OK, like alot of other Evo owners. I occasionally get the P300 misfire code. It usually happens if Im cruising on the freeway and downshift into 4th. The car starts stumbling and does not want to accelerate at all. If I lift off the throttle and get back into it, it seems to clear right up. Everytime it does this it throws the same misfire code.BTW... I've check: plugs, wires, coil packs,intake scoop etc..etc.. and they all check out fine.

Now my car has done this intermittently pretty much since it was new.I now have 37,000 miles. No biggie. I would just clear the code and deal with it again in a few months. Latley it seems to be getting worse for whatever reason and another thing I have been noticing is it only does it with a full tank of gas.

So reading through some of the forums and doing some research I see that some people have ran into defective fuel pumps and Im starting to think maybe some of these misfire codes are coming from bad fuel pumps or possibly bad fuel pressure regulators. So for anybody out there who is running a walboro pump and/or aftermarket FPR I would like to know if your having the misfire code issue or any hesitation as a result of it. Any other opinions on this issue are welcome as well.

Thanks!!

 

This is probably your answer: https://www.evolutionm.net/forums/sh...ad.php?t=88937

 

Yea, I read through that thread earlier. Good info. but it seems like alot of peopl are still having the same problem even with the Mitsu. re-flash.

Im just trying to narrow it down to the actual problem. It seems like a fuel issue to me. I could be wrong, but its pretty suspicious that mine only does it after a fill up. Maybe a venting issue in the return line(vapor lock) or something like that.

I think I may do a little testing by running a temporary fuel pressure guage into the cabin of the car and see what it does when this happens.

 

I used to get the code all the times although I had no real misfire or stumbling . I was very surprized but I started to see when I am getting the code exactly and finally I was able to solve my issue. I only get the code if I slam my hood half way through while the car is on. I know this is not your case but I want to help you. Now I just lay down my hood then I press it down.

 

Are you still stock? I'm bone stock except for the filter and bushings and haven't had this misfire at all, and I have just over 16k miles. Are those that are still stock having the problems?

 

I have an Exede piggyback and Vishnu T-back, but it was doing the same thing when it was stock.
Im running the stock box and element also.