The Stovebolt Site Forums The Shop Area Engine Shop Sudden, Severe Knocking

Today I was driving my 66' when suddenly it began knocking severally for no apparent reason. Since the tank was nearly empty, I drove to the gas station and filled it all the way up with 93 octane gas without ethanol, no change in knocking. Even if you hardly give it any gas, it will begin with what I assume is, severe knocking. It has been around 3000 miles since the last oil change, the truck it not smoking and has plenty of power. The motor is a 350 V8 with 4 barrel carb and an RV cam with around 15,000 miles on it. Any ideas? Thanks

If it is doing the knocking during acceleration it sounds like timing problems. Maybe the distributor has turned and the engine is knocking due to pre-ignition. If it is just knocking all the time, get a mechanics stethescope and try to locate the loudest sound to determine where the noise is coming from. From your note it sounds like this is happening when you give it gas under a load. Timing probably.

Yes, it is only during acceleration. Thanks for the reply, I was planning to give timing a go, the distributor may have just came loose which would explain how sudden it came on.

yea, i would bet on timing. mine knocked under load, so i retarded the timing, and even had to go to a colder spark plug.

leonard

Check to make sure your centrifugal advance in the distributor is working correctly. They can freeze up due to rust and/or dried grease. I also had an HEI distributor that had enough end play that it would jam the advance mechanism - sometimes. That was tough to troubleshoot.

regards,
Leon

I did some adjusting but couldn't quite get rid of the knocking. I will look into replacing the centrifugal advance as Leon suggested. The inside of my distributor is quite rusted; it looks as though someone had left it out in the rain for a couple of nights. I may just invest in a new distributor, I was planning to replace it eventually anyway.

There is a huge difference between knocking in a motor from a spun rod bearing etc. and the rattle sound from pre-ignition. If you think it is a real knock you should drop the oil pan and check for a loose piston rod. If you keep driving hoping it will go away you may find an errant rod sticking out the side of the block. Trust me, been there, done that.

What Dan's talking about is a distinct possibility. I was thinking about your problem while driving to work the other morning after I posted. My 2000 Silverado pings lightly during low speed slight acceleration. My 69 C-10 pings rather badly if I don't feed it 93 octane, again mostly under slight acceleration. I wouldn't call either of these a "knock" since I have actually heard a connecting rod knock. The pinging I experience comes from multiple cylinders, so its a higher repetition rate. The rod knock repeats once per engine rev, so its much slower. If you actually are hearing a rod knocking, Dellman, then I fear there's no easy way out.

If its making the noise at idle, under no load, then it almost assuredly not pinging.

regards,
Leon

Sounds like a broken valve spring.

It makes no "knocking" at all at idle. You can rev it up as high as you like and there isn't the slightest sign of the noise. The truck used to knock (or I guess ping) under heavy acceleration soon after I had the motor rebuilt but I had a mechanic do a tune-up and it completely went away. Also, when I adjusted the timing the other day, it actually effected how much pinging it made. The motor makes more of a "rattle sound" as Dan said, than anything else, I guess this is a good sign?

Yesterday I noticed that there didn't seem to be any vacuum from the distributor; it wouldn't even pull a small piece of paper towards it. I ran to the local parts store and bought a new vacuum pump, installed it, no luck. There still doesn't seem to be any vacuum. Could it be that the mechanism that actuates the pump is faulty? Would no vacuum cause pinging?

The vacuum comes from the intake manifold and actuates the vacuum advance on the distributer. The line from the intake will be what has a vacuum. Not the fitting on the distributer.

If there is no vacuum from intake, the line is either broken or plugged up.

Having no vacuum to the distributer vacuum advance device will probably cause pinging.

Pinging results from too much advance timing. If there is no functioning vacuum advance, you would tend to run retarded, which would not result in pinging. Ported vacuum would only come from a carburetor, never from a fitting on the intake manifold.

Stuart

Well now I am embarrassed. Shows how much I know about motors. Thanks for the replies; I will see what I can find out tomorrow.

Dellman,

In the recent issue of Skinned Knuckles, I read an article on servicing the timing system. They gave the following simple test for the vacuum advance module on the distributor:

(from memory:)
Disconnect the vacuum line from the carburator at the distributor.

Manually rotate and hold the distributor in its usual direction so it's fully advanced (like it would be at full vacuum).

Hold your finger over the distributor vacuum port (on the advance module on the dizzy - not the vacuum line from the carb ;-)

Release the distributor body (while holding the finger over the port). The vacuum advance should hold the disributor in it's rotated (advanced) position. When you release your finger, it should only then rotate back to home position. If it rotates back immediately when you release the body, it indicates the diaphram in the advance module may be failed.

I thought this was an elegant, simple diagnostic test. Haven't tried it myself, yet, but hope it helps?!

-Michael

Thanks Mike, that sounds like a handy tip that I will try. It seems though that I may have found the problem. I remember noticing that when I replaced the vacuum parts, the rotor didn't seem to be rotating very freely. I took the cap and rotor back off and tried to turn the "dog ears" (The place that rotor bolts to, I guess that's what they are called). The dog ears had hardly any slack and wouldn't let me turn them as far as to where they would spring back like they are supposed to. I called the mechanic and he said that they can freeze due to rust suggested that I try WD-40 and try to work them back and forth until it became free. So far I have had no luck, before I got feed-up, I sprayed a heavy dose of penetrating oil in hopes that it will soak in over night and free it. If I get mad enough, I'll just by a new distributor, I've been wanting to anyway due to all the rust inside of mine. I still would rather have this one going though.

If you're like me, you'll get a new one, install it and get it running great, then tear into the old one to learn what was wrong and how it works!!! Not a bad idea, really . . . I've taken the distributor apart to varying degrees a few times. Nothing too scary, but digital photos help. A good cleaning will likely do wonders. . .

When you remove the old distributor, be sure you mark somehow it's alignment, so you can match the new one to be firing in the right location.

-Michael

Is the noise a "pinging" or "knocking"?
Well, Let us know what the problem is when you find it. There is plenty of people who can learn from the experiences of others in Stovebolt.

It’s defiantly just pinging. I can't get this thing unfroze. I am going to buy a new distributor probably within the hour. Anyone have any suggestions?

So I finally got my new distributor, a Procomp Thunder series with clear cap and rotor. I installed my new distributor and fired the truck up but I just can't seem to get the timing right. If I set it 6 or 8 degrees left of 0, the truck will run but has no power and misses terribly. If I set the truck even higher, it stops starting due to kick-back. Anything else that I've tried results in missing and pinging. Is there some trick that I haven't tried? The old distributor was without doubt bad. Is it possible for a slipped timing chain (just brainstorming) or any other reason?

Sounds like you got a couple of plug wires reversed.
Easy enough to do.

You've already done better than me when installing a chevy distributor. I don't think I've ever got one in the right way the 1st time. Always 180 degrees out. I crank, it backfires, then I take the distributor out and turn it around, then it works. Except for the time where I thought to myself "I always get this backwards. I'll just flip it around before trying to start it". Yep. You guessed it. That time I had it in right the 1st time.

regards,
Leon

Haha, you seem to have the worst luck Leon. Thanks for the reply. It can't be the spark plug wires because I labeled them way back when I had the motor rebuilt.

Leon got me thinking. From everything that I've looked up online, my sparkplug wires have always been hooked up wrong, now I am really confused! Anyway, when I get off work I will try to figure out what is up.

Easy part with wires is checking proper order. Follow the wire from the plug in the front-most cylinder to the distributor cap. Then follow the wires, in clockwise order, around the cap. Should match the firing order (molded into the side of the intake manifold, driver's side, by-the-way:) 1-5-3-6-2-4. 1 is the front, 6 is the Rear.

Harder part is seeing if the distributor is timed properly to fire #1 cylinder at the proper time. If you are comfortable determing where Top Dead Center is, this part isn't too hard, either. If NOT, well, post again.

Post your results for our studio audience! ;-)

-Michael

If you are in gear step on brake and load the motor what happens to the noise. How about if you are revved up and back off of the gas

Thanks for the replies but I fear that I may have found the problem, or at least nearly. It turns out that the fact that the motor is missing is due to the fact that there is absolutely no compression on cylinders 5 and 7. Now, seeing as the two cylinders are next to each other, it seems likely that it could be caused by the blowing out of the head gasket. I haven't driven the truck much since the problem began (maybe at most two, 10 mile runs to town and back with a break in between making them 5 mile runs) so I am hoping that I have not driven it enough to warp the heads or burn out anything. It's time to tear the head off and see what can be done, wish me luck. I forgot to mention that after the motor began acting up, I gave it an oil change and there was no foam or anything that would have been a sign of water in the oil.

I had the exact same thing happen on a 1995 GMC with a 350. I didn't know what all the noise was. Tore it down expecting to find something terrible but it was just a head gasket.

Thanks, that really does make me feel much better.

Head is off. Gasket is blown alright. Now it is time to clean up everything and "straight-line" test everything to see if anything is warped, wish me luck, just keeping you guys updated.

Alright everyone thanks for your help, the new gasket is installed and the truck is running flawlessly. I am surprised that the distributor also happened to be bad. I've never had so much power before. Thanks.

Thats good news, glad to hear you got it going and your happy with the results!