Keep it til it blows or replace it?
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New User
Joined: Jan 2004
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Keep it til it blows or replace it?
I am new to this group and am impressed by the amout of knowledge possesed by the users so I am asking for your opinion. I recently purchased a '77 f100 with a 300 I6 and a 3 speed manual tranny. The engine has a strange knock, almost sounds like a diesel at high rpm. Mainly at the top end of first and second. Not noticable at Idle or at cruising speed. I thought it might be main bearings but the person I bought it from said it was the throw out bearing. Just for kicks I checked the compression and had 100/100/108/105/102/100. Seems a bit low to me but this is the first 300 I have owned. I have heard they will not stop and I don't depend on this truck so I just want to hear from some more experienced owners. Thanks for your time.
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New User
Joined: Jan 2004
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No, it is to low to be in the head and honestly it seems too loud to be a lifter. It's low and tward the rear of the engine. That is why the previous owner thought it was the throwout bearing. It does have some minor lifter tap but I probobly just need to adjust them. The engine seems strong and starts every time with out fail. Has me confused.
Senior User
Joined: Aug 2003
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Sounds like a decarbonising treatment is in order. Seafoam is one product. GM top engine cleaner is what I used to quieten my Knocking ( sounded like piston knock ). Instructions on the can say you can pour it directly into the throttle body. I used a piece of vacuum hose to suck it right out of the can into the manifold. The key to this treatment is the soak period when you shut the engine off which will allow the detergents to get to work on that carbon. Much more effective than any gas treatment. I would try this first before any internal work.
Elder User
Joined: Sep 2003
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I would bet that the sound is one of the rear rod bearings knocking.
No amount of stuff from a can is going to fix that problem.
Your compression looks a little low; but, if this was taken "Dry" it looks OK.
Good Luck.
No amount of stuff from a can is going to fix that problem.
Your compression looks a little low; but, if this was taken "Dry" it looks OK.
Good Luck.
Junior User
Joined: Oct 2003
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Look at your oil pan see if it is bent in.
I had this happen once on a 77.
We put a new clutch in and made the mistake of raising the motor with a jack on the pan.
It made a sound like a rod at the back of motor
I had this happen once on a 77.
We put a new clutch in and made the mistake of raising the motor with a jack on the pan.
It made a sound like a rod at the back of motor
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Senior User
Joined: Oct 2003
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From: Central Coast, CA
Your compression numbers are about right compared to my 225K mi '79. I'd expect it to be rod knock (as mentioned). Expect quite a few miles before she's gone.
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Senior User
Joined: Oct 2003
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From: Elyria, Ohio
Don't overlook a broken motor mount that let's an exhaust pipe
bang on a frame member when you place the engine under load.
Also a vibration damper that is rotting, coming apart, and allowing inner and outer metal to clank under speed. These sounds can be deceiving and we think of the worst when they start.
bang on a frame member when you place the engine under load.
Also a vibration damper that is rotting, coming apart, and allowing inner and outer metal to clank under speed. These sounds can be deceiving and we think of the worst when they start.
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Joined: Dec 2002
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From: Remsen, NY
Rule of thumb:
* Bad rod bearings are quiet under load, noisy at idle and no load.
* Bad main bearings are noisy under load, quiet at idle and no load.
Check this out:
http://www.remanufactured.com/Engine...gnosis_101.htm
I had something this fall that sounded to me to be too loud and deep to be anything but bottom end. It was bad at idle. I had a more experienced ear listen and insist that it was lifter noise. Remember the lifters are low in the engine, it's an old-fashioned (tough) pushrod engine. They're about where your rods are height-wise. I put a cup of marvel mystery oil in the crankcase against my better judgement and the noise almost immediately went away.
It's a snake oil treatment and there is risk that it will knock too much crap loose and plug the filter up, and I wouldn't go over a cup of it, but if you're not depending on the motor 100% anyway you might give it a try.
Now, I know my motor is not permanently cured and it's going to need some bigger help but this will help me get through until the spring when my hands won't freeze to the metal as I work.
* Bad rod bearings are quiet under load, noisy at idle and no load.
* Bad main bearings are noisy under load, quiet at idle and no load.
Check this out:
http://www.remanufactured.com/Engine...gnosis_101.htm
I had something this fall that sounded to me to be too loud and deep to be anything but bottom end. It was bad at idle. I had a more experienced ear listen and insist that it was lifter noise. Remember the lifters are low in the engine, it's an old-fashioned (tough) pushrod engine. They're about where your rods are height-wise. I put a cup of marvel mystery oil in the crankcase against my better judgement and the noise almost immediately went away.
It's a snake oil treatment and there is risk that it will knock too much crap loose and plug the filter up, and I wouldn't go over a cup of it, but if you're not depending on the motor 100% anyway you might give it a try.
Now, I know my motor is not permanently cured and it's going to need some bigger help but this will help me get through until the spring when my hands won't freeze to the metal as I work.
Last edited by gatesj; Jan 23, 2004 at 09:57 AM.