code P0304, the engine began running rough under low RPM. Limped it back home and parked it.

Swapped coil packs 1 and 4. Cleared the code and restarted the car. Threw code P0302.

Bought all new duralast coil packs and NGK iridium spark plugs (pre-gapped). Old plugs were covered in black grime, all 4 of them. Cleared the code and restarted the car. Threw P0302 again.

Bought seafoam fuel injector cleaner, ran it through the fuel system through the gas tank. No change.

Bought seafoam spray top-end cleaner. Sprayed the whole can into the throttle body as instructed, with the car running at 2,000 RPM. No change.

Bought a compression tester. All 4 cylinders are at 180 Psi. However, the brand new plugs I just inserted are covered in black grime. The only engine wear that has occurred on the new plugs is a couple idles to clear codes, and a drive around the block to get the seafoam spray through the system.

What could the problem be? I have a mobile mechanic coming next Wednesday, but until then I'm totally struck. Any input would be greatly appreciated.

 

I'm probably wrong but IIRC stock head studs are torque to yield and cannot be reused safely.

A couple of things:
1. Log of issue (if it runs shitty at idle that should be enough to let us know what's going on). Log everything you can.
2. Pull the intercooler or intercooler piping/couplers to check for oil that could be coming from the turbo.
3. If results from item 2 are good, then check for excess oil from PCV system.
4. Leakdown is a better test of engine health than compression.
5. Pictures of plugs.

 

I agree with Enki. Do a cylinder leak down test. Check terminal tension/connector health on your coils, check for moisture and stuff in the injector connector on your intake manifold.

It is odd you had a 304 code then after swapping coil packs for 1 and 4 it moved to cylinder 2. Try swapping 2 and 3. Pictures of the plugs will help. Maybe you cracked a plug by accident? It happens sometimes. It may not even look cracked it could have slight carbon tracing up the porcelain.

 

Along with what has already been said, a couple things.

1. You said the plugs were pre-gapped. Pre-gapped to what? Seems like a lot of the NGK plugs are indeed pre-gapped... but not for this car. Verify the gap.
2. There are certain parts where it's generally accepted that OEM is the only answer, timing components and coil packs are some of those parts. Since the misfire didn't move to the coil that you initially swapped around, and putting the Duralast coils on didn't solve it either, I would switch back to the OEM coils.

 

Agreed on going with OEM coilpacks. NGK plugs are gapped pretty wide IIRC at around .043" which is way out of spec. You want to generally be in the .024-.026" range.

Also Enki is correct about the head bolts being TTY and not reusable so that should be addressed at some point as well.

From Edge Autosport:

 

Check the gap on your sparkplugs and inspect the wiring harness for the coil packs very thoroughly

 

Mobile mechanic came and did an inspection.

Results: camshaft positioning sensor was throwing a code I was unable to see. The misfire code was active, as was the camshaft code. There was also a pending misfire code, telling me that the misfire was intermittent based on the sensor issue.

Mechanic said engine sounded healthy snd that the problems were simply communication errors from failed sensors.

The recommendation was to replace the Camshaft positioning sensor, crankshaft positioning sensor, and knock sensor. Mechanic also said to replace oxygen sensor for a “fine tune”

I bought an o2 sensor, crankshaft sensor, and camshaft sensor from autozone. I haven’t got a knock sensor yet as that isn’t an issue, but rather a good idea to replace after 120,000 miles.

Any insights or recommendations for additional maintenance or replacements while I’m in there? Or tips for replacement process? Thanks for all the help, it’s greatly appreciated.

 

Sadly auto zone sensors tend to be as good as non oem coil packs. Thry are shit.

 

Order from edge auto sport then? Or is there another auto store I should pickup from ?


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You should check your wiring diagram. If the crank and cam sensor are on the same 5 volt reference there would be a code for both. Do actual electrical diagnostic instead of listening to a shady tree mobile mechanic who is advising you to throw parts at it.

 

They shouldn’t be on the same circuit, the camshaft sensor is located on top of the engine by cylinder 4. The crankshaft sensor is located by the timing chain cover under one of the pulleys


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Before you go throwing Autozone sensors at it, let's go back to the plugs. Did you either:

A: Personally gap them yourself before installing, if so, what did you gap them to?
B: If you did not, did you pull the plugs and verify what the gap is? Somewhere around .026-.028 should be good for a stock motor.

Even if you bought them from a vendor that pre-gaps them to the MS3 specs for you, sometimes they're off or they just plain forget to do it before sending it out. It's such a simple thing to check, do that first. Once you know that the gap is good, then you can start going down the rabbit hole of sensors.

 

When I rebuilt the engine 6 months ago, I replaced the plugs with ngk iridiums from autozone. It has ran perfectly until this issue, which leads me to believe the factory gap from the plugs works just fine.

Also, wouldn’t that lead to a random misfire, or a constant misfire on multiple cylinders?


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The off the shelf gap from NGK is typically .043. This is straight from the factory service manual for the MS3:



It's your car, if you don't want to rule out one of the simplest and cheapest (free if you already have a set of feeler gauges) things we could be ruling out right now, that's your prerogative.

 

I had a plug which had been in my car for 30k miles which decided to open up it's gap to about 0.06 and it only showed up as an intermittent missfire at medium rpms and heavier load. Took the plug out and regapped no issue for the last 15 k miles

I don't understand why you would be opposed to taking the 20 minutes to check.

Also if they were gapped by the manufacturer they were definitely gapped incorrectly

 

I will definitely do so this afternoon, I was just confused on how that could be the problem. Thanks for the explanations, I’ll be picking up a feeler gauge and gapping them once I’m off work


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Good call Even if the gap isn't the problem, having a gap that far off is a problem. Verifying/correcting that gives us a good baseline to do further diagnostics from and lets us rule that out at as a potential cause at least.

 

Put your OEM coil packs back in as well after fixing the gaps.

 

Will do, thanks. Any other maintenance while replacing the camshaft sensor? It’s only a $50 part and autozone has the oem part number in stock and it’s right on top of the block.


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Do the free/easy stuff first