I was out tuning the carb on the truck today; specifically I was fiddling with the secondaries and trying new springs from the secondary spring kit I just bought.

Part of the tuning required me to run the truck hard to see if the secondaries are opening when and where they should. On one pull, I ran the engine to about 5,000RPM before shifting, and afterwards I have a strange engine vibration and noise that I can't quite pinpoint.

The truck has a pronounced vibration / shudder that can be felt from under the hood at low RPM, and you can hear / feel it in the truck cab if you're at 2k or higher. While working the throttle from under the hood, it almost seems like the vibration is coming from the front of the engine, sort of like the fan is out of balance or something. I'm certain it's not a miss. And the noise it makes has me thinking it's the fan also... but how would I diagnose this? Oil pressure, water temp, engine power/output are all fine, so I don't think it's anything internal or related to the engine mechanicals at all. The noise is not mechanical, it almost sounds like a bad wheel bearing type noise, a low growl, but it's coupled to that vibration that I feel.

Anyone else have anything similar happen to their truck? Any ideas on how I can track this down would be appreciated. It's my daily driver, and although I drove it the whole way home (25 miles) in this condition, I don't think it's good for it to run it this way for long periods.

 

Take the belts off and see if the fan clutch or water pump bushings are wasted by trying to move the fan blade up and down.

You might also check that the timing marks are still in the right place by finding TDC #1.
It is not too uncommon for the harmonic damper to come apart.

 

Also, you said it's not a miss but I would still verify that. Loosen all the sparkplug wires on top of the dist so you can easily take them off, start the engine, and with a glove and some insulated pliers, pull each wire off individually. They all should make it run worse. If you have one that doesn't make any different, that's the dead cylinder.

I am wondering if you didn't bend a pushrod.

 

Oddly enough, 95% of the vibration had cured itself when I went to start the truck up the next morning. I haven't experienced it (to that degree) since, though I do seem to think it's running a tad bit rougher than before.

I think I'll pull the fan belt off and see if it's related to the water pump.

I still don't think it's a miss, but I may try pulling the plugs off one by one if it doesn't turn out related to the accessories at the front of the engine. Thanks for the help guys, hope you all had a good Christmas!

 

I see people with bent pushrods here on the forum but I used to take my engine above 5k regularly and have never had a problem.

The crappy RV cam I installed in 2008 really falls off above 4,500 and this makes short shifting a necessity.

Hope you find the cause, and it's an easy fix.

Merry Christmas and a Happy New Year to you!