304 v8 mysterious noise

[Binderman rex]

would it be possible to have rods knocking in an SV and still have good oil pressure? The noise only comes as you increase rpms. No noise at a steady rpm, and it sounds like it’s all 8. I’d compare it to sounding like a diesel as you rev it up. But only for a few seconds! 70K miles, 50 psi oil cold, almost 20 at 160 degrees. Gonna drop the pan next when I get time, just looking for a little insight

 

[Dana Strong]

The noise only comes as you increase rpms. No noise at a steady rpm, and it sounds like it’s all 8.

Any chance it's from preignition? If you think that's a possibility, you might check the timing and retard it a bit to see if the noise stops. Even if that isn't the cause, the test is quick, easy and doesn't cost anything.

 

[Binderman rex]

That is a possibility I hadn’t thought of, I think I’ll pop the cap off quick too and make sure the advance isnt stuck.

 

[kevingweq]

Sounds a bit like piston slap, Kinda low miles for that issue though.
Any chance the engine has been run without oil/oil pressure for any length of time ?

 

[mallen]

What is your base timing set at? Set it at 0 degrees to start and see if that cures it. Leave it at 0 degrees until you get the problem fixed. At that point, you can try advancing it and see if you can get a bit better fuel economy and power. If setting the timing to 0 degrees does not fix it, check that the vacuum and mechanical advance are working.

Then "read" the spark plugs to see if your running lean. (And of course look for any other soghns of anything unusual) A lean condition can cause ping under load. If it's lean, and it used to be ok, the carburetor is probably dirty. It will require a rebuild. Spraying a can of cleaner down the carb will do nothing as that just cleans off the throttle plates. The dirt (if it's a dirty carb) is in the jets and passages inside the carb.

And yes, you absolutely can have great oil pressure and have a locking rod. Bad oil pressure can cause a rod knock, butrod knocks can happen without bad oil pressure. But this does not sound like that. A bad rod does not come and go. A knock during acceleration generally falls under "knock" ,"ping" and "detonation"

 

[Jeff Jamison]

Have you checked to make sure you are not low on oil.

 

[Binderman rex]

Ok, I’ll try to cover everything here! As far as I know, it’s never been run without oil. Truck sat in storage for 28 years and the PO dumped gas down the carb and fired it prior to me buying it. That’s the history I know on it. I haven’t checked the timing yet. I put new plugs in it, the Old ones looked good. I did put a kit in the carb, the bowl gaskets shrunk so bad it wouldn’t hold gas. I’m running it off a 5 gal can right now. As far as oil, I had changed it so that’s right at the full mark.

 

[RinTX]

Well detonation pinging sounds likely but ... Was a pre-oil done before this engine was started after sitting so long?
I’d check the timing with a light to see where it is. Then I would retard it some to see if it corrects the problem.
Let us know where the timing was set.

 

[mallen]

Ok, I’ll try to cover everything here! As far as I know, it’s never been run without oil. Truck sat in storage for 28 years and the PO dumped gas down the carb and fired it prior to me buying it. That’s the history I know on it. I haven’t checked the timing yet. I put new plugs in it, the Old ones looked good. I did put a kit in the carb, the bowl gaskets shrunk so bad it wouldn’t hold gas. I’m running it off a 5 gal can right now. As far as oil, I had changed it so that’s right at the full mark.

Did you clean a the passages in the carb really well. Carb cleaner won't disolve done if those. In fact b12 chemdip won't either. The old stuff would but the modern stuff took all the good (and insanely toxic) stuff out. This is probably a good thing. There were things in there that, being a chemist, I would not handle without double gloves (a neoprene glove with a nitrile over it) under a fume hood. (And even then I'd be careful) It still amazed me that back in high school shop we all had to go buy a gallon of it. But the problem is, it worked. And the new stuff doesn't. On my edelbrock I had to clean the passages out with some wires and some of those tiny brushes they sell as an alternative to dental floss and compressed air.

 

[Binderman rex]

Well detonation pinging sounds likely but ... Was a pre-oil done before this engine was started after sitting so long?
I’d check the timing with a light to see where it is. Then I would retard it some to see if it corrects the problem.
Let us know where the timing was set.

I doubt it was pre lubed, it was ticking pretty good for a while when I got it running. I have a feeling I’m gonna be pulling it out and going through it.

 

[Binderman rex]

Carb was surprisingly clean when I took it apart, blew through everything multiple times to be certain. Aside from the noise, the truck starts and runs great!

 

[Binderman rex]

Having started it tonight at 47 degrees out, I’m thinking the noise is rod bearings.

 

[Dana Strong]

Having started it tonight at 47 degrees out,

Base timing for my 304 is 0* BTDC. I don't understand how your timing could be 47*.
Did you try retarding it some and driving to see if that changed the sound?

 

[1975IH200]

He means 47° Fahrenheit (temperature).

BTW, am I missing something? I haven't heard the noise. Is there a video I missed?......or a link to a video or .wav file or similar?

 

[Ron A]

Never hurts to drop the pan and look. Might want to check the torque converter bolts if you have an auto. Worked on a Chevy once with a bent dust cover, you would have thought the engine was flying apart. New dust cover and the engine was fixed.

 

[mallen]

Having started it tonight at 47 degrees out, I’m thinking the noise is rod bearings.

I agree, first thing to check is timing. Time it off cylinder #8. Also check and double check and triple check the firing order. By check, I don't mean "follow it with your eyes. I mean trace it with your fingers, never taking your fingers completely off the wire. Trace each from the cap to the plug and make sure they are all where they should be.

 

[Binderman rex]

I’ll throw a timing light on it this weekend when I have more daylight

 

[Binderman rex]

He means 47° Fahrenheit (temperature).

BTW, am I missing something? I haven't heard the noise. Is there a video I missed?......or a link to a video or .wav file or similar?

no video, I could probably get one if I remember my phone when I’m out working on it. It usually laying on my work bench

 

[mallen]

I’ll throw a timing light on it this weekend when I have more daylight

Cool. The best way to fix this is to do things in order
Diagnose rather than just blindly jumping in. It starts with the timing. Once thats set to something that you KNOW should work, if it doesn't, you then can figure out why it doesn't. I doubt it's a rod knock. A rod knock shouldn't just go away at idle. A ping or knock however often shows up when you accelerate. (You CAN actually have ping and knock at idle. But that's not what you have, or at least you have not said that you heard anything like that)

Next, have a helper slowly rev the engine. You need a tachometer as well as your timing light. Leave the advance disconnected with the engine side of the hose plugged so you don't have a vacuum leak. (This should already have been done when you checked base timing. ) Take note of the timing at idle and in steps across the rpm range up to red line. Compare this to the listed specs of the mechanical advance in the manual.

Next, at idle connect a handheld vacu gauge to the vacuum advance can. You don't need to rev it for this one. Just let it idle. Record the advance at various levels of vacuum and compare it to the listed specs.

If all that looks good reconnect he vacuum advance.

Next examine the color of the spark plugs. Do this one at a time, removing one, examining it and replacing it. Google up some pictures to diagnose with. This can spot a lean condition but is nod definative. For example, everything could be good EXCEPT for during acceleration. This can be cause by things such as the accelerator pump being clogged or a secondary jet being clogged or incorrectly sized. It can even be caused by the fuel you use. After you have done these simple checks, if you don't find the problem, try a tank full of premium.