sudden knocking noise
Thread Starter
Intermediate
Joined: Oct 2010
Posts: 29
Likes: 0
From: Bay Area California
sudden knocking noise
so about 2 months ago i got my 74 dialed in with a fresh tune up. changed oil, plugs, cap n rotor, new electronic ignition, set the timing to 36* and the motor has been running great. it starts easy every time and runs like a champ...
yesterday i was driving and going uphill i rolled into a stop and i started to hear a loud knocking noise, my house was only two blocks away so i tried to idled it home but it wouldn't run for more than maybe 10 seconds at a time unless i gave it some gas. i limped it home the two blocks and once i got in the garage the motor gave out.
can an engine knock develop this quick? the oil is full and very clean looking (just pulled the dipstick to see, i will drain oil this weekend)
neighbor said it could be fuel related since the motor didnt want to stay running but the loud (really loud) knocking noise was something i have never heard. maybe timing got out of wack somehow?
motor work is relatively unknown we have had the car about 3 years and previous owner didnt have much info on any mods to the car... as far as i can tell it just has a aluminum intake, edelbrock carb, and possibly a slightly hotter cam.... guys have any ideas or suggestions as to where to start diagnosing?
yesterday i was driving and going uphill i rolled into a stop and i started to hear a loud knocking noise, my house was only two blocks away so i tried to idled it home but it wouldn't run for more than maybe 10 seconds at a time unless i gave it some gas. i limped it home the two blocks and once i got in the garage the motor gave out.
can an engine knock develop this quick? the oil is full and very clean looking (just pulled the dipstick to see, i will drain oil this weekend)
neighbor said it could be fuel related since the motor didnt want to stay running but the loud (really loud) knocking noise was something i have never heard. maybe timing got out of wack somehow?
motor work is relatively unknown we have had the car about 3 years and previous owner didnt have much info on any mods to the car... as far as i can tell it just has a aluminum intake, edelbrock carb, and possibly a slightly hotter cam.... guys have any ideas or suggestions as to where to start diagnosing?
Melting Slicks
Joined: Nov 2010
Posts: 2,216
Likes: 113
From: Michigan
Cut the oil filter and look for bearing trash. If your lucky it's the fuel pump and if it'll start you can use a stethoscope and listen to the pump.
My 2nd stock replacement fuel pump knocked for awhile and I finally just put an aftermarket pump & return reg on instead of another stock pump. I got maybe 5,000 miles out of both auto parts store pumps and the 1st one blew the diaphragm and filled my engine with gas.
My 2nd stock replacement fuel pump knocked for awhile and I finally just put an aftermarket pump & return reg on instead of another stock pump. I got maybe 5,000 miles out of both auto parts store pumps and the 1st one blew the diaphragm and filled my engine with gas.
Thread Starter
Intermediate
Joined: Oct 2010
Posts: 29
Likes: 0
From: Bay Area California
The vette is at my parents house and I won't be there until Saturday so I'll post up a video over the weekend.
The knock was loud at idle and was louder as the rpms went up, as far as oil pressure I did no look at the gusts when this occurred. As soon as I heard it I knew something was up so I limped it home. The guage does not work too good, when the motor is off it shows 40psi and when running it will be pegged a little past 80psi so I know it's not that accurate but has oil pressure
The knock was loud at idle and was louder as the rpms went up, as far as oil pressure I did no look at the gusts when this occurred. As soon as I heard it I knew something was up so I limped it home. The guage does not work too good, when the motor is off it shows 40psi and when running it will be pegged a little past 80psi so I know it's not that accurate but has oil pressure
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Le Mans Master
Joined: Sep 2003
Posts: 8,812
Likes: 2,029
From: eastern NC
A spun bearing will normally cause the oil pressure to drop from normal readings, sometimes significantly. If it were me, I would pop a valve cover (or two) and see if a rocker arm or stud has failed.
Cost is minimal and they can make a lot of noise when they let go. It will also cause a noticeable loss of power.
Good luck... GUSTO
Cost is minimal and they can make a lot of noise when they let go. It will also cause a noticeable loss of power.
Good luck... GUSTO
Thread Starter
Intermediate
Joined: Oct 2010
Posts: 29
Likes: 0
From: Bay Area California
disconnect and plug up the vacuum advance, set the timing gun advance to 36*, rev the motor to about 3500 rpms and turn the distributor until the timing mark on the balancer is on the 0 on the timing tab.... this is how i set up my 70 elco motor and it has been flawless for 2 years.
Thread Starter
Intermediate
Joined: Oct 2010
Posts: 29
Likes: 0
From: Bay Area California
A spun bearing will normally cause the oil pressure to drop from normal readings, sometimes significantly. If it were me, I would pop a valve cover (or two) and see if a rocker arm or stud has failed.
Cost is minimal and they can make a lot of noise when they let go. It will also cause a noticeable loss of power.
Good luck... GUSTO
Cost is minimal and they can make a lot of noise when they let go. It will also cause a noticeable loss of power.
Good luck... GUSTO
Drifting
Joined: Sep 2000
Posts: 1,492
Likes: 12
From: Milwaukee, WI / NYC, NY
Weird- my 78 did the exact same thing today. Barely made it home, and pushed it down the driveway and into the garage. My mechanic, who is a C3 guru, came by and though either rod knock or fuel pump.
Hopefully it's just the pump. Engine is an L48 that has never been cracked open other than putting on an L82 aluminum intake and air cleaner.
Like you, car was just tuned up, new plugs, wires, rotor, cap, etc... Engine oil is full and clear.
Had just bought a tank of gas a few miles before this happened at a place I haven't bought gas from before. I'll let youknow what they find.
Hopefully it's just the pump. Engine is an L48 that has never been cracked open other than putting on an L82 aluminum intake and air cleaner.
Like you, car was just tuned up, new plugs, wires, rotor, cap, etc... Engine oil is full and clear.
Had just bought a tank of gas a few miles before this happened at a place I haven't bought gas from before. I'll let youknow what they find.
Thread Starter
Intermediate
Joined: Oct 2010
Posts: 29
Likes: 0
From: Bay Area California
Found the problem... Pulled the drivers side valve cover off first and found a rocker that was a little loose but nothing out of the ordinary....
Pulled the passenger side off and a rocker arm fell out when I pulled off the valve cover lol exhaust rocker arm broke at the seat on the #4 cylinder.... I'll upload pics later tonight.
Can I just throw a new rocker on, adjust, and call it a day?
Pulled the passenger side off and a rocker arm fell out when I pulled off the valve cover lol exhaust rocker arm broke at the seat on the #4 cylinder.... I'll upload pics later tonight.
Can I just throw a new rocker on, adjust, and call it a day?
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
Found the problem... Pulled the drivers side valve cover off first and found a rocker that was a little loose but nothing out of the ordinary....
Pulled the passenger side off and a rocker arm fell out when I pulled off the valve cover lol exhaust rocker arm broke at the seat on the #4 cylinder.... I'll upload pics later tonight.
Can I just throw a new rocker on, adjust, and call it a day?
Pulled the passenger side off and a rocker arm fell out when I pulled off the valve cover lol exhaust rocker arm broke at the seat on the #4 cylinder.... I'll upload pics later tonight.
Can I just throw a new rocker on, adjust, and call it a day?
Are you running cheap aftermarket aluminum rocker arms? When it comes to rocker arms you get what you pay for...................
Thread Starter
Intermediate
Joined: Oct 2010
Posts: 29
Likes: 0
From: Bay Area California
Here's what I was looking at when I pulled the valve cover
Here's the rocker
Didn't find much shrapnel it looks like it was a clean break
Last edited by 70elcoss; Jul 19, 2015 at 05:56 PM.