How do I find the source of a knocking sound?

ATOMICDR

MEMBER
1984 MERCURY CAPRI
126,000 MILES

This is the RS model with the 5.0 and a 5-speed. There is a continual knocking sound coming from somewhere in the engine/trans? With the use of a stethescope we narrowed it down to the harmonic balancer, which in fact was bad but not the real source of the problem. We thought it might be coming from the bell housing area. We removed the valve covers and all the rockers/lifters seem to be functioning properly. The noise is constant at any rpm. It does not get any louder with use or at a higher rpm. You can rev it up quite high and there is no sign of weakness. It almost sounds like a rod bearing but not as loud. Could a cam issue cause this? Is it possible in this engine a wristpin in defective? What else should we look at?

5 Replies

HMAC300

MECHANIC
48,601 POSTS

Best ways are to checkc oil pressure by mechanical gauge, if it's low something is going on in the bottom end. If you short out each cylinder one at a time then that wold tell you it's aproblem in the cylinder that you tested. Menaing rod bearing or piston. But if the sound doesn't change when you incresase or decrease the rpm it 's hard telling what it is. Eng trans problems should increase or decrease as rpm goes up or down. It cold be lifter or valve guide noise. It cold also be piston slap but that wold increase or decrease with rpm. It might be a wrist pin but shorting the cylinder would let you know that. Fords never really had cam problems like chevys so I don't think so without actually hearing it and doing tests can't tell you much. One thing that i'm pounding my brain here is there was some problem with carbon brekaing off pistons and making noise, normally a decarbonizer fixed that. It's the same at ford or gm dealers you spray in carb let soak for a couple hours and then start the car an ddrive it.

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Sunday, April 22nd, 2012 AT 9:05 PM

ATOMICDR

MEMBER
41 POSTS

I appreciate that lead on the decarbonizer route. That's at least a cheap way to deduce one more possible cause. I forgot to mention that we did try the disconnect for each plug wire to remove the combustion in each cylinder and listen for a reduction in the noise level but that proved to bear out no change in noise volume or frequency. We are really stumped with this one.

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Sunday, April 22nd, 2012 AT 9:46 PM

HMAC300

MECHANIC
48,601 POSTS

Without hearing it i'm blind on this. Try disconnecting the fan belt and seeif the noise goes away, that eliminates pumps an dalternator. If not that then look for a worng bolt hitting something, etc.

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Sunday, April 22nd, 2012 AT 10:55 PM

ATOMICDR

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41 POSTS

We took the belt off right away just to assure us there was no alternate noise or combination of noises causing the issue. We'll check for a possible bolt hitting but it does sound internal.

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Monday, April 23rd, 2012 AT 1:19 AM