I had this problem last winter. Car does random misfire and goes into limp mode with a loud exhaust backfire. I have cold air intake, test pipe and CBE at that time. I end up changed a faulty MAF sensor to fix it. There was no any code, but i monitored the MAF reading on my ECUTEK APP and found the problem.

 

I have exactly that... I'm fixing the exhaust leak. Then I'll do that. If that doesn't work... Running it into a wall to get insurance money Lmfao

 

You should send you car to a performance shop and let them diagnosis the car. I won't let dealer do those. find a good mechanic is very important.

 

Also try to wash the maf sensor with maf cleaner. cuz they will be dried oil stick on them...

 

I am starting to wonder if there is a connection between having test pipes and the random mis-fire codes.
Over the last few years, when trying to help multiple people on Facebook, Youtube, and various forums to troubleshoot this, once common mod seems to be test pipes.

Most think the rear O2 sensor is just there to test for proper catalytic function, and this was true in the past.

However, when the front O2 sensor (narrow band) changed to an air/fuel sensor (wide band) many manufacturers now use the narrow band rear o2 sensor to check and adjust for any lean or rich bias the AF sensor might develop over time.

AND with the test pipes, most people add an O2 extension trying to trick the ECU into thinking the slow switching rear O2 (if it does actually switch) is actually a functioning catalytic converter.
BUT this may be telling the ECU that the front AFR sensor has a rich bias (due to the lack of oxygen in the o2 extension and the rear O2 reports a rich condition) and leans the mixture.

It would be interesting (and a huge pain in the but for someone) that has this issue, to remove the test pipes and put in the EOM cats to see if the P0300 goes away.

Thoughts?

 

There was no exhaust leak like i thought, and now i am resetting my computer every other day. It throws the code within 60-120 seconds after i cold start the car. If the light does not go on within that time frame, it does not go one at all while driving and i have driven 100 miles with no issues. I was debating changing out the MAF, and RockAuto has 10 different ones for sale ranging from $60 all the way up to OEM $280. Any recommendations on aftermarket MAFs, brands, etc.? Last thing i wanna do is drop 500+ on 2 MAFs that i dont need...

 

Which MAF did you go with? OEM or aftermarket? Where did you get it at and how much did you pay?

 

I have re-read everything, and have some thoughts to add:

You can use OBD Fusion's ~ $14 "Enhanced Diagnostic" add on to view the bank 1 and bank 2 MAF voltages under different engine loads, as well as just about every other engine sensor.
(I have verified this on my test bench with a 08 G37 Coupe ECU)

Has any work been done on the ECU? You state the vehicle is a 2008, but the ECU ID (JK75D) is from a 2007 G37... Kind of weird.

Has a compression test been done? I think it is time to look at mechanical reasons for a misfire.

Also, keep in mind how the ECU determines a misfire.
It looks at the angular acceleration of the crankshaft for each for each power stroke. If there are differences, it declares a misfire.
Is the accessory belt in good shape?

 

I just helped my friend's 2012 FX35 last month, both mass air sensor low-performance codes. we changed air filters, maf sensors, tested leaks, nothing worked. In the end, idle relearn fixed it!

 

I am having similar issues. I have added the data log. I cant seem to find the issue. I have long tube headers with a cat back exhaust. I made sure the flanges were flush and no exhaust leaks. It drives but at high RPM check engine light blinks. Hope someone will be able to help me out. I have done a lot of work to try to find the issue. Even did an engine swap and still having the issue. I put the long tubes on made sure the nuts were torqued to spec. I'm at a loss right now I cant identify what the issue is. I'm trying to learn how to tune. But unsure if it just needs a tune or if there's something more major going on. I moved the MAF sensor to see if the problem would move banks and it never did.

 

I've got a p0300 code for multi misfire but they are very minor you can't feel them. I did do a log. Inspected exhaust for leaks. Going to change the plenum gasket throttle body gaskets and clean them again but I feel like it's something else. Would someone be able to take a look at the log. Thank you. I'm new to this form.

 

Your short term fuel trims are all over the place... they cycle between -25 to + 25. The should be more in the range of -15 to + 15.
The blinking CEL indicates a cylinder misfire. My guess would be that happens when the fuel trim is +25 (remember a + fuel trim is adding fuel to correct a perceived LEAN condition)

Closely examine every part of the intake system and check for any rips/cracks in the rubber tubes/hoses. You might want to spray something like brake clean on the intake, with the engine at hot idle. If the engine RPMs increase there is a intake leak.

I don't have any experience with long tube headers, but I do know they typically place the pre cat AFR sensor and the post cat O2 sensor VERY close to each other.
With no catalytic converter and the sensors being very close to each other, the signals sent to the ECU are not what it expects and then weird things happens.

You might want to ask for help on the HPT forums top understand what you can tune out or turn off.

 

Keep in mind, every misfire sends raw fuel to your catalytic converter, even if you can't feel it. Over time, that will shorten the life of the catalytic converter.

Fuel trims and AFR looks good. This makes me think fueling and intake vacuum are not the problem.
I would have a look at the condition of the spark plugs and check the ignition coils for small while lines on the rubber boots. (that indicates arcing to the spark plug tube)

 

So I'm running motordyne catless downpipes. I do get the CEL on cold winter starts and when flooring it randomly will go into limp mode flash the check engine, might just be pulling timing due to knock, and then stop flashing and start driving normal again after a minute or 2. Spark plugs and coil packs were replaced less then 10k miles. I did use z1 high spark coils idk if that would cause an issue. I thought it should be better. Also did oil pressure test. Idle is about 20 psi and at 2k is 55 psi and 80°C. I did order new throttle body gaskets, plenum gasket and new maps. 103k miles if that matters. Cannot get re-tuned at the moment since clutch is slipping at peak torque. I'll do a cold start log maybe it will be more noticeable. Thank you for your help.

 

Hello everyone, I'm still having troubles with a p0300 code. Although I did find new codes too but after I threw parts at it. I replaced injectors upstream and downstream O2 sensors on the passenger side both maf sensors, upper plenum gasket and still have p0300 on cold start and randomly under higher load. Any advice on what some of the codes mean.

Not sure what U122A-05 is