nikkonikko

Okay...I have been having a persistent P0300 Random Multiple Cylinder Misfire code and at this point I am unsure as to what to try next. I am going to go through what I have done thus far and give all information I think would be helpful in determining the problem.

I have a 2003 Evo that I got with about 52,000 miles on it just over a year ago. Around 55,000 miles I got a CEL for a P0300 Random Multiple Cylinder Misfire. I looked extensively into the problem and saw on various forums that that was a common problem on the Evo 8's and that if there was no noticeable misfiring and it wasn't persistent if reset, you could just let it go. I knew at the time that the fix was getting the ECU update at a dealer (or doing your own tuning with a Tactrix cable, which I did not want to mess with). So I reset the CEL and didn't get a code again until somewhere around 65,000 miles. Reset it again and didn't get a code until late July this summer around 78,000 miles on a road trip to Colorado. When I got back home in St. Louis, I took the car to a Mitsu dealer to get the ECU update. I had no real indication as to whether or not that fixed the problem as I was only getting the CEL every 5,000-10,000 miles but I figured that it would solve the problem as I had never actually felt the engine misfire while driving it.

Then I moved to Denver for school and began having a hell of a time getting my car to pass emissions due to excessive Hydrocarbons and NOx. Around the same time I got a CEL, P0300 again. So I figured next possible problem was ignition so I replaced ignition coils and plug wires with OEM equipment. I replaced my plugs (which had only about 8,000 miles on them) with four NGK BPR7ESs. Still didn't pass emissions, so I checked fuel trims (which were normal) and EGR system which passed systems readiness on an code scanner. I figured the only thing left was the catalytic converter so I replaced that and voila I passed emissions. I wasn't too bummed though because I figured that replacing the ignition components, along with the ECU update that I had done back in St. Louis had to have solved the P0300 problem.

Now, this morning on the way up the mountain I got another CEL and I just checked it...of course its a P0300 misfire code. It should be noted that since I have gotten the ECU update, I can definitely feel the motor retard the timing semi frequently (every 10 miles or so under load) AFTER the CEL has been tripped. If I reset the CEL, I will not trip it again for potentially hundreds or even thousands of miles and I will never feel it retard the timing as long as the CEL is off. Once the CEL is tripped, the system seems to become much more sensitive and slows the motor temporarily (I can definitely feel it) fairly frequently until I reset the CEL again.

I have no idea what to replace next. Keep in mind that the car lived its life up to 79,000 miles at nearly sea level in St. Louis, where 93 octane gas is also plentiful. I am now 1,500 miles of driving into Denver which is over 5,000 feet above sea level and where 93 octane gas is literally nowhere to be found. I have put Shell 91 octane in the tank since I have been here. 100 octane is available for mixing at a Shell near my house, but it is 7.49 a gallon right now which a little concerning on the wallet. Is it possible that the 91 octane gas is causing higher ignition temps and that I need spark plugs with an 8 heat rating and not a 7? Is it gas related? Is it something I have not even looked at yet?

My Evo has no considerable mods. Turbo-back 3 inch exhaust, boost gauge, Exedy twin disc clutch (old owner replacement, I'm not complaining). Let me know what you guys think...I appreciate any help.

nikkonikko

Also, today I checked the freeze frame data from when the CEL tripped and noticed the following fuel trims. I don't know that much about fuel trims but these seem odd to me...

SHORT TERM FUEL TRIM1: -5.1%
LONG TERM FUEL TRIM1: -2.2%
SHORT TERM FUEL TRIM 3: -100.6%
LONG TERM FUEL TRIM3: -100.6%

614erich

Dont know if it will help but check it out

http://www.aa1car.com/library/us10325.htm

My03evo

iTrader: (1)

The gas link was a good read...

Deff something wrong with those LTFT's tho

raulthode

One of the most overlooked cause for misfires, is a dirty fuel injector. Good practice and as preventive maintenance, is to remove them and have them cleaned, at least once a year.
Especially if you question gasoline quality. Note: cleaner additives can only do so much in preventing; nothing beats a good ultrasonic bath....