Thread: 350 small block knock at idle rpms

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i have a chevy 350 small block that knocks at idling rpms. when the engine is cool, there is no knocking, but once it gets warmed up it starts aknockin'. once you get above idling rpms, the knocking goes away completely and the engine runs very smooth. the knock is pretty deep sounding and sounds as if it is coming from the driver side, in the very back of the engine. the engine also tends to overheat at high speeds regardless of a good cooling system.

any info would be greatly appreciated,
thanks,
shad

Couple more questions. Automatic or manual? Does it do it in gear or in nuetral or both? Could it be the torque converter, or a loose torque converter bolt if it is an automatic?. Have you gotten under the car to try to locate it? Have you had the carburetor or intake manifold off lately? Any chance something small could have fallen into an intake port and is sitting on top of a piston? (been there done that with a piece of a cotter pin. Made a knock like you decribe. But my knock did not go away above idle).

One thing to try is a mechanics stethascope to try an locate it. if you don't have one, a long extension or breaker bar placed right by your ear and then the other end moved around against different parts of the motor will transmit the sound. Just be careful around moving parts.

As for the overheating, describe what you mean by high speed, and how long before it over heats?

Pat

Is it louder under the car or under the hood? If its under the car then in most cases it will end up being a connecting rod bearing. If it seems about the same up top as the bottom then its most likley piston slap.

Both are comon on high miles motors. Most SB see this condition towards the end of their life cycle. First the noise starts just when its cold, then it takes longer and longer before it goes away till one day you notice it doen't go away. I have seen cars in normal (not hot rod) service last 40,000 mile after a noice started and most of the time repairs are made for piece of mind.

Course this all assumes we are talking a older motor. If we are talking a late model ya got to get if figured out. These tightly toleranced precision engines are not very forgiving. They will run strong for near 200,000 miles but once something goes wrong it goes quick.

Yo !
I have 90 Corvette, 6-speed, 42,000 miles.
My brother owns a 90 Corvette, 6-speed, 44,000 miles. Both standard 350s.

Both of these 'Vettes started this stuff in the last year:

- only "knocks" when warm, at idle;
- if you shut it off, then turn it back on, "knock" goes away
- matters not if clutch in/out, and/or in neutral.
-sounds like it's coming from "bottom" of the car....

No answers here. Misery loves company, but we'd rather have a solution than just company !!

I can almost guarantee you your idle oil pressure is low. Don't worry, your engine still has plenty of life in it, just don't expect it to hold up if you start increasing the power.

well if i where you guys i would be taking the engines down pulling them apart before they do to much damage to them selfs and bored them .30 over to clean them back up get new pistons, get everything checked out and rebuild if you have to so that knock will go away. if my 327 started doing that i would tare it down right away not waiting for a month or to still driving it

Cd

Change the oil, and filter. Use a quality 30 wt. like Valvoline....then see if it still knocks & let us know.

thanks for all the replies guys. i clicked on the option to be emailed when a response was posted and never got an email, so i just assumed that no one had replied.

Ok, i'll try to answer all of your ??'s

year is 1970, dont know the miles, 650 holley carb
i have tried different brands/weights of oil with no change in knock.
it is an automatic transmission, and does not knock when idling with the car in gear. the sound appears to be louder on the bottom of the engine.

i thought it was a rod, but several people have told me that a rod would continue knocking through high rpms.

the car will overheat at highway speeds. i have done everything i can think of to fix the overheating problem.

i have done the following

-freshly cleaned/rodded 3 row radiator
-new water pump
-new thermostat(ive tried 160 and 180 temps)
-electric fan mounted on front of radiator
-replaced flex fan with a clutch fan
-new radiator hoses
-timed with timing light

what else can i do? would a shroud help?

thanks,
shad

Yer Fan shroud directs air threw the radiator, yes it helps alot!!

As for the knock, is it metalic? My truck sounded like it was going to throw a rod, then I started looking at it, well I'll be an exhaust leak, not a bad one so it sounded like a knock.

Otto

thanks for all the info guys,

here's an update on what ive done and whats happening.

the engine is from a 1978 camaro. i dont know the history as to whether it has been rebuilt, how many miles, stock or other wise, etc...
Firts of all, let me say that i doubled checked the distributor and sure enough it was firing well past #1 at tdc. I pulled the distributor and set it back one tooth. now the timing mark lines up good with the timing tab and balancer...that problem solved. also, the knocking has stopped...that problem solved.
BUT IT STILL OVERHEATS!!! Now, if i let it idle long enough, it will overheat to about 220-225, but only gets up to around 240-250 at highway speeds(above 50-60mph).
I tried a new temp guage...same temp readings(220-250F). I am running about a 40:60 ratio antifreeze:water now.
i replaced the timing chain/gears with a double roller. i double checked the t-stat. it's good. i checked the vacuum advance and it was toast(no suction at all) so i replaced it.

Today, i let the car idle with the front end raised about 6 inches to try and get any air out. After about 7-10 minutes, the temp was up to about 220 and the water began going way down and then coming up and spilling out of the radiator, then back down and then up again. After a few times of it doing this, i shut the engine down, and then all hell broke loose. antifreeze/water shot out over a foot high for about 3 or 4 seconds, and then settled back down...i swear i heard the thing laughing at me! Anyways, i put the cap back on and cleaned up the engine area a little bit, and a little while later i noriced that the top radiator hose was completely collapsed! The bottom hose has a spring in it, but not the top. ive never heard of anyone having to put a spring the top hose. there must be an underlying problem to it. could it be that my water pump is the wrong water pump and turning the wrong direction???

thanks,
shad

ok guys,
i did a compression test and all check out good. all cylinders read between 155-165psi. i tested the engine cold and hot. all cylinders still consistent. however, on a good note and maybe this could help lead to an answer...

it's been a couple days since i have driven my car. each time i work on it, i check the coolant level, which always drops a little, even though i havent driven it and there is none on the ground and none in my oil. each time i top it off, only to come back later and find the level down a little bit, anywhere from 1/4-1". not much, but still enough to indicate that thier might be air locked somewhere, which is gradually escaping. today in an attempt to rid the sytem of air, i drove on a steep up hill. about 1/4" way up the 50 yards or so hill, the temp dropped by about 20 degrees. and stayed there for about 15 minutes of driving around, then it started to get a little hot again (about 215). but never got any hotter than that. nothing like the 230-250 that i used to rock up to!! i am thinking that either there is still more air that needs to be gotten out of the system, or there is something that is pumping air into the system.

what do you think?
what else can i do to get the air out of the system?
how do i tell if there is something that is causing air to pump into the system?

thanks,
shad

not to ask a dumb question but did you leave the cap pff untill the t stat opened and ran through.

if this doesnt work either do pressure test on the cooling system if it holds, then i would do a block test to see if you have a cracked head or bad gasket

yes, i did leave the cap off until the t-stat opened. the water pump is new and i am running a 16lb cap. whats the best way to go about checking for a cracked block, etc...

thanks,
shad

also, i wanted to mention that there is an overflow spicket on the side of the cap nozzle, but i dont have a resevoir tank...i just attached a hose to it and let it drip on the ground. should i have a resevoir?

thanks,
shad

a block test is using a chemical in a test tube and sucking out antifreeze to see if there are any hydrocarbons in the coolant.
this will tell you if any cylinder is leaking into the coolant system.