Hello, just so you know, pulling the spark plug wire off a coil like that can damage the coil, it stresses it too much, but that noise sounds mechanical, when youre taking spark away from that cylinder there isnt as much pressure in the cylinder due to combustion not taking place. You might try pulling the spark plugs out, and turn the engine over by hand slowly and see if you notice any noise. I have heard serpentine belt tensioners make a semi random tap like that as well, although I dont think its a belt tensioner, but you can run the engine with the Serpentine belt off for a few seconds to see if the noise disappears, but you seem to be isolating it when disabling the ignition. Have you kept up with oil changes as well?
At least take the valve covers off and rotate the engine by hand, make sure there isnt a broken valve spring, lifter or anything on the top end causing the noise, see if any of the rocker arms are loose. It is note worthy that the noise at idle is not consistent, only at higher rpm its more constant. Check the cam lobes for any wear or damage, with the valve covers off you should also be able to see the timing chains and check them for any slack on either side. If you have a stethoscope or long handled screw driver you could identify the area where the noise is coming from better.
Having the spark plugs out will make it much easier to turn the engine over and possibly hear the noise from that cylinder as well.. It could also be piston slap against the cylinder wall. It sounds so much worse when you raise the rpm, made me cringe a bit. I would also say maybe a timing chain component except it really does seem to be effecting just one cylinder. Ive heard many of these Grand Marquis make these noises before, but it was always more of a constant rapping type of noise, whereas the high rpm causes this more.
Your question regarding the ignition wire being disconnected is, this is a waste spark system, meaning one coil is firing 2 cylinders, When one cylinder is on its compression stroke and another is on its exhaust stroke. The coil fires, spark travels through one plug wire to the spark plug, travels through the engine block to another spark plug on a different cylinder, through that plug wire and back to the coil pack. This car being a V8 has two coil packs with 2 coils per coil pack, heres a diagram of the coil packs. Cylinders 2 and 8 share a coil.
https://www.2carpros.com/articles/engine-noises
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Wednesday, April 16th, 2025 AT 1:08 PM
