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1999 Jeep 4.7 V8 top end noise and dead cylinder

It was running on 7 cylinders and had not been tuned in probably 50,000 miles or more. I recently bought it at auction. Changed spark plugs and 1 coil for the dead cylinder. Rear cylinder on passenger side was not firing. Compression test shows 150 psi that cylinder. Fairly loud mechanical pecking from that area which was isolated with a stethoscope. I suspect a flat cam lobe or broken rocker arm. What do you think?

I think it’s time to pop the valvecover and see what’s happening. Might as well pop both side while you’re at it.

Sincere best with this.

Might simply need a valve adjustment. Hopefully it isn’t the head gasket springing a leak. Sometimes that will cause a sort of popping sound at first. If the manual says it’s time to retorque the head bolts, might be worth the effort to bring that scheduled maintenance up to date in any event.

It is a mechanical noise that you might hear if you had about 1/4 inch of valve lash. It is surely related to the failure of that cylinder. As for scheduled maintenance…who knows? Oddly the car was very clean and did not appear to have near the mileage I now know it has, 201, 654. Could not tell at the auction because the battery was dead after being confiscated months before and the pedal covers looked like new, probably were.
These SOHC motors have a history of dropping valve seats, but 150 psi compression seems to eliminate that horror.

I will bet it is a stuck valve lifter.

Collapsed exhaust lifter is far more likely considering the mileage.