Both times i've had it, were when cruising at 70mph (bang on) on the motorway after a couple of mins.

If I am above 70 or below, then it's not a problem.

Also a mate of mine with an Evo 5 (e7 ecu conversion) was plagued with it, everytime he went out in the car it would throw up the P0300 code and drop onto 2 or 3 cylinders.

In the end he went back to a 5/6 ecu.

 

The Evo 7 code does have some significant differences from the Evo 8/9 code. I kinda suspected that the P0300 test was only written into the USDM ROMs, but if you've seen it with cars running non-USDM ROMs, I must be wrong. A different periphery bit may be controlling the test with the non-USDM ROMs.

 

My car is down for engine overhaul otherwise I could have confirmed this fix withing a few min. Good stuff! Thanks OP!

 

It seems to be all E7/8 and 9 JDM roms that have bits 4 and 5 set to 0 by default.

So maybe it is only the USDM and Euro 260 models (similar to the USDM 8) that have a test.

 

0x89DF (0b1000 1001 1101 1111) for my 99860002 EVO 7 ROM.
So Bit 5 clear, bit 4 set.
In my ROM the bit 4 is not checked alone. Only together with bit 5.

 

Yes 99860002 is a UK / Euro FQ3xx rom, so changing it to 0x89CF will clear the P0300 test for you

 

I've grown so used to the light I don't know what I would do if it turned off.

 

I had been testing 0x8201 and 0x0001 for this but hadnt any confirmed success or failure. Good to see you had it well in hand man

 

Ever since you modded my ROM months ago I havent had one since. I used to get one in 10-15 mi. of driving. Not to mention the other mods. Couldnt be happier.
Adam

 

Thanks Mr Fred, relief at last.

 

Awesome Adam I forgot I had modded yours...actually I think I owe you an email anyway...need to go look at my other email account (sorry )

 

No P0300 for me last night or this morning! Generally that would be enough time to get the code, but if it doesn't come back tonight or tomorrow I'll know for sure.

Thanks for figuring this out!

 

mfred, how does this affect the OBDII readiness state? I need to smog my evo soon.

 

Shouldn't affect readiness as there is no "not ready" or "readiness" state for the misfire test.

 

I'm going to try this today, I'll report back in a few days whether it comes back or not!