I have a 2003 evo with only 50k miles on it, ibought it about 8 months ago and it ran perfect. Outta no where one day it started missing under boost, I figured it need new plugs, I tossed in some new plugs and it ran a lil better but still missed at high rpm. A few weeks later the plugs must have fouled out cause it was running like crap and sounded like a lawn mower. I changed the plugs again and its the same. Could this be a bad coil? The is running stupid rich too I can smell the gas strong. The car will start but it runs super rough but it can hold a idle, check engine light is on but the reader says its just a misfire. I think its something else since its running super rich. Its fouling plugs really fast. Maybe mass airflow sensor? O2 sensor? I unplugged the mass airflow and started it, it spat the code for thr sensor but ran about the same, rough smelly and lawn mowerish but held a idle. Any input on what kind of tests I can run to help diagnose the problem would help a lot. Only mods are a 3 inch turbo back and a larger intercooler and piping(its not crazy huge)

 

Boost leak check should be first thing you do.

 

The misfire code should show which cylinder to head you in the right direction.Do a comp test on all 4,quick and easy,and go from there.

 

+1on the boost leak test.

 

im going to build a boost leak tester tommrow and try that, missfire code started at cyl #3 and then became #1 and #3 cause its fouling plugs i pulled one out and it smelled like gas.

 

+1 Check all your intercooler pipes/clamps.

 

If it's running lean enough to cause a misfire at idle, it should trigger a
lean cond code. I'm betting on a bad coil or wire. Easiest way to check would be to pull injector harnesses one at a time. When you pull one and nothing happens, you found the problem cylinder(s). If it is a coil, it's affecting two cylinders. If it's a wire, it should only be affecting one. Does the car still have power at WOT?

 

car tuned? what boost.

 

Cars at stock boost. I did a boost leak test today and found one very small leak from one of the vacuum lines coming off the intake mani. I fixed the leak and took it for a spin around the block, now this is when it gets weird. The car still runs like a lawn mower at idle, and when driving normaly it backfires and sputters quite a bit, it doesn't stall or smoke really, just runs rough. The funny part is when I get into the boost a bit it actually runs better? It sputters a bit when the turbo 1st starts spooling but after I would say 6 or 8 psi the car takes off and runs a ton better, huge difference. I'm stumped, maybe bad gas or a clogged fuel filter? Oh and the check engine light turned off by itself, I didn't reset the ecu or anything, just shut off and hasn't come back on. This car is weird man lol. I plan to take it to my mechanic in the morning and seeing if he can make sense of it.

 

Clean and/or replace the plugs. Sounds like you fixed the problem but your plugs are all screwed up from running rich so long.

 

Funny because I had the same problem with my 8 too last week! I took my plug out and gap them t0 around .27 .28 and made sure i didn't have water in the combustion chamber, happen to me last year. I and use my torque wrench to put my spark plug back and now it all gravy!!!

 

My mechanic suggested changing the plugs and wires again and running some injector cleaner/octane booster through. I ordered the plugs and wires and I'm picking them up tommorow. Thanks for the help guys ill post if that fixes the problem.

 

Swap the coils to see if misfire moves. If it does its a bad coil

 

I had a similar problem with a few weeks ago. I was getting misfires at WOT and full boost. I actually delt with this problem for a few months before I got it figured out. I tried changing spark plugs twice, had the car retuned, the car was even at the shop for two weeks to fix some exhaust leaks. In the end my big problem was.... A dirty air filter!! Initially I thought the filter was fine but after I looked at it a little closer, I saw that it was a K&N and the filter is supposed to be red not dirty gray!!! I changed the fliter and the car runs like a champ. Of course I feel like an idiot for not checking it sooner but sometimes it's the really simple things that we overlook that are the source of the problem.

 

Does your car have a catalytic converter? If so, you might want to take it easy on the driving until you fix it to avoid clogging it.