Titus210 Posted December 20, 2018 Share Posted December 20, 2018 I have a 98 Legacy GT Limited that is throwing a cylinder 4 misfire erratically, as well as a few other problem areas that I am trying to diagnose. I've been searching everywhere for what the normal reading should be using an OBD2 scanner, but I'm not having much luck finding a list or reported readings that others have taken. If you have done normal data logging, what have your readings? Or what is a good place to look to find what the normal specification should be? Looking specifically for calculated load percent, short-term fuel trim and long-term fuel trim, map, ignition Advance, MAF, absolute TPS O2 sensors. Thanks for the help. Also if any of you guys have had this erratic cylinder 4 misfire come up, were you able to resolve the issue and if so, what did you do to get the code to go away? Link to comment Share on other sites More sharing options...
doublechaz Posted December 21, 2018 Share Posted December 21, 2018 Mine is a 2.2 NA, but I have live data up pretty well all the time. MAF is about 2 at idle and tops somewhere over 100 scaling pretty close to linear. At 5500 ft ASL, I used to get about 75% to 80% at WOT redline. I'm not sure what all was less than perfect back then. Now I run polished heads/intake, straight exhaust, and I get to 100% load a while before redline, and then it just stays on 100 on the way up. Spark coasting down a mountain I've seen go as far as 49*, but normal driving is much less. I think 14 at idle, but I don't usually watch this one, so I'd have to get back to you. LTFT idle is around 0 for me, but anything that is actually driving runs 10% to 15%. But mods. TPS is 0% at idle, and 100% at WOT, linear between. O2s run about .500 at idle, and bounce around from about .8 to .3, but I don't watch these much as I have a wideband. As for the misfire, if you didn't already, start with the plugs and wires. Mine used to come loose until I learned to use a paperclip or something under the edge of the boot during install so that it didn't trap air in there that would later push it back off. Link to comment Share on other sites More sharing options...
Titus210 Posted December 21, 2018 Author Share Posted December 21, 2018 Contributing V.P., thanks for the reply. I've literally gone through and done nearly everything- plugs, wires, coil pack, all sensors on top of motor and a completelydifferent intake manifold with different harness. I've worked on these cars for a good while, hanging done numerous motor swaps on mine and other's Subaru's as well. Never have I had a misfire so sporadic in it's doing, or come about but not fully act as a misfire. And the craziest part about it is that this is a fully rebuilt motor with only right over a thousand miles on it. Even has good psi and within spec for compression on all four cylinders. Link to comment Share on other sites More sharing options...
doublechaz Posted December 21, 2018 Share Posted December 21, 2018 I had a somewhat mysterious misfire a while back from one cam being jumped compared to the other and the crank. But it was a good deal more common than what I think you are describing. Magnetic dust on the cam or crank pickup? Sketchy ground connection on the shield for the cam or crank sensor? It gets a lot harder to figure out when it's like this and you've done lots of stuff already. I ended up with a portable oscilloscope checking what various signals looked like going into my ECU to figure mine out. Link to comment Share on other sites More sharing options...
Setnev Posted December 21, 2018 Share Posted December 21, 2018 One tooth misaligned will cause a decrease in performance and produce random misfires but normally won't throw a code. 2-3 teeth retarded will cause significant issues and cause it to fire the spark when the piston is already on it's descent causing it to backfire out the exhaust and cause pre-detonation in the opposite cylinders. 4 teeth or more will cause internal damage to the valves and piston. Lets take this back to basics. There has to be a common denominator for the seemingly random events. Not to mention, cylinder 4 is notorious for running hotter than the rest of the 3 cylinders. First, what manufacturer and part number did you use for your spark plugs. Weird? I know, but it matters. These engines need to be running platinum or better plugs. Double or quad tip spark plugs do not play nice with these motors. Colder, or hotter, spark plugs also change how combustion is made and is as important as the gap of the plugs. If you relied on the manufacturer to trust the plug gap is correct and just installed them as is, you did it wrong. Always verify plug gap, even on pregapped plugs. About 25% of the time, they're wrong. That's 1 in 4 on a 4 cylinder engine. (see the correlation). Since you report good compression, I can assume there's no burned valves, which is common on cylinder 4. You're going to want to do some further investigative work. A misfire is due to faulty spark. Swap your plug from cylinder 4 to cylinder 2. See if the misfire follows. If not, swap the wire between 2 and 4. See if it follows. If not, it's not a spark plug issue. Fully test the coil with a voltmeter. Test each pole to see if the resistance is within spec. EGR system malfunction, faulty injector, and incorrect AFR in the air intake system caused my vacuum leaks can also cause a misfire. Is the only code P0304? Link to comment Share on other sites More sharing options...
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