Friday, May 15, 2009

Motor Clunk

If you look closely at the last post, where we listed what was left to do on the car, you'll see at the top of the list it says "motor clunk".

Well, that piece of tape is there to address the minor issue that Astrid broke the car at the last track day. Her aggressive driving and constant accelerator abuse was just too much for poor Ling Ling. She's taken all the abuse a panda can, and is now making decidedly not good sounds from the engine compartment.

(ok ok. it's probably not Astrid's fault, but don't tell her that)

What's it sound like? I'm glad you asked. Here's a video:

E30 Rod Knock? from Adam Lazur on Vimeo.

With the race rapidly approaching, we checked the valves. They looked fine. Hell, after an adjustment, they're the quietest E30 valves on the planet. Unfortunately, quieter valves just made the bad noise sound louder.

So we recorded the noise and appealed to the internets. On the internet everyone has an opinion. 3 main causes were proposed:
  • broken rocker arm / valve train problems
  • rod knock
  • piston slap
To address the first, off the valve cover came (again). Maybe there was a tiny tiny crack that was missed before? Many minutes of scrutiny on the valve train yielded nothing.

The cylinder compression was tested, and it was fine. Further lending credibility to the theory that it was not a valve issue.

To differentiate piston slap vs rod knock, we were informed of the "pull a plug" method. The procedure is to pull the wire from a spark plug, and if the tone of the sound changes, that's your bad rod bearing. The theory goes that by removing combustion from the cylinder, the piston will still slap since it's moving, but the rod will knock less since it's under much less pressure.

Before trying the plug trick, the car was taken by the shop of an experienced mechanic. After 30 seconds of revving and listening in the lot, said mechanic diagnosed the noise as rod knock and promptly prescribed a new motor. Sad panda.

After the car was back home with a diagnosis, we tried the plug pulling for a 2nd opinion. While the motor was running we pulled the wires from the cylinders, one by one. Cylinder 1 is fine. So is 2. Boy this is boring. 3, 4, 5: all fine. Maybe it's actually piston slap after all? Oh oh ... cylinder 6 it is!

Woohoo, just like we thought, bad rod bearing in cylinder 6!

Oh crap.


This is the last weekend of work. We were planning to do some minor tweaks on the car, throw it on the trailer, and knock back a coupla cases of celebratory beer. Instead we're going to pull the oil pan off the motor and hope that it isn't too screwed up.

If all goes well, celebratory beer will be shared with grease under our finger nails and the ever so faint tapping of an E30 valve train in the background. If not, we'll hit something with a hammer and start scheming for a whole new motor.

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