Q: After installing new spark plugs they soot up after 70 miles

High mileage engines leak oil into the combustion chamber through worn out valve stem oil seals, worn valve guides and ill fitting piston rings. If a substantial amount of oil is entering the combustion chamber the plugs will quickly foul. You might also have leaking fuel injectors and/or the air fuel ratio is off allowing excess raw gas into the combustion chamber. That overly rich running condition will foul the plugs as well. These are the more common causes.

There are other possibilities but a mechanic should do a thorough check of the engine to see if perhaps changing the valve stem seals and correcting any operating condition (e.g., too rich) might make the engine usable for a while longer. If the 400,000 miles is on the present engine, it might be necessary to rebuild it to fully resolve the plug fouling issue.

If you plan on getting this repaired, consider YourMechanic as one of our mobile technicians can come to your home to diagnose the misfiring firsthand and help you make the necessary repairs.