This motor only has 40k on it, and they are mostly easy miles. I have been having an occasional stumble and rough idle-misfire problem for the past couple weeks. I think it's probably the MAF, but I haven't had a chance to confirm that yet. Yesterday morning I was coming around a turn at around 15-20 MPH and the car began to stumble and misfire. I put the clutch in and coasted down the block because I wanted to listen to what it was doing. The Idle bounced between 700 and ~200, almost stalling, but I never touched the throttle (wanted to see if she'd right herself). Then, quite suddenly and loudly she began to knock. I milked it the block and a half back to my driveway and took a listen... It didn't sound like it was coming from the heads like a shot lifter, it sounded internal – like a dull clack.
I was able to isolate the noise to the pass side, but still sounds like its in the block and not the head. Against my better judgment, I drove it to work anyway, and when I got there I took another listen and heard... nothing. No noise, nothing unusual even when revved >3k.
So I started it in the afternoon yesterday to leave work.... and theres the knock again, not as bad, but still quite loud. However, after 10 minutes of listening to it while she warmed up, the knock had disappeared and was imperceptible yet again. I drove it home, and still didn't hear it in the driveway.
My gut feeling is that it must be a rod knock, but there are a few problems with that... This engine has never been oil starved in it's life (that I know of, I got it with 7k and now only has 40-ish), and my OP gauge says I have 70-80 PSI most of the time. Doesn't it sound unusual for a rod to let go so suddenly like that? I guess that could happen if the oil pressure dropped to zero and the bearing became oil starved and then spun... but that's pretty fast. I've also not heard of a knock going away at all RPMs when warm, but I can see how that might happen (in theory).
I'm going to check the pas. side lifters tomorrow (driver side done in June) and find out for sure if it's a lifter or not. I took a short video of the knock, but it sounds allot sharper on the video than when you're in front of it... even sounds more like a lifter on here... I know it's tough to diagnose noises over the Internet, but here goes:
Thanks for any help in advance!! I really hope I don't have to end up rebuilding this motor.... I've never abused it, so I really am at a loss about how this happened...