Looking for diagnostic help please. 2 years ago at 60K miles I had my first miss. 1st dealer, plugs and wires. 2nd dealer, alternator and battery. I replaced 1 coil pack and the plugs again and that worked. At 70 and 80K I had to do plugs again. At 90 once more, and noticed cly 4 has been oil fouled last 2 times. Now at 93K miles it is at it again. A local shop says it is common for a carbon build up on the lower piston ring to make that ring stick on cyl 4 and 8. He flushed the motor with some additive. It still misses, and now there is actual oil on the plug. He said it needs 3000 miles for the additive to work. I ran it today on the highway for 100 miles and it seems to be getting better. His final solution is a lower rebuild. Does this make sense, or am I being sold a story?

 

IMO everybodies guessing so far.
Do a compression test of all the cylinders and compare to the cylinder fouling the plug.
If that cylinder is low, add some #30 motor oil thru the plug hole and do the test again.
If the compression level rises a considerable amount you can be safe in assuming the rings are worn or broken permitting crankcase oil into the combustion chamber in excess amounts. That would mean a rebuild/replacement.
Adding the oil for testing, results in a hydraulic seal of the top two rings.
Note: Alway do this test with wide open throttle so the engine can draw a large amount of air to compess for the cylinder under test and all other plugs out.
AND keep all plug wires away or grounded because on your engine the ignition will still be active and fuel sprayed into the cylinders when cranking over unless the injector plugs are removed.

 

I did a compression test on that cylinder and got 103 pounds. I was too lazy to do the rest, it's cold out.

 

That pressure level is to low.
Make sure you do it all correct or it has little meaning.

 

I have a threaded gauge on a hose, but I can never get the threads started. The only way I can get it to work is with the rubber forcing cone. It seemed to seal allright. I guess I'll have to do them all and compare. What should the pressure be?

 

That number does seem low, but what you are really looking for is variation. If that cylinder varies more than (I think) 10% from the other cylinders, you have found your problem.