dont know whats wrong but sounds like a massive lower end knocking type noise dont know if its exactly from the motor itself but when it idles it just sounds horrible. if its possible id say it sounded like a connecting rod was knocking on the inside of the block but when gas is given the motor runs awesome and strong. any ideas?

 

Sounds like a loose bearing cap. Heres a test you can do. In the lower gears, like 1 or 2, go driving on the highway. Bring the rpms up and keep going fast in second gear, once youve picked up speed, lift off the throttle, and listen for a knock. If you hear a knocking as soon as you lift off and it dies after a few seconds, its probably a loose cap on your connecting rods.

This happens, because if you imagine the load direction on the crank shaft, the connecting rod, and the cap, when you accelerate, the piston pushes down on the rod, the rod pushes down on the crank, and your main cap has NO load on it. When you decelerate, now the power isnt driving the car, its the momentum, so the crank is spinning faster than the pistons. Therefore, the reverse load occurs, you now have no load on your connecting rods, and all the load is on the bearings. If the bearing is loose, it will knock.

 

alright i will try that thanks

 

my brothers 78 305 developed a similar problem. It was an occasional knock at idle, never at throttle, and always on hard deceleration. Another way to test it is to downshift, from 2-3 lets say, but its a bit harder on the car, but much easier to hear.

 

ok so i didnt drive it but i did rev it up and all through the rev it ran fine and when i let off, the noise didnt come till it was back at idle...so no noise while rpms are dropping but when at idle its there...i will take it for a spin in a lil bit

 

Not quite, you cant just rev it in neutral or park and expect to hear the noise. First off, the tick isnt loud enough to hear over the engine neccessarily, and second there is no load.

When you free rev up, the connecting rod exerts force on the crank so the bearing is free of load, and when you release, it all comes back to idle speed ALL at once, the crank and the rod, and the bearing are all going the speed, so you wont hear a tick.

You need to either downshift, or do what i said IN gear. When the engine is reducing revs under loaded conditions, the wheels are spinning faster than the engine, so in essence the wheels are keeping the revs up. This is why a car doesnt reduce down to 1k rpm from 5k rpm, just because you lifted your foot off the gas, the wheels are still keeping the revs high. It is this condition that causes the "reverse load". The load is now exerted from the crank on the bearing, since the bearing wants to slow down, but the crank doesnt.


This is the same reason your bike pedal has a clutch. If you've ever driven a bike with no gears, and direct drive chain, youd understand easier. When your stopped, and pedal hard, your foot (kind of like the connecting rod) pushes down on the pedal (kind of like the crank), so you pedal fast and get up to speed. Now lets say you start going down hill, and you dont need to pedal anymore. However, since the bike is direct drive (no gears), eventually your feet cant keep up with the spinning, and the wheels and pedals are now spinning faster than your feet. SO, the pedal will exert a force on your foot. (ie: reverse load). Ofcourse bikes have clutches now, so its not quite the same.

Similarly, if you did the same thing with a direct drive bike, with the rear wheel lifted off the ground, you wouldnt notice as large of a force on your feet from the pedals. Same thing in a car, you cant expect to hear it in neutral or park.


(note
i read it over, and i found something that may confuse you. When i say your leg exerts a force on the pedal, i mean when the pedal is in front part of its cycle (going DOWN). Ie: you are giving it power. When i say the pedal exerts a force on your leg, i mean when the pedal is in the "rear" part of the cycle (coming UP).

A crank to connecting rod force is similar, except, the crank will PULL and PUSH on the connecting rod. PULL (on the bearing) when its going down, and PUSH when its going up. This is what causes the TICK. But when you are accelerating, the pistons are moving faster and faster, so it is ONLY the connecting rod that exerts the force on the crank, therefore, no tick on acceleration.


Hope that helps.

 

ok thanks for the explanation...i had just thought id hear thenoise cause it isnt just a tick its litterally a knocking sound that you can hear at idle from 30-40 feet away...this is why i at first thought maybe exhaust was hitting the floor but i checked that and nothing....

 

Hmm, so by knock u dont mean a really loud tick? Is it more like a "clunk"?

Oh the limitations of text forums IF you can record it maybe, it would be helpful. I dont like the fact that the noise is intermittent. Other than the noise, how is the actual driveability of the car?