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Old 05-27-2022, 12:05 AM   #1
big_krish
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Default 2004 STI P0303 misfire + no start

I just swapped a new type RA short block into my '04 STI, and it was running great for around ~200 miles until I let my friend drive it. Told him to not go above 3-4k RPM for the break in, but he ignored me and revved to around 7k on the highway. Check engine light started flashing, we pulled over and the engine stalled and no longer started. I had it towed back to my place and the code came back as cylinder 3 misfire (P0303).

The cylinder 3 spark plug was completely bent (the cathode was bent and contacting the anode). I replaced it, but still no start. I tested all of the sparkplugs and coil packs, but they appeared to be working (not sure if the spark was weak, this could also be a problem). I checked the timing, and it was perfectly in time. The engine also turns smoothly (no valve grinding noises).

I did a compression test, although I'm not sure I can trust the results because I used the crap O'Reilly rental gauge. Cylinder 3 compression came back zero, but once again not sure if this is the gauge leaking as well.

I ordered a borescope to check for obvious piston or valve damage, but the fact that the engine still turns smoothly and cranks without any noise makes me wonder if something else is the problem. I haven't checked injectors yet, but I doubt that's the problem since I cleaned them during the swap and there is a strong gas smell when I crank without the plugs in.

The only thing left is really major piston or valve damage, which I'm hoping is not the problem since this engine is brand new. The heads and valves are from the old engine and were professionally machined and refurbished. Will update with pictures from the borescope hopefully tomorrow, but any ideas in the meantime? Also I guess one lesson is to never let your friends drive your car even if you trust them lol
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Old 05-27-2022, 06:25 PM   #2
big_krish
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Well rip, it appears that the #3 piston is cracked. Since it's a type RA block meant for the 18-20 STI, there's no warranty on it either in my car. Really not sure how this happened either.
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Old 05-27-2022, 06:50 PM   #3
big_krish
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I'm now convinced that the octane rating of the gas could have caused this. I'm not entirely sure I filled up 91, but could putting 87 have caused detonation at full boost?
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Old 05-27-2022, 07:38 PM   #4
benflynn
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Yes, but unless full boost is 25 or so I wouldn’t except it to bust a piston so quick, or the comp to be 0, either way, there is an issue and that thing needed forged pistons anyhow
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Old 05-27-2022, 07:39 PM   #5
benflynn
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I wouldn’t think 7k was an issue either, not all engines get broken in easy

Cracked piston wouldn’t crush a plug unless a part of it came off.
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Old 05-29-2022, 10:27 AM   #6
BlackFighter
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2004 WRX STI Black
RalliSpec Shortblock

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Who did the install, a shop or yourself? Might have jumped a tooth and timing is off.
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Old 06-03-2022, 12:34 AM   #7
big_krish
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I did the install, but the engine was perfectly in time. I got the block apart and the (very mild) scoring is present on all 4 cylinders. On the piston that cracked, the valves took some damage but the cylinder itself is fine so I'm just going to visit my machinist.

More importantly, I still don't know what happened. I don't feel good about putting the engine back together until I know for sure what happened. Here's what I know:
- Some oil splashed out from the AVCS solenoid area on the 2/4 side. Not a lot, but at a high enough pressure to spray on to the windshield fluid reservoir. I think this is unrelated and probably just because of a bad O-ring.
- The oil I drained was completely clean, and the pickup was fine. The oil pump is new.
- The heads were rebuilt by a trustworthy shop, so I don't think anything there failed either. Apart from damage from debris, the valves looked fine, and the cams spun freely.

I'm really not sure what happened, could I just have gotten a bad block? That seems unlikely to me, but I'll ask my machinist to inspect everything and update.
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Old 06-09-2022, 02:46 AM   #8
big_krish
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Ok well I think I finally have this figured out. My machinist said that the failure looked like overheating likely because of a lean condition. Although there were no related codes, the leads of the front A/F ratio sensor had melted together because the blue insulation pulled back (probably when I was putting the engine in). The signal wires shorted, and conveniently higher voltage tells the ECU that the exhaust is running rich. If the exhaust is running rich, it reduces the fuel quantity. This made the car run super lean at high boost because the ECU incorrectly thought the car was running rich when it wasn't. Hence the overheating and melted #3 piston. Well, a new sensor is in order and hopefully I'll have this running again once the block is back from the machinist.
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