AuthorizedUser

I have a 2010 G37 Journey 4 door with the Sport package and around 85,000 miles.

A couple of years ago, I started the thread Backfiring under hard acceleration as I thought I had a misfire problem. The responses helped me see that this isn't a misfire since have never had a SES light. Yes, I do have a code scanner, and it pulls nothing interesting.

I think what I have is actually an exhaust leak. Here's what I know:

• The sound resembles a misfire through the carburetor on my old '74 Chevrolet Nova. Note: resemble ≠ identical!
• It is proportional to engine effort or throttle position. I think this helps my case. More throttle = more exhaust back pressure.
• It is worse with lower temperatures and absent when ambient temperatures are around 80F or higher.
• It stops a few minutes after the engine is warmed up. It takes much longer than when the engine fully reach operating temperature. (Yes, I know that the engine reaches full operating temperature a few minutes after the coolant temperature reaches normal temps).
• No SES light, ever.
• No noticeable effect on engine performance.
• Occasionally, after fun driving that has brief hard acceleration, I get the sulfurous burp. Not sure this is related.
• Two treatments of fuel injector treatment had no effect.
• No other obvious symptoms.
• Doesn't happen if I re-start the car when it is still pretty warm.

Does this sound like an exhaust leak? What should I check?

My theory: Some exhaust components take much longer than the engine to reach normal temperatures, especially in cold weather. One of these components has a leak that self-seals once it gets warm enough. But which components?

SonicVQ

Buy OBD Fusion (available for iOS and Android) and run a diagnostic report when the engine is fully warmed up at idle.
This video will tell you what the Mode$01 PID (engine sensors) are and what value they should be:

Have a close look at your total fuel trim and in the last 2 pages of the report, look at the mis-fire counters.

Typically, dirty MAF sensors will overestimate the amount of air at idle and underestimate the amount of air under load.
This will cause fuel trims at low engine speeds to be negative and the fuel trims at higher engine speeds to be positive.

AuthorizedUser

Thanks. I'll need to re-locate my OBD2 Bluetooth adapter first.

Should I use driving behavior that causes more of the misfire/exhaust leak/whatever this is? Or drive normally?

SonicVQ

To start, I would drive normally to warm up the car, then run a diagnostic report t idle.
(make sure to save it and email it to yourself, see this for a quick overview: )

On the report you emailed to yourself, open it and under the Mode $06 section, look for the the "misfire counters" THIS will tell if the car is misfiring.
Post your fuel trims (short and long) and misfire counters and this will help to give a sense of direction.

AuthorizedUser

Thank you. Found my reader and purchased the OBD Fusion app. The car has been off for an hour, but I went to the garage, turned the ignition to ON without starting the engine, and connected my reader.

The misfire counts for all cylinders is zero on all 12 readings. (Half the readings are EWMA for each cylinder over past 10 drives, and other half are for prior and current driving cycle misfires for each cylinder.)

I'll get trim info tomorrow when I arrive at work; obviously it's zero right now.

AuthorizedUser

In this morning's drive, all $Ax misfires were zero except:

• $A4 - Misfire Cylinder 3 Data (TID $0C - Misfire counts for last/current driving cycles): 1 misfire
• $A6 - Misfire Cylinder 5 Data (TID $0C - Misfire counts for last/current driving cycles): 2 misfires

Fuel trims:

• Short term fuel % trim - Bank 1: -9.38%
• Long term fuel % trim - Bank 1: 0.78%
• Short term fuel % trim - Bank 2: -6.25%
• Long term fuel % trim - Bank 2: 0.78%

The video recommended adding the two long and short numbers together for each bank and that you're in good condition if the sum is between -10% and 10%. (I am not sure I agree with that since these are different measurements, but whatever.)

The car had run for 17.5 minutes on a 6.5 mile drive. Ambient air temp was just below 40F. I did not get into it hard, but on a handful of occasions, mainly in the first 1/3 of my drive, I caused the exhaust leak-like sounds due to moderately aggressive use of the fun pedal.

SonicVQ

With a misfire count that low I don't think you have a misfiring issue.
The reason you add the short term to the long term fuel trim is to determine the TOTAL FUEL TRIM for each bank.
Your fuel trims seem to be in the OK range (total fuel trim around -10 to +10%)

Everything seems to point to a small exhaust leak.
Maybe record a video with your phone under the car, and revving the engine.
In your other video, I didn't hear anything unusual.

Or you can just do nothing... Small exhaust leak always become larger exhaust leaks and then they are easier to find

AuthorizedUser

Thank you. Does the occasional sulfurous burp help narrow where the leak could be?

The intellectual challenge with adding the two trim figures is you're adding a long-term average to a short-term average, if I'm understanding the figures correctly. That doesn't make sense to me.

SonicVQ

Yes, the odd exhaust burb mean you have an exhaust leak somewhere
You might want to get under the car (jack stands, ramps, hoist, oil change pit) with the engine running and you will be able to hear where it is coming from.

As for fuel trims:
The fuel trims are not TWO different averages. They are ONE average over TWO different time frames.
The main goal of the short term fuel trim is to get to ZERO. It does that by increasing or decreasing the long term fuel trim.