I did a compression test on my truck however im not sure if i did it right. i pulled all the plugs out. then i disconnected the fuel pump relay and the ignition control module. but i forgot to put a brick in the gas pedal to keep the throttle blade wide open. does that matter? anyway you will notice three psi values for each cylinder. that is because i hooked up the gauge and cranded the motor, then went to look and write down the value and left the gauge there with the pressure in it still and i went back and cranked some more. i did that three time for each cylinder. and everytime i did that the pressure went higher. why? also, is there something wrong with my middle cylinder on the passenger side? heres the values starting with the passenger side first since i did those first.

front cyl 175/185/191
middle cyl 150/162/173
rear cyl 176/196/205

here are the drivers side:

front cyl 170/176/185
middle cyl 170/176/181
rear cyl 170/172/179

 

I think you are only supposed to crank the engine over a few times and take a reading. The middle cylinder is a bit low. I think a healthy engine is supposed to have all cylinders within 10% of each other from highest to lowest. Possibly valve not seating properly or rings leaking due to wear or scuffing/scratching. Could even be cam lobe wearing down. There is no need to hold the gas pedal at the floor. It looks like you did a competent test, although a bit of overkill, but the middle cylinder should be investigated. If you put some lightweight oil like Marvel Mystery Oil in the center cylinder and crank it again and the pressure comes up close to the others then it pionts to the rings as being leaky. To test valve seal you would have to make sure the cylinder is on top dead center with both valves closed and hook compressed air to the plug hole and listen out the exhaust and intake to see if you hear any air escaping. Could pop off the valve cover and measure the rocker travel with a dial indicator to verify cam lobe health. Just a few backyard diagnostic tricks that are virtually free to check it out with. Good luck.

 

i did squirt some straight 40 weight oil into the middle cylinder that was lower but the compression values did not change. if anything it seemed a bit lower.......

 

The compression on #4 does seem a bit low, however, the average on that cylinder is still within +/-10% of the average (taking all of your values and averaging them together).

If squirting some oil into the cylinder did not affect its compression reading, then the cylinder has one or more of the following problems:

• Slightly Bent Valve
• Burnt Valve
• Head Gasket Leak
• Cracked Head
• some other problem...

If the value had of gone up with some oil in the cylinder, then it would indicate that you have a ring problem.