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I'm looking to purchase a PSM '04 STI from a guy in Long Island, NY. I went with my brother so he can look at the car for me since he's much more "mechanical-savvy" than I am. We found out that the car has a CEL throwing only one code which is "P0304 - Cylinder 4 Misfire Detected."

The owner was kind enough to let my bro run multiple test on the vehicle so he could diagnose the issue. We were assuming a worst case scenario of a worn piston ring and/or ringland failure but the compression numbers came back at 148psi on cylinder #4 on a somewhat warm motor (car was turned off for about 30 mins prior to the compression test at 38f weather). My bro also checked for any signs of smoke coming out of the oil filler tube and exhaust and none was existent. He also swapped ignition coils from cylinder #2 into cylinder #4 and the misfire didn't follow the swap. At this point, we're assuming the worst case scenario could possibly be caused by a bad valve/seal. My bro also did notice that the spark plug on cylinder 4 was darker than all the other spark plugs leading him to believe it could be caused by rich fuel conditions (possibly a faulty injector?).

The motor is fairly stock as far as I can remember with 138k on the clock, no mods were done to it other than an upgraded wastegate. The ECU is running off a TurboXS piggyback system (information on the tune is not available as the owner claims he purchased the car like that). The only other mod is an aftermarket catted DP along with a full cat-back exhaust (brands unknown). The motor pulls strong on the test drive, however, the misfire is evident during idle where it would...idle, pop pop, idle, pop pop..etc.

I'm hoping the community could chime in on some other test we could perform to get a better idea what I will be getting into if I do decide to purchase the vehicle. Any feedback is greatly welcomed and appreciated, thanks!
 

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1999 RSTi coupe
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402 Posts
You seem to have all the bases covered. Last thing would be to replace the spark plugs and finally the fuel injector if the issue persisted. With the compression #'s coming back promising then I would think it would be a pretty safe buy.
 

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ok terrific.
'95 STi RA rep, rhd yo!
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5,223 Posts
could be a tuning issue.
Thank you for that insightful post.

OP, the compression test isn't worth much alone, especially without testing at least the adjacent cylinder. Retest and do a cylinder leakdown test if you can. I'm willing to bet it's a hasty sale because of a broken ringland, which I've seen most often occur on that cylinder. A cylinder leakdown test is the best way to tell for sure.
 

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needs more cowbell
95L, 07 2.5i, R6
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9,315 Posts
+1, just assume the cost of possible ringland damage into the sale. If the guy wants to sell it he'll bargain. The seller will have to pay either way, unless he finds a buyer who jumps the gun.
 

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there was a reason I stated it could be a tuning issue.
I have had Clark Turner "road tune" one of my STis (never again) and he did a terrible job, I had tons of issues with the car and many CELs and lean conditions (actually since I did not rag on the car was the only reason he engine did not blow according to the real tuner whom saw the terrible job he did). Once I had the car retuned by a real tuner, no more issues.

Next time before you dismiss someone's comments, maybe you should ask "why would they think it could be a tune" all you guys keep jumping to piston issues, when he is clearly stating lean conditions. if he had piston issues, he would have white a bit of oil smoke when in mid to upper rpm ranges, much lower compression numbers than the rest of the cylinders, and lots of knock sensor triggers. he does not seem to be having those issues. (yes again from experience.

OP, before you go trusting half these guys, do yourself a big favor, find a real tuner and have them take a look at your tune, at the same time do a complete motor compression test. if that cylinder is more than 15 lbs different than the others, you have a mechanical issue of sorts. It could be anything from HG going bad to yes possible piston issues.

That being said unlike you monkeys whom jump to worst case before looking at alternatives, I did not want to jump to a conclusion before it being looked into. telling someone who has 1 cel he has a blown motor is about the worst case of troubleshooting I have ever seen. you guys should be ashamed...
 

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2000 Impreza Outback Sport
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Sure it could be a tune... why bother diagnosing it?

The OP was interested in purchasing the car... why go to all of that trouble for a purchase?! The car has issues, so unless you are prepared to correct them, stay away...
 
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