1983 300 6 compression test
 

Hi, i have a 1983 ford inline 6. I did a compression test on it today and the first cylinder was 80 psi where every other one was 158 psi. So i redid the first on with a little squirt of oil down the cylinder and the compression was still 80 psi which leads me to believe the rings are fine. I next did a cylider leakage test and i pressurized the first cylinder and found no bubbles from the radiator but i had a lot of air coming from the oil fill on top of the valve cover. This one has me puzzled maybe a problem with the piston? please let me know what you think. thanks alot, mike

Generally, if you have low compression in a cylinder and then fill it with oil and still have low compression, it means that the valves are bad.

Engine Compression Testing

Both your tests indicate bad valves in that cylinder. A leakdown test will give you audible evidence of where your pressure loss is. Hissing in the crankcase is bad rings, hissing in the head is bad valves. And bubbles in the water is a blown head gasket or possibly a cracked head. Seems though like you have a pretty sound engine that just needs a little valve work. Could easily be fixed for a small bit of money by a machine shop.

Is this a daily driver or has it been parked for a long time?

I would drive it hard and then retest #1.

Thanks for the advice. The truck sat for 8 years but in this past year ive put 14,000 miles on her.

Try that leak down test with the air cleaner off and throttle plates open and listen at the carb and the exhaust.I guess you could maybe have a bad valve guide or more letting air into the valve cover,but you may have something up with the piston,crack,broken ring,but I would check everything else first.Maybe when you remove the head for the valve work,you may see something,but do all the checking first,once the head's off,the checking is over.