the assumption has been that the test itself causes a stumble from what others said in the past. I'm not sure I subscribe to that theory though, it's just what people though a while back.

the easiest thing to do is disable the test routine and see if that fixes your problems. If it does not then you probably have a real issue.

 

Having the P0300 trigger at highway cruise speeds which cause the stumble many many have experienced. If you simply get off the pedal and back on again the car will usually accelerate normally again. The CEL will stay triggered though

People that have disabled the code routine have no longer had the stumble during Highway cruise situations.

 

yeah that's what I remember hearing ^^ for some reason I didn't notice a problem with it came on with my 05 EVO 8. I disabled it anyway because the SES light was being set off randomly.

 

This

 

oops that should have said "i've NOT been"

 

I used to get the stumble just about every ten seconds on the highway. I have not had it since I disabled the p0300.

 

P0300 can have a variety of causes. For some it's bad plugs, bad tune with bigger cams, or a harmonic condition from the drivetrain that is resonating to the knock sensor.

In the case of the harmonic, generally the car runs, idles, and drives great otherwise, and the fault can be consistantly repeated by holding a steady speed between 2800-3500 rpm in 5th gear, and sometimes in 4th as well. Sustaining a certain RPM in that range for 5 seconds will fault the test, throw the code, turn on the CEL, and the ECU will then respond by cutting power drastically. Lifting off the throttle triggers the ECU to 'reset' the power cut and you get full power back until the condition is detected again. To confirm this, shift out of 5th, release the clutch in neutral, re-engage the clutch, and go back into 5th and try to produce the fault again. You may find that doing this out-and-back-in will reduce the frequency of occurance, perhaps even eliminating it for as long as you stay in 5th and keep cruising. It would seem as if you've found a "sweet spot" with the synchros and/or tranny, and thus confirms the harmonic condition. If repeatedly going out-and-back-in makes no difference at all, perhaps you need to look toward cams and/or tuning to resolve a real misfire problem. Since a tranny rebuild is costly, and since the fault is being falsely thrown by the confirmed harmonic, this would seem to be a justified case to disable the P0300 test for the short term. The real fix is to have the dealer upgrade the ECU firmware (reflash per tech bulletin) to handle the harmonic condition while keeping this test enabled for real misfires, or perform some expensive work on the tranny to get rid of the harmonic. I realize we're all out of warranty, and that the dealer may not touch the car if you've put in a custom tune, and even if they will, they'll wipe out your custom tune in the process. If you can get them to reflash the ECU, save your ROM before and after and retune the car immediately after confirming that you no longer can reproduce the P0300 fault. If you get the fault again after retuning, then the tune is to blame and you can fix that.

My point to all readers is simply to make clear that disabling the P0300 test hides the underlying problem. If you're getting the P0300 fault under conditions other than the above described harmonic, pursue other methods to correct the actual cause. If you ever get a real misfire and this test is disabled, you won't get the notification and the ECU won't react to save the motor.

 

havent had the code in almost a week!!!!

 

I too had this issue mostly during highway crusing. After doing the mod I no long have the "Stumble" or "Miss". Car runs great and has been for over a month since doing the mod. I do believe it is due to drivetrain noise as my tranny is very noisy and in need of Mr. Shep to work his magic.

 

Mine was plugs, swaping from copper to irridium NGK 8ES's fixed the problem.

 

im going to try this on my 06 IX. i get the code withen 5-70 miles of driving. i never have had any driveability problems just that the light pops up

 

Was plaqued with this on a customers evo5 with 7 ecu fitted ,eventually his gearbox 3rd gear gave up we fitted another box and bye bye po300 .
had it on my own evo4 also using 7 ecu but bpr8es plugs cured that

 

looks like I'll be trying this sometime this week...

 

very helpful

 

Still works!! Two years of constant code clearing drove me crazy! Thank you mrfred, you've brightened my day