Grating engine vibration - only occurs sometimes. Ideas? Pic inclosed!

[jesterbot]

Ok, I recently installed a 255lph fuel pump and full UICP (TB to SMIC) - those are the only recent changes I have made, so it's either involved in that or unrelated.

This morning was a typical example of what will happen:

I started the car - it was cold outside - everything ran/idled/revved just fine. I begin driving to work thinking maybe the phantom problem had resolved itself (I didnt experience it yesterday). After about 5 or 6 minutes on the road as my car is finally warming up, I am slowing down in neutral for a light and as the rpms drop down through the 2500-1000 range I feel this vibration/shuttering. It feels like something rubbing against big rubber teeth (pic enclosed below) - that kind of steady shutter. As it drops to idle it feels OK. When I rev up (still in neutral) it begins the shuttering around 1000rpm, gets really bad around 1500, then slowly clears up as I rev past 2000-2500. As I go at the light (in 1st now) it does the same thing.

By the time I get to the next light (or two), the problem has gone away. Usually once it hits it stays with me for the next few minutes until I get to my destination - but this morning it went away rather quickly.

Here's what I relate the sound/feel to, to give you an idea ("vibration" and "shuttering" can bring to mind all sorts of different ideas...):

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Let me know what you think it could be??

Thanks guys!

 

[heavyD]

Maybe a possible balance shaft belt problem. It may have skipped a teeth or two. I got this belt replaced with my timing belt and the idiot mechanic installed it off by two teeth and it had this same type of problem. But you say it goes away so it could be something different.

 

[jesterbot]

Yeah, it never occurs directly after a cold startup - it always springs up a few minutes later. And according to this morning, its not like an issue that occurs whenever its warmed up, because it went away after a minute or two. I'm going to try and record the engine sounds as well... normal idle/rev compared to when this crap is going on.

Could it be a fuel management issue? Maybe the pump is having fits? A boost leak wouldn't do this to my knowledge... as this seems to be rpm related, not vac/boost related.

Help!