Opinions on knocking sound on I6 250

[slaminum69]

Here is my situation, I have a I6 250, its been rebuilt with all new parts and maybe I have 1000 miles on it, there has been this knocking sound I cant get rid off. I have changed out the lifters, rockers arms and its still there. I did track it down to the 5th cylinder, tightened the valve till it went away but then it begins to misfire, loosen it up and the knocking is back, repeat and same situation. I am having a hard time thinking its the cam, but you never know, today I am going to pull the lifters out on the cylinder again and take a look at the cam to see if there is any distortion. Also I noticed on those two rockers arms there was alot of air bubbles in the oil coming from the push rods. So I am looking for some inputs from the I6 gurus on here, Thanks in advance. Dennis

[luvbowties]

Lifter, hopefully; or cam lobe, hopefully not---but if you replaced lifters, hope seems to be fading...
sam

[Wrenchbender Ret]

It may be a worn valve guide.

[68gmsee]

If replacing the lifter does not fix it, take it to a mechanic or other person that has experience to see if they can determine what it may be. I would even get a second or third opinion.

Engine noises are very difficult to pinpoint unless you've had a lot of experience with engines. What may sound like a knocking noise to one person may be desicribed as a ticking or clicking sound by another. Also, the pitch of the knock determines where it's coming from. A deeper knock usually comes from a main bearing and a higher pitch knock cab be a rod bearing or wrist pin. There's also piston slap which makes a different sound.

[awbrown]

short the plug wire and see if you get the same results as you are getting by messing with the valves .. I bet it is piston or wrist pin related ,the old motors manuals told how to tell the differance between noises by when the happen .. when does it make the most noise ??? reving , coasting ? accelerating ??

[GASoline71]

Lifters "tick". Rockers "clack". Rods "knock".

Lifter tick will usually go away after the lifter finally gets pumped up with oil. Rocker arm clack is constant and changes dramatically with RPM. Rod knock on the big end of the rod will normally happen at deceleration or can be subtle at idle. If a wrist pin is going... it is pretty much the same as when the big end end goes... but will not be as pronounced.

There is also pre detonation... that is a loud "pingish-knock" that happens during acceleration.

Edit... Exhaust manifold collectors can leak at the donut gasket and give off a noticable "tick", that can be mistaken as internal engine noises as well.

Gary

[Rufton]

I would dump the oil and cut out filter element then inspect for metal flakes.

[pgaffaney]

Hello Dennis,
I wanted to follow up on this post you had a couple of years ago. Were you able to resolve the issue? I'm running into the EXACT problem you posted about. I have about 1,500 miles on a rebuilt I6 250 (1969 Chevy C10) when a "ticking" noise started. "Ticking" occurs at idle to about 1800 RPM. The "ticking" goes away at cruising speeds 45-60 MPH (above 1800 RPM).

With use of a stethoscope, it sounds like it is in the valve train, number 5 piston. I'm able to get the "ticking" to go away by adjusting/tightening the rocker arm nut while engine is running, but the engine begins to choke out.

I replaced all lifters, inspected rocker arms & push rods. The cam looks okay based on a video scope. Oil pressure is good, fuel pump checked out, no exhaust leaks. My next step is to pull the oil pan to check the bottom end.

If ANYONE has more thoughts or ideas it would be appreciated.

Thanks.

[nonstop]

Have you checked the stud? Could it be pulling out of the head?

[pgaffaney]

Yes, I checked the studs as well.

[Chevyrestorerman]

My guess is cam walking. Any one check cam shaft end play while they rebuild their engine. Pull the distributor, check your distributor gear. Do the easy stuff first.