A 2000 Ford 250 with a 5.4 and 242,000 miles. Spark plug on #8 blew out of the head and then was driven for 40 miles. Insert was put in,new plug and coil wires were installed for #8. Now we have low compression on #6,80 pounds and readout shows a misfire on #5 and #6 cyls. The oxygen sensors are also showing up on the readout. The truck ran fine untill the pulg blew now it runs rough like a diesel that has jelled up, it surges then cuts out. Fuel filter has been replaced with no improvement. The engine seems to run better at 3000 to 3500 rpm’s. We have checked for spark and fuel seems to be ok, thanks for any help.
When a situation like this happens that cylinder is is NOT firing. The problem with this is it can cause a cylinder wash situation. This is where raw unburned gas drips down the cylinder walls washing off the oil. You may have a scared cylinder wall or even a broken ring.
The engine is never going to run right until the reason for the low compression is determined. A reading of 80 PSI is horrible.
A compression test of all cylinders should be done and if they’re low, and considering the engine has almost a quarter of a million miles on it, it’s time for another engine. No sense in throwing parts at a junk engine.
You can spend lots of money trying to diagnose the problem, but after 242k miles I would just have a rebuilt engine installed. It’s served you well, time to be changed out.