Car: 2002 Wrx 95k miles Problem history: first started a couple of months ago when i was sitting at a stop light and had the CEL come on, reached in glove box and grabbed my scanner and got the code P0304. did some research and didnt find much... so i did some easy stuff and swapped the injector and coil from cyl 4 and cyl 2. that didnt help anything. so then i went to my friends shop and did a leak-down test. that test showed 70% leak-down in cyl 4... found that air was rushing out of the intake tube. so after some thought about how things work, i looked at the timing and that all seemed fine. so went to the next step that seemed smart and took the driver side head off and brought it to a machine shop. they looked at it and said everything was good, vacuum test showed good and so they went ahead and took the valves out to see if maybe a valve was bent and everything was fine. so i put the head back on and put everything back together and now it still has the mis @ idle. the car seems to run great but the CEL comes on and says cyl 4 mis.... im lost, i dont know whether it could be bad rings and maybe the pressure is coming from the crank case breather system? any suggestions would be great! maybe there is something i am missing. im not saying i am a SUPER MECHANIC but i do know a lot about cars. if there is something i may have missed just let me know. even if you have stupid questions like, "did you do the timing right when u put it back together?" Thanks to anyone with suggestions! hope i can figure this out soon!
Yup, did that too. Had to Put in a couple new wafers for those to on reassembly. Also I have not done another leak-down or compression test since I put the motor back in/together. Have not had time.
You mentioned that the cylinder head shop said the valves were fine but you still had low compression in cyl. 4. This makes me think your problem isn't your valves/head. When you say you felt air rushing through the intake tube, do you mean the turbo inlet? Air could be going past your rings and out the crank case vent into the inlet if that is the case. You can try cracking the throttle to see if the air flow changes as well. One additional test i've done to see if the rings are the most likely culprit is dripping a bit of oil in the cylinder before a compression test. Typically compression will shoot way up if it's the rings.
I looked at it and it seemed good but it did rock say up and down, by that I mean it pivoted on the topic the rod. But I didnt know if that was bad. You know I thought it might just be that way... I'm not an internal engine person to be honest
Zombie: I have heard of that oil trick with the ring but have not yet done so. And I was thinking that the air could e coming from the PVC system. I'll have to test next time I have shop time
If you had performed a vacuum test, and it showed that it was in good shape (17-19 ingh) with a steady needle, that probably means that you didn't have a valve related problem. There is still a chance, but not a very good one. When you say "air rushing back into the intake tube", what intake tube are you talking about? If the intake manifold is still bolted on the heads, then air rushing back into it at the throttle body won't be a tell-tale sign of what cylinder has the problem. Not sure what you are using to pull the code from your ECM, but if you have access to view the "freeze frame data", that usually gives you an idea of what to look for. Your fuel trims, and MAF readings can give you a big heads up to what kind of mis-fire you have.
Manifold was bolted on and by intake tube I mean the tube pre-turbo. Where the maf clamps to. I didn't wen thing about the butterfly being closed so it was...
Oh and as far as pulling codes I just have a cheapo code scanner. No freeze frame available. I have a friend with some snap-on scanners. The thing that suck is this mostly all happened when I was still living in Duluth, I just moved to Eden Prairie last week.
Did you do a compression/leak down test since re-assembly? Assuming the Cyl 4 low compression still exists, and it's not the head (again, assuming the head work was thorough), and the timing is correct, then it kind of has to be the rings. You should not be able to move the piston almost at all, hard to describe whats good or bad, kind of takes experience to know.
Umm, the piston just moved a touch, and I haven't had time to redo the leak-down. But Midwest(in Duluth) did the work, assuming they are good.
