This is compression data from my 72 F100 with a 360 . The truck has 100K miles on it, I don't know if it has ever been over hauled, but the heads I beleive are not original. I have two sets of data one from Sept 3rd the other from Jan 5th. When I originally took the compression it was crummy, so I added some JB Engine Treatment. Thought it couldn't hurt. I would like someone to give me a few ideas on the engine, should I rebuild it or just run it. The truck is just a run around rig that I would like to rebuild and restore.

Sept 3 data Jan 5 data

Oil In Cylinder Oil in Cylinder
#1 90 100 100 105
#2 105 120 130 130
#3 85 105 125 125
#4 54 75 75 85
#5 105 115 120 125
#6 70 80 115 95
#7 70 80 125 125
#8 115 110 125 105

Any recommendatins on what to do, a total rebuild or are the head just bad on the lower cylinders. I am going to get a vacuum gauge and do some more checking, but would like a little input from some of the more knowledgable people on the list. I know that the good cylinders are probably low, but what about the # 4 cylinder any thought on what is going on.

Also do you think that the JB's was responsible for the increase in compression. I had it in the engine since Sept 3rd, but only have about a 1000 miles on it since then.

Thanks,
Bob

 

The first two sets of numbers are from Sept 3, first one's are without oil in cylinder, second set are with oil. The third and fourth set are the same from Jan 5th.

Bob

 

Why are you taking a comp. test? Is the engine running bad now or just FYI? If you are not going to rebuild it now and it runs ok then leave it alone until you decide to rebuild it, if its running bad now then I would pull off the head on the side with the low cylinder, check the valves and mic the bore to see if its standard or has been bored. When you did the test did you remove all the spark plugs? If you didn't then the engine was turning against compression and draining the battery , this will sometimes cause low readings on the last cylinders you did by the engine cranking slower.I would warm the engine,remove the sparkplugs,do the compression,then squirt the exact same amount of oil in each cylinder as you do them then compare them , then find a buddy with a leak down tester and do a leak down test.

 

In addition to removing the spark plugs when doing a compression test, the carb should be at wide open throttle. And, like Terry said, always test a warm engine.

Jason Kendrick

 

I am doing the compression test for peace of mind and just to get an idea of the condition of the engine. The engine was warm, all spark plugs remove, and the carb was wide open. I get the feeling that the low cylinder is due to valves, but wanted to see if others thought that also.

Bob

 

Get the a spark plug adapter that lets you connect an airline as if you were going to use air pressure to hold the valves in place so you could change the springs.

With air going to the bad cylinder, listen for air leaking at he tail pipe, carb and oil filler cap. The cause of your low reading will be obvious.