98 Impreza RS 2.5 P0301, 302 & 304 codes intermittantly

Hello, I bought my impreza RS with a bad knock sensor and a broken heater wire in #2 O2 sensor. I repaired those issues. I also did an initial retune including plugs and oil....the original plugs looked good but were extremely worn. Those being fixed, I have had no problems until I replaced the exhaust with a 2.25" (stock exhaust mainfold/header remains for now) piping, high flow cat, resonator, and cannister type muffler. At the same time I replaced the intakesystem with an aftermarket cold air intake reusing the stock MAF meter and sensor. I cleaned the sensor and did a computer reset. All this gained me about 3MPG which I realize is a bit over what I should expect but was only getting about 23MPG hwy before. I am 48 years old and do not drive the car hard or race. Now the problem...

On my first road trip after doing the above changes (about 600 miles total trip mileage) after a near full throttle acceleration trip through the gears I received a p0301 code which when cleared has not returned. All has been fine for the past couple of weeks driving the car to work and back to the tune of about 300 miles per week...Today while cruising to work at about 75 MPH (70 mile one-way trip) about 5 miles short of work I got a CEL which when tested showed 2 codes. p0302 and p0304, 2 instances of each (4 codes total, 2-p0302 and 2-p0304). Since I carry my reader with me, I immediately checked and then cleared them. About a mile later the CEL flashed again and the reader showed that the same codes had been thrown once more. I realize in order to fully clear the codes you must restart the engine but when the p0301 cel came on the first time, I cleared it while driving and it did not return since.

I am wondering if the coil pack (which appears to have been replaced at some point) might be causing problems and if so is this know to be a weak area of the engine? I noticed several car parts outlets that sell the coil packs only offer 3 month warranties on them...

The car runs fine and I have not noticed any misses or other abnormalities other than a rare low/lopey idle sometimes which is cleared by quickly reffing the engine as though to clear out a flooded/'fuel loading' condition (maybe not but that is how it presents itself...will idle at 4-500 rpm and lopey as long as allowed but if revved and alowed to idle will settle back into the normal 750-800 rpm smooth idle). This rare instance may or may not be related to the cause of the CELs but before spending $100 on a coil pack, just wanted to get some thoughts about the likelyhood of it being related to a weak coilpack and/or how long the life of a coilpacks is.

thanks,
Les

Actually as you said could be the coil packs, is not a bad Idea to change it, but much better if we can be sure the coil is producing these before changing it.
Before you replaced or modificated the intake system, did you were having the missfires ?? Does the missfires appeared inmediately after the modifications ?
Start by checking if there is no losen wire/harness over the coils, then check if there is no losen or dammaged injector´s # 1 #2 # 4 /wire harness. Maybe when you were tuning the engine you accidentaley dammaged wires.
Also check if your intake manifold was well attached and tightened to the engine, check if it is not leaking air..

Keep posted ¡

Tnx for the response.

The coil packs being at least $100 per set it will be pretty expensive to buy a set just to try, especially in light of the intermittent nature of the problem. I was just wondering how reliable they are, in gereral. I e-mailed a friend with t newer WRX and he said he has never had an issue with his on his fairly highly modified engine so I am thinking they may be pretty reliable.

As I have stated in my reply to the other post....follow up on another thread by a guy with a 2.5 with same codes looking for help where you replied that I should check my injectors but I had not given the entire recent mod story...the CEL did not occur until after the intake and exhaust mods. the p0301 code that did not return after being cleared happened about 70 miles after both mods...I did the exhaust and drove it to work and back (150 miles) without problems. The CAI arrived and I installed it just before leaving on a road trip over Memorial Day weekend. I got up and and left on the trip after only having driven about 25 miles on the CAI + exhaust mod....I had no further CEL since last Friday (1 week and probably 650 miles) until the 2 consecutive sets of P0302/P0304 codes today while cruising at 75 MPH on the highway.

I have checked and rechecked the CAI tube "joints" and other connections (hoses, gaskets, etc) and all are tight...the engine idles fine (except for the rare lopey idle issue I described which is cleared by revving the engine to "clear it out" and letting it idle back down).

I had not even approached the wiring harness to the injectors or the coil during my mods but have just gone out checked them and found all connections to be secure, clean, and dry to the best of my "checking ability"... any intermittent connection excepted, of course.

I am still leaning toward checking fuel pressure tomorrow and MAYBE then going with a coilpack tryout...problem still remains that it is intermittent so unless I find an obvious issue (low fuel pressure, broken/loose connection/ crack in the coilpack, etc) I could have to drive for a month on new "try out" parts without issue before i would have a clue that it is fixed...unless drivability changes enough to tell that something wasn't right because it feels/drives differently after any changes.

Keep the ideas coming...I'll line them up in order and go through them until I run out of ideas or fix the problem.
Les

can you monitor fuel trims?

No I cannot... I did check the fuel pressure and found it to be well within specs...about 38psi with engine off key on. 28 psi at idle and jumps to as much as the max of 38psi when revved or throttle held open.

Also the resistance values were good on the soil pack...I have known them to change once the engine gets a little hot so i will check but don't know that I have ever actually "caught" one until it was actually acting up which mine does not regualrly do.

I drove home today (70 Miles) without any CEL, btw and fuel MPG was 27 1/4 on the latest tank. Up from 23 in stock form and 26 afte the addition of the CAI and 2.25 exhaust with hi-flo ocnverter, resonator and cannister muffler (no headers, yet). All else is stock with new NGK plugs.

Les