P0302 Slight misfire at idle or low rpm. Runs great on long drives!

[SpeedyMatt]

Ooohhh this one has been a tricky one! Time to get input on this ‘07 CX-7!

The situation in short: 125,000 miles, all stock, slight misfire in cylinder 2 at start-up or at idle (and very mild most of the time). Runs perfectly fine otherwise... but the longer a drive, the rougher the first minute the next morning.

Already done/tried:
Cleaned the MAF
Thought it’d be my coil since my plugs are only 4,000 miles, so I put in a new coil. Didn’t work.
Swapped plugs with cylinder 3 (and realized they are Denso and gapped at 0.038). The issue didn’t follow to 3.
Put in a new NGK plug (the 3787), gapped at 0.028 (they came out that way instead of 0.32. Figured that’d be perfect)
Already replaced all injector seals 4,000 miles ago when I had compression issues in cylinder 3.
Almost through a first tank with a bottle of Techron in the tank

Freeze frame data (if of any use):
Fuel sys 1: CL
Fuel Sys 2: NA
Call load: 53.3%
ECT des F: 120.2
STFT B1 (%): -6.3
LTFT B1 (%): 18.0
MAP (inHg): 18.3
Eng rpm: 851
Veh speed (rpm): 851
Veh speed (mph): 4
Spark adv: -2.0
IAT deg F: 93.2
MAF (lb/min): 1.0
TPS (%): 15.7
Run time (sec): 58
Fuel press (psi): 1008.0
Command EGR (%): 0.0
Command EVAP (%): 0.0
Fuel level:12.5%
Warm up DTC clear: 1
Clear distance (miles): 2
BARO (InHg): 29.5
CAT Te 11 (deg F): 629.8
ÉCU volts: 13.684
Load value (%): 43.9
EQ ratio: 0.991
Real TPS %: 6.3
Ambient temp deg F: 93.2
TPS B (%): 15.7
Acc pedal D %: 31.8
Acc pedal E %: 20.4
Command TAC %: 7.5

Thoughts? What do you suspect, and what would you recommend trying next (injector? Valve cover gasket? Other?)

Thanks in advance for your input!

 

[neganox]

1. Take all denso plugs and throw them in the garbage before they fall into your cylinders.
2. If you're at stock power levels and using stock temp plugs, .034 is the stock gap.
3. If you bypass the cold start routine does the misfire occur? (hold the gas pedal down slightly while cranking and then let off once the engine fires)

 

[SpeedyMatt]

Thanks Neganox for brainstorming on this! Much appreciated! Totally agree on the Densos, was planning on that as soon as I’d resolve this issue (the NGK is already in for the specific cylinder that misfires).

I’ve seen a number of gaps recommended, but came across this manual page for Gen 1 CX-7 showing 0.028-0.031. Where did you get 0.034 from? (See attached)

Good idea on the start-up routine. No CEL this morning while doing it but I could still feel a very very slight misfire at the red lights (but I am talking about super slightly more vibration; not your typical misfire shaking the car). The CEL comes on perhaps once every 10 starts or so, and usually within 3 minutes of run time (58s, 48s, 6s, 35s for the latest four P0302 CELs. 152s before I had changed the plug). I will continue to try that for a few days (especially after a longer drive this weekend)

What do you suspect?

Thanks!!

 

[neganox]

Thanks Neganox for brainstorming on this! Much appreciated! Totally agree on the Densos, was planning on that as soon as I’d resolve this issue (the NGK is already in for the specific cylinder that misfires).

I’ve seen a number of gaps recommended, but came across this manual page for Gen 1 CX-7 showing 0.028-0.031. Where did you get 0.034 from? (See attached)

Good idea on the start-up routine. No CEL this morning while doing it but I could still feel a very very slight misfire at the red lights (but I am talking about super slightly more vibration; not your typical misfire shaking the car). The CEL comes on perhaps once every 10 starts or so, and usually within 3 minutes of run time (58s, 48s, 6s, 35s for the latest four P0302 CELs. 152s before I had changed the plug). I will continue to try that for a few days (especially after a longer drive this weekend)

What do you suspect?

Thanks!!

I've gotten CELs on cold days while doing a cold start, but I had my VTCS flappers removed, so that could've contributed to it. No CELs ever when I switched to the warm startup only routine.

I think cobb recommended .034 for stock heat range plugs. NGK recommends .032

 

[SpeedyMatt]

Evening update: bypassing the start-up routine did not work tonight. P0302 back :-( Any other thoughts? Continuing to behave well when moving; only identifiable when stopped, especially when not in neutral/Park. I have additional sets of freeze frame data, so let me know if that’s of any help. Thanks again for thinking through this!

 

[Ssinstaller]

Next step would be to swap in a known good injector. Or if you can afford the downtime send all your injectors off to be cleaned and tested.

 

[Mauro_Penguin]

Just a thought but have you checked the crank position sensor for any crud build up, or timing position?

One day I had some dirt on mine and it caused the engine to stumble heavily and stall at idle and low RPM.

I sprayed it with some MAF cleaner and haven't had an issue since.

 

[SpeedyMatt]

Thanks, I wouldn’t have thought of that possibility.

Just a thought but have you checked the crank position sensor for any crud build up, or timing position?

One day I had some dirt on mine and it caused the engine to stumble heavily and stall at idle and low RPM.

I sprayed it with some MAF cleaner and haven't had an issue since.

Didn’t you get a P0300 or perhaps even a code specific to the sensor though (rather than cylinder-specific misfiring code)? Very curious about it... Mine is so slight that it doesn’t seem nearly as bad as the issue you had encountered but that could nevertheless still be the same issue. Thanks!
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Next step would be to swap in a known good injector. Or if you can afford the downtime send all your injectors off to be cleaned and tested.

The injector is indeed what I had in mind (I plan to do a compression test to put my mind at ease before that too, but I am intrigued by the CKP thought).

Do you, or anyone here, have instructions on getting to the injectors (and removing it without having a CP-E tool)?

Also, any recommendation on places to send them for testing/cleaning. Located in SoCal but don’t mind shipping... Thanks!

 

[Mauro_Penguin]

It was a misfire code but it was so long ago I don't remember.

As far as getting the injectors out, corksport sells a tool for $40. If not you can use a vice grip with slight pressure, and while rotating the injector, give it a slight pull. It will take forever, but you will get the job done.

@Raider who did you send your injectors to for cleaning?

 

[SpeedyMatt]

Interesting find as I keep on chasing this resilient annoying yet insignificant P0302... gold star for Neganox as I kept experimenting with the bypass of the cold start routine (aka AWS, Accelerated Warm-up System). After another month I can confirm that bypassing that routine works, all the time. No CEL ever when I do that (the time that I reported it not working must have been because I had started it 10 minutes earlier without bypassing it properly and probably had a pending code). Starting the engine cold in the morning without bypassing that routine earns a guaranteed P0302.

Also learned from a buddy in engine R&D that this routine changes the timing of the ignition (to warm up the catalytic converter) and basically makes the engine run on the edge of stability and a slight issue would indeed trigger a misfire. Incidentally I came across a TSB for the recent Mazda 3 and I believe CX-5 (Not the Speed 3) that describes the same issue and directs the techs to clean the intake valves and reflash the engine.

Do you guys believe in this, and do you think a can of CRC could be good enough to clean the air intake valve from the carbon deposit to do the trick? (If so, would you spray that in the hose right at the end of the inter cooler??) I’m sure a proper scraping job of the valves would be much better but that’s also much more involving...

PS: I also did 4 tanks with Techron. No changes.

 

[neganox]

In-tank valve cleaning solutions wont work because direct injection. The only way to clean your valves is the scrape/media blast method while the intake manifold is off.

Running catch cans, removing / disabling your EGR system, and spraying 100% methanol all do a good job of lessening the carbon buildup on your intake valves.

Glad the warm startup trick worked for you as well!