Thread: knock noise

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I have a 68 c20 with as best as I can tell is a 76 350. Compression is good in all cyclinders from 170-180. after the engine warms up I can hear a knocking sound, actually more like a hollow tinging sound, kinda hard to describe that increases when you up the rpms. Using a stethoscope it seems loudest at the water pump but not the block itself, but the noise is still there when I disconnect the fan belt?? anybody got any ideas?

Thanks,

Milo

have you put the steh on the pan yet?

Mechanical or electric fuel pump? mechanical fuel pump or fuel pump pushrod could be possible near the water pump, but so could a bunch O' other things too.

Could also be an exhaust leak at the manifold(s) or a cracked flexplate.

By the way, for you fellows who are going to buy a new flexplate, shop for an SFI 29.1 rated plate (drag race specification). It will be made of thicker material than the run-of-the-mill chinesium plates that will crack so easily.

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Chinesium...must be between taiwanium and unobtanium on the periodic table.

I have not put the steth on the oil pan but good idea as well as checking the fuel pump (mech). Whats an easy way to check for a leak at the manifold or cracked flexplate?

Thanks,

Milo

Could be loose timing chain slapping the timing cove.....te

40 year old harmonic dampner-main bearings--cam lopes gone( front of engine gets oil last)
Broken piston skirt, wrist pin or rod------short out cylinders one at a time to see if knock can be located in and certain cylinder

Easy way to check ex. Manifold leak is

#1 listen. A length of pipe makes a poor man's stethoscope. DON'T put it directly on your ear, looking for exhaust leak!

#2 feel for the leak with your hand. Don't touch the manifold, It's hot!

Welp, so far the noise is loudest at the fuel pump, so maybe I will get lucky. Going to replace it and see where that gets me!

Thanks

Just take it loose and remove the rod----------enough gas in carb to run for a bit----

On the passenger side of the SBC the upper water pump bolt hole aligns with the fuel pump rod, often this hole is used with a longer bolt to hold the rod in place when changing the fuel pump. If this longer bolt is left in place after changing the fuel pump it can cause noise such as you are encountering. And, in a worst case scenario it can cause damage to the cam’s fuel pump lobe. Loosen this bolt, if the noise goes away replace the bolt with a shorter bolt.

Its not the water pump bolt-its the upper one of two that are down between the crank and fuel pump boss-you will usually find a short bolt threaded in the hole without anything being attached. There are two holes there vertical about 2 inches high and 2 inches inside the pump mounting surface. I'd post a pic but I won't until they fix this place to where we can post modern size photos from modern cameras

OK, so I took of the fuel pump and the upper bolt (the one you guys are talking about) and sprayed some lube in the hole, just to make sure it was the same oil galley and yep it ran out the fuel pump mounting hole. The bolt seemed short enough and I didnt see any damage to it but.... the fuel pump push rod seems to be stuck. Tapped it a few times wouldnt budge so I took some vise grips and tried to turn it and pull it out, wont budge. Should I crank the engine a couple of times and see if that pushes it out? Kinda hard to see how the push rod could get bent?

Sounds to me like your fuel pump rod is bent, binding in the hole. I would lock a pair of vice grips on it and either lever it out or rig some sort of slide hammer to get it out. I'd be very concerned that the lobe on the cam may be toast, and I also wonder how it moved enough to work the mechanical pump if it's stuck that tight? This may be the start of a very bad day for you, my friend. If you're very, very lucky the old rod will come out and a new rod will work just fine. Once you get it out chuck it into a drill press to see if it runs true....