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I posted a video today on youtube. Sometimes, noises are hard to capture, and this is an example of such. Here are the symptoms. It's a 2001 that's been sitting on it's center stand for about 6mths on a battery tender. The owner starts it weekly, and allows it to run until the temperature gauge moves to the center mark. He reports SeaFoam was added to the gas months ago. While it was in his garage, he's heard the noise for two startups, so I did a house call.

What I heard was a loud snapping noise like plug wire arching to ground. While cranking it was slow turning over. Once started, there was backfiring in the exhaust. The snapping noise was only there for a few seconds. After 3 more starts, it is now gone. The video captures 2 snapping noises as it is turning over at the 3 sec. mark.

His noise is caused from a sticking valve, or slow moving valves.

Up date

After capturing the above video, the noise went away. In spite of my recommendation, the customer chose to assume that the noise was gone for good, but today it reoccured. This time I was able to pull the FI/IGN fuse to turn off the ignition system and fuel system, and re-capture the noise on a 2nd video with the FI and IGN system disabled. Keep in mind that the noise is inconsistent and being able to capture any video takes luck.

My customer has now authorized further diagnostics. It's a good thing, because not knowing what's actually causing it can lead to catastrophic failure ... valve fully open and colliding with a piston !!!

I've since pulled all buckets and shims under the right cam. I was looking for the obvious ... a valve keeper or shim that had dropped out of place ... no problem found. However, with my thumb, I was able to push in on all 3 exhaust valves but not the intakes (intake and exhaust valves springs are the same, so the effort to push open all should be the similar).

Once I found no shim or keeper out of place, I reassembled and measure valve lash. What I'm looking for is an excessive wide gap indicating a valve is bent or not completely closing ... nothing stood out.

I then took a compression test on all 6 cylinders. While cracking for a compression test on #5, for a second, I heard a similar "snapping noise" from the left head. Compression was good on all 6.

Thus far, there is mounting evidence that the intake valves do not want to move open (hard to push open compare to the exhaust valves), and once open, some or all are slow to close (snapping noise). Nothing yet to indicate that there is a bent valve; however, next I'll do a leak down test to know more.

For more info on this repair, it is probably best to read my original post in full length.
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