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2005 Impala SS 129,000 miles in June 2021. 86 Olds 442 with a '68 Olds 350
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Discussion Starter · #1 ·
Sure enough, I discovered that an oil soaked , collapsed front engine mount will cause knock. With a 3.5" pulley, 3" DP, front powerlog, FWI, 180 stat, 104 ALs, I was getting up to 2 degree of knock only on the freeway from 75-95. Strange this was, it always started at 5300 rpms (playing back data from aeroforce gauge). Never any knock going 0-60. Then one day, I started getting up to 2 degrees 0-60 and up to 4 degrees going 75-95. I tried torco octane booster but really no change. I played it safe and put the 3.6" pulley back on it. Changed original plug wires, original O2 sensor and 6 year old fuel filter. The fuel filter helped a little but I still had more knock than I should have.

Recently I noticed the steering wheel vibrating and a vibration in the cab accelerating then when I was under the hood, the front of the engine looked lower than the back. Looked at the front mount and it had chunks of rubber hanging out. Changed the mount . With the 3.6" I recently put on because of the excess knock, I took it out on the freeway and 0.0 knock 75-95. I put the 3.5" back on and did the same thing. 75-95, and 83 degrees/humid outside, I had 0.0 knock. Seems like if you're chasing knock or get excess knock out of the blue when you had little or no knock just the day before and your car is ancient, that front motor mount seems like something to check before throwing parts at the engine. If the front of your engine seems a little lower than the back, that mount is trashed.
 

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Not out of the realm of possibility, since all of these lower engine mounts are hydraulic on the W-body's.

I cut one apart on my car last year, and you can see the cross-section in this video, and there's definitely potential for solid metal contact internally: (ignore corny rock music as I was trying to be entertaining)

 

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2005 Impala SS 129,000 miles in June 2021. 86 Olds 442 with a '68 Olds 350
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30 Posts
Discussion Starter · #3 ·
Cool video. That corny rock music seems about standard issue for DIY automotive project like this! Being a RWD, V8 hot rod guy for decades who just recently started messing with FWD/superchargers, I wasn't even familiar with hydraulic motor mounts. Changing a mount on a RWD 86 cutlass was a bit easier than this 05 Impala .
 
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