78 silverado 350 knocking

normally while idling there is a faint grinding/knocking going on. driving i cant hear it (i assume its drowned out by the engine) but returns when i stop or slow enough. havent been able to locate the source other than that it seems to come from the passenger side.

that was my only problem until....

today after driving on the freeway for about 10 min a new and atrocious noise showed up. its a terrible clanking/knocking that increases as i apply more throttle. if i let off to a coast it diminishes to basically a loud version of the first noise i described. i got off the freeway and it continued but after about 5 min the clanking stopped completely and returned to the initial state in the first section. i rode around town a bit and everything seemed normal. on my way home the exact same thing happened. loud noisey clanking and when i got off the freeway, within 5 min it was gone. during the freeway portion of this fiasco the engine did "buck" a little bit and high throttle would keep it from bucking (though the noise got worse of course)

any ideas whats going on? thanks.

how is your oil pressure on the freeway? what grade of oil are you using. if its got a lot of miles i would run at least 10W-40. if you are going very fast and the motor is revved pretty high for long periods, thats a no-no. i think people have told me that small block chevys will start to experience crankcase windage problems above 3500 rpm or so, which isnt too hard to hit in top gear with a 3 speed th350 or th400 automatic on the freeway. basically the crank is spinning so fast that it is starting to whip up the oil in the crankcase and i believe it can cause a drop in oil pressure. i suppose running a quart less of oil would help if this is what's happening to you.

thats interesting, though i dont have a change in oil pressure. im also not sure what oil is in it, i just got it 2 weeks ago. im changing it this weekend but i bough 5w30 synthetic blend high mileage. dont recommend that?

let's review some facts:

1) you just bought the truck 2 weeks ago.
2) you have a loud knock from the engine.

from what you're saying, it sounds like you might have a bad connecting-rod bearing. as bearings wear out, clearances increase. the gap from the clearance is responsible for the rod slapping into the rod journal on the crankshaft, thus producing a knock every time the crank rotates.

unless you have an oil-pressure gauge, there's no way to tell how much oil pressure you're running. only the "idiot" light (oil light) will light up, and usually only under severe circumstances of low oil/oil pressure.

if it is a rod bearing, the motor is going to sieze up pretty soon. bad bearings will cause "bucking" and significant loss of power as the bearing starts to fail completely. the wear increases friction, which makes it harder for the crank to spin. the friction acts exactly like a brake does and is not "even", that's why you had to increase throttle when the bucking occured.

it could be something else, but it's probably a rod bearing, especially in a 30 year-old truck.

to help diagnose the problem, drain the engine oil and look for fine metal particles. the particles will look like very small, shiny "glitter." if you see glitter in your oil, you have significant engine damage, time for a rebuild or a different motor. my preference would be not to use 5W-30 synthetic in that motor, "normal" 10W-30 is probably best.

good luck.

Also, cut the oil filter open and lay the paper element out. Look at the particles in it. If there is "glitter", pull the engine without even starting it again. If there is no copper-looking stuff, but still significant metal, using a small magnet, see if it's iron/steel or aluminum. Aluminum won't stick to the magnet. More than just a slight amount of iron is very bad. It usually means it's grinding up crank journals. Aluminum "peels" could be the edges of bearings, or piston skirt. Not good, but possibly not as catestrophic as the other two mentioned.

I agree, it sounds from your description, a rod bearing.

Jim

well i got a pressure guage and it doesnt change when this occurs and is the rod bearing issue consitstent with the noise going away off the freeway? ill take your word for it and ill certainly look for the shavings later today when i do the oil.

hi there heres some info from personal experience that may or may not help you:


low deep knock when idling usually is rods

low deep knock only when slowing the enfine back down is usually mains
rattling in bottom end when revving up is also usually rods


deeper and lower it is then the more likely its mains worn out

oil pressure will be low all the time if the bearings are severely worn in which it sounds like at least some are

tired old engine

7 psi per 1000 rpm is stock normally in these sbc engines in good condition

good luck