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Old 07-08-2008, 12:53 PM   post #1 of 40
chrpmaster
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Default strange knock from Chevy 350

I just bought an old Chevy pickup to haul stuff. Its a 1976 C10 with a 350 auto 2 wheel drive reg cab long bed. Its in pretty good shape with low miles.

It runs well with lots of power but one issue that is bugging me is when its idling it sounds like it is knocking inside the engine. At first I thought I figured it out when I saw a vacuum hose going from the air cleener to the bottom back of the carb was not connected. Reinstalled that and it quieted down. But now it still comes back sometimes when it is idling. Increase rpm's slightly and it quits. No other simptoms that I can see. Is this just a timing issue or maybe a weak valve lifter or tappet or something? One mechanic suggested some kind of additive to put in the oil (or maybe the fuel). Supposedly this would better lubricate the weak somethingorother and quiet it down. Can anyone help me out here?

I did add Seafoam to the first tank of gas just to make me feel better but I can't say there was any difference. I think this is supposed to be a good carb/fuel system cleaner outer. (don't you love those technical terms?) Would Marvel Mystery oil be good to add to the fuel? It says so on the bottle but its kind of pricey to just pore in for nothing. I did put some in the oil when I got it remembering someone telling me it helped their sticky lifters quiet down.

Thanks
Andy

P.S. yes I will post some pics if anyone is interested.
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Old 07-08-2008, 01:01 PM   post #2 of 40
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Default Re: strange knock from Chevy 350

Is it a knock or more like a ticking. Ususally a knock means lower end, crank bearing etc.
If it's just and occasional tick, it's most likely the tappets/rockers.
I know most sbc's when idling the oil pressure drops off, sometimes on the older ones the oil (idiot) light would ficker. Not a big deal once rpm's come up as you describe. Tough to say without hearing it. Probably not a big deal seeing it goes away a r's come up.

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Old 07-08-2008, 01:11 PM   post #3 of 40
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Default Re: strange knock from Chevy 350

My guess is you got loose bearings, and the oil psi drops as you come down to idel. If you want it to last a wile longer, try running a little thicker oil, and upping the idel speed a tick.

My old convertible does the same thing. Mostly on hot days, with a hot motor. raised the idel speed up a little, and it works great.
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Old 07-08-2008, 05:10 PM   post #4 of 40
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Default Re: strange knock from Chevy 350

Munderhill I think its more of the tappet/ticking sound and its more annoying than dangerous sounding.

Paul I was messing around with the idle and got the noise to go away but of course when I'm at a light with it in gear the sound comes back. I will look into the heavier oil. I just had it changed with 10w30 but maybe I should look at 10w40 or even 15w45? I wonder if something thick like the old STP engine additive would help (or hurt).

I remember the old engines watching my Dad adjust the tappets after rebuilding and engine. Is that still done or did the hydraulic lifters eliminate the need for that? I'm wondering if the tappets on one or two cylinders worked loose enough to prevent the self adjusters from being able to take up the slack.

Andy
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Old 07-08-2008, 05:15 PM   post #5 of 40
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Default Re: strange knock from Chevy 350

You dont have adjustable tappats on that motor. Not uncommon to have a flattened cam lobe on a old SBC. Or maybe its sludged up a little, and not getting a lot of oil to the top end... Or could be a leaking down lifter... well you get the point.


Me, I would just drive it.
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Old 07-08-2008, 07:48 PM   post #6 of 40
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Default Re: strange knock from Chevy 350

Actually, the tappets do have an adjustment to "center" them for the plunger travel.
With the engine warmed up, you loosen the rocker arm screw until it just starts to clatter. You then SLOWLY tighten 1 turn.
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Old 07-08-2008, 08:32 PM   post #7 of 40
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Default Re: strange knock from Chevy 350

Your truck has spent a lot of time sitting around over 32 years, unless low miles means 300,000. As others have mentioned some of those engines were known for soft camshafts and the related trouble that causes. I had a 75 350 4 barrel that was running fine one second then wiped 3 cam lobes off and bent a few pushrods in the next instant, while turning all of 1800 RPM. It gave absolutely no indication of problems until that moment, good oil pressure, smooth running and decent power for an engine with 115K on it. I put a 400 small block in it and immediately wondered why Chevrolet bothered to make a 350.

I suggest using 15w40 Rotella T in it at the next oil change, and then just drive it. The mysterious noise will either go away or get worse, maybe suddenly like mine did. When (if) it gets bad enough to be a serious concern, rebuild it or even put a whole different engine in there. Just about any V8 Chevy from 1955 to 2008 has been put in a 1/2 ton truck at one time or another, take your pick and it will be good for another 32 years or so.
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Old 07-09-2008, 06:13 AM   post #8 of 40
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Default Re: strange knock from Chevy 350

Substitute 1 qt oil for a qt bottle of Lucas Oil Stabilizer. I do agree with the "soft cam" theory and worn lobes. My youngest brother had a 73' Monte Carlo with a 350, and the original owner had to replace the cam at 50k miles for the same thing. The bad iron might had carried over through 76', not too sure.
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Old 07-09-2008, 08:00 AM   post #9 of 40
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Default Re: strange knock from Chevy 350

May not even be close to your problem,but I have a 98 with a 350.I have used 10w30 in just about everything I own.I noticed a tick when first cranked and I changed to 5w30 as book said and low & behold,tick gone?? I guess it needed thinner oil on startup to oil valve train.I would suggest a "clean-out".We use to mix 1qt.mineral sprits,4qt.trans.fuild crank it but just let it idle a few min.(it'll smoke a bunch)then change oil and filter.Then run a good deterget oil.This is old school,and may not suit all folks,something to think about.
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Old 07-09-2008, 08:06 AM   post #10 of 40
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Default Re: strange knock from Chevy 350

Thanks guys. It sounds like I just need to put up with the sound. Like I said its not so bad I'm concerned about the engine grenading. I just like my engines to run smooth. I will try different weights of oil and additives to see if that quiets it down.

Wayne990 by low miles I meant that it has less than 62k original miles on it. So lots of setting under a tree. It was fun to read the original sales reciept that showed the buyer having heavy duty rear springs install by the dealer. I know the guy who originally bought it and he was a farmer who probably just used it to haul grain wagons relatively short distances. I am the third owner. The second owner bought it about 3 years ago and the title said it had about 47k miles on it when he bought it. I bought it a couple weeks ago.

Andy
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Old 07-09-2008, 08:58 AM   post #11 of 40
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Default Re: strange knock from Chevy 350

Quote:
Originally Posted by Bill Kapaun View Post
Actually, the tappets do have an adjustment to "center" them for the plunger travel.
With the engine warmed up, you loosen the rocker arm screw until it just starts to clatter. You then SLOWLY tighten 1 turn.


this is likely the problem I've seen several where the locknut on the rocker has backed off pull the tappet covers and try it. Hopefully that cures your problem. If the noise comes back it may be neccesary to replace the nut or stake it to the stud with a center punch
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Old 07-09-2008, 12:18 PM   post #12 of 40
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Default Re: strange knock from Chevy 350

By all means, check/reset the valve lash per specs. Once done, use Lucas Oil Stabilizer. It works well! Used it for several years now in many different applications from gear boxes to engine, both gasoline and diesel. I use their synthetic formula on lower mileage vehicles and equipment. Their Trans Fix formula is a must have for manual transmissions using ATF fluid as the lubricant. Will really aid in smooth shifting. All of their products work well. I highly recommend them. They're worth the price.
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Old 07-09-2008, 01:12 PM   post #13 of 40
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Default Re: strange knock from Chevy 350

Would be a good time for a cam/intake/head swap on the old girl.
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Old 07-09-2008, 01:46 PM   post #14 of 40
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Default Re: strange knock from Chevy 350

Other sources of knocking or clicking noises on a small block Chevy V8 are :

1. Fuel pump pushrod worn,or the rocker arm in the fuel pump has a busted spring,worn pivot points,etc..this sounds like a tappet ,but morphs into a knock at times,loudest at idle,goes away just above idle..you can run the engine briefly after removing the pump to see if its the cause of the noise,but usually the wear on the pushrod or pivot on the rocker arm is readily visible upon removal..

2. Timing chain!--many GM V8's used nylon coated aluminum cam sprockets,that shed the nylon teeth,and let the chain slop around!-I have owned a few that actually wore a hole in the timing cover and oil started leaking out!..be aware if you find this on your engine,you'd better be prepared to drop the oil pan and replace the oil pump,as well as the timing chain and gears,since every motor I had that "ate" the cam gear,had nylon gear teeth get past the oil pump screen and got crushed in the pump gears..a metal oil pump drive shaft coupler is a good idea to replace the cheesy nylon one used as OEM,and only costs 5 bucks..


3, Wrist pins,or collapsed piston skirts,and cracked pistons all sound very similar,but usually increase in volume as RPM's increase,sound more like a low pitched "Thock-Thock" noise at idle,like your tounge hitting the bottom of your mouth ,or a rapid clicking noise that seems to be half as fast as the crank is spinning..

5. Valve springs,rocker arms,or retainers can cause clicking noises--I have had more than one motor that the rocker arms hit the inside top of the valve cover too,had to massage the cover with a mallet to get rid of the worrysome knock!..valves ARE adjustable,and yes,Chevy's always had "soft" cam lobes,but mostly on their 307 and 305 V8's,not so much the 350's and other size small blocks..its easy to tell if one is worn,the rocker arm wont move much compared to its neighbors,if you start it up with the valve covers off--but be aware oil will sqirt everywhere!..I can "hear" a Chevy with a bad cam lobe a mile away--by taking off the air cleaner,and flooring the accelerator,you'll hear it "gargle" through the carb at high RPM's if a lobe is flat..90% of all the cams I replaced in Chevy V8's,had the #1 exhaust valve lose its lift--this causes pressure build up in the cylinder,and when the intake valve opens,the built up exhaust that couldn't get out escapes intead through the carb or intake!..and you can "hear" it..


I'd guess it's probably just stuck lifters or oil deprived parts from sitting so long,that is causing the noises in your case--oil probably got gummy,and parts may have had all the oil drain off and gotten a bit rusty as well..

I'd try adding a quart of Marvel Mystery oil to the crankcase,and run it for a week or two,then change the oil--the 15W-40 diesel oil will be good for it
,it has higher detergent content than regular oils,plus has more anti-wear additives..might not be good to use in a sludged up motor though,since it could loosen up heavy deposits--but ANY oil will,and I think an older engine will appreciate heavier oil than thinner oil...

Lucas makes good products too,I've seen them work when others did little or nothing..

Robert
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Old 07-09-2008, 02:29 PM   post #15 of 40
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Default Re: strange knock from Chevy 350

It could be a gummed up hydraulic valve lifter.Or one that only taps at low engine speed and low oil pressure, could be that it is starving for oil, I had a 1976 4x4 200K miles with a 400 CSB that did that. I found that the oil suction screen on the oil pump had fallen off into the pan and when it got almost a qt low the lifters would rattle.
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