City driving was fine, even under boost, but on the highway I threw p0300 random misfire and started misfiring around 70mph. If I let off and then on again it would be fine, but if I tried to keep a constant speed it would misfire. Read through the forums seen there was a TSB out for this common issue, decided to take it to the dealership.

Dealership tells me the TSB is up to date on my vehicle. So I’m really not sure what else it could be. Any help in the right direction is very much appreciated before the stealership drains me out.

2003 Evo 8 (previously had sat for more than a year without an engine/trans etc.)
- 77k miles
- ETS upper intercooler piping
- K&N typhoon intake (stock MAF)
- ACT HD clutch less than 1,000 miles
- Straight piped
- EGR delete
- APS BOV
- Everything else is stock and no other CELs aside from p0300 random misfire
- Spark plugs, ignition coils, all oils changed within 1000 miles

 

have you done a boost leak test?

 

I’ve been meaning too, bought some of the things to make my own boost leak tester but then the air compressor crapped out. So in short no, it hasn’t been tested for a boost leak.

 

I would definitely be doing that as a first course. Boost leaks = vacuum leaks, and they both mess up fueling.

 

Will look into that thanks. With boost/ vacuum leak would I be getting the symptoms I described? Where I can boost and everything at low speeds but at highway speeds is where I get the actual misfire symptoms.

 

What spark plugs and gap? Also oem ignition?

 

Small leaks effect low airflow volume like cruising the most.

 

BPR7ES and gapped to about 25 if I remember correctly. Looked it up on here what most people run on stock setups and did that. And yes oem ignition.

 

Ahhh okay I see, thanks definitely doing a boost leak then once I get the car back from the dealership who seem to be at a loss after 2 days

 

When the dealer said the TSB is up to date, what does that mean? After some testing the TSB required replacement of the transmission input shaft and 5th gear. Was that done?

 

Hey finally something I can post about! I too had this same issue (highway p0300 code). I did a ton of research on the problem and came to the conclusion that the HD clutch was the problem (I have the Exedy Twindisc HD). It creates a vibration that the ECU thinks is engine knock or misfiring and puts the engine into limp mode. It's not actually knocking/misfiring the ECU just thinks it is. The fix for it was to tephra mod the ECU and delete the p0300 code. I have not had a single code thrown since the code was deleted.

 

Appreciate the response, although I have a ACT HD clutch that’ll be something I look into next, I’m assuming it might just be an aftermarket clutch thing on the 8s that causes this p0300. But I also recently just did a boost leak and had a few leaks: boost control solenoid, throttle body and a vacuum line of the manifold. I have yet to hit the highway and test her out to see if after sealing the air leaks I still throw the code. Will update it this thread.