Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Loud knocking when truck starts
View Single Post
  #10 (permalink)  
Old 01-28-2013, 07:37 AM
cobalt327's Avatar
cobalt327 cobalt327 is offline
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Posts: 5,031
Wiki Edits: 1616

Thanks: 128
Thanked 616 Times in 565 Posts
Originally Posted by Chevyman87 View Post
Hello all I am starting to have an issue with my 350 SBC that literally just started today. It all started when I pulled over at a gas station after driving 30 minutes on the freeway to fill up. After starting the truck, I heard a loud knocking noise that went away after about 30 seconds or so. I noticed that the noise seemed to be coming from the driver side of the motor. I also noticed that the sound was loudest if I stood where in between the cab and the bed of my truck. Anyway, I pulled the driver side valve cover and checked the pushrods by twisting them with my index and thumb and some of them were loose. After tightening them up, I started the truck again and the problem went away. However, on my way home I stopped by a relative's house and after getting back in my truck and starting it, the sound came back. Except this time it lasted longer but eventually went away. I don't hear it while I'm driving only when I've shut the motor off and let it sit for probably 10 minutes and start it up again. Does anyone know what it might be?
Adjust the valves correctly. The push rods normally will be loose after the engine's been turned off for a while, so that's not how to tell if they need adjusting. Plus the lifter has to be on the lowest part of the cam (the base circle) when you adjust them.

You need to check the flexplate to be sure it's not cracked. Hearing the noise where you're hearing it (between the cab and bed) would mean it might be coming from aft of the engine. You'll need to get under it (block the wheels, always be safe!). With the engine Off, and using GOOD lighting, visually check it over, especially at the bolts holding it to the crank. Then use a short piece of 2X4 or a rubber mallet to rap on it to see if you get an odd sound- if it's cracked you should be able to hear it. You can also use a long screwdriver or pry bar to tweak the flexplate to see if it's cracked. Don't beat/bend it out of shape, but it can take some amount of force to get the crack to show itself.

One other thing that's often mistaken for a valve or engine knock is an exhaust leak at the header or manifold. I've heard leaks like that you'd SWEAR was a metal-on-metal noise, but it wasn't.
Reply With Quote