Compression test, any tips?

[Danglerb]

It doesn't sound too hard from what I have read so far, but since I haven't done it before, I'm checking.

Warm up the motor, shut down and let it cool (I have a box fan I plan to use). Once I can do it without frying my flesh, pull the plug wires and then all the plugs, and disconnect both plugs on the ignition modules. With the throttle wide open *** give each cylinder 5 or 6 compression pulses with the meter screwed in and record the values, plus maybe the number of pulses for the pressure to stabilize.

Start with a full charge on the battery, and maybe leave the charger connected during the test.

Maybe do a second round if any cylinders were questionable.

Check, possibly regap the plugs and put them back in the same cylinders.

See if it still runs the same.

*** with WOT do I need to pull the fuel pump fuse, I remember something about WOT doing something about the fuel.

[carmen78sc]

Sounds like you got it nailed. Do it WOT on the motor as close to operating temperature as possible without burning yourself for the most accurate readings. Disconnect fuel and spark as you don't want unburned fuel to "wash" the cylinders. Don't crank for more than 10-15 seconds at a time to give the starter a break.

[Herr-Kuhn]

You should really do a leak down. I recently let a friend borrow my leak down tester. He used a crank unit and it showed good numbers...he then decided put the car back together (new intake manifold gaskets) and he still had the same problem (coolant blowing by). He then ran the leak down test and confirmed his worst fear...blown head gasket. He had to re-work the entire top side to change the head gaskets (he had thought his issue was the intake manifold gasket). Needless to say he was not happy.

Do it right and run a leak down test. The crank-up units don't begine to tell you the whole story. I built my own leak down tester with some left over regulators and control flow valves...it works really well. You get to see what is leaking...rings, valves, head gaskets, heads, etc.

[Danglerb]

I've found two schools of thought on the leak down vs compression only testing. Clearly doing both is the most reliable, but a good case can be made for compression testing being "enough" provided no other special circumstances exist.

Any motor with boost, or boost planned, or with a problem like your friend had needs a leak down test, same for any racing application where ring wear or condition needs to be checked.

I'm not too sure what the compression test is going to tell me, since I haven't been able to drive the car even a mile, and it was sold as non starting by the previous owner and parked for at least 6 months, maybe longer. If pulling it myself doesn't seem that bad, I may say screw it and just clean it up on the outside, put on new hoses and drop it into my 83 and see what some Italian tuning can do (fresh gas and vroom). If a few tanks don't "fix" it, then maybe I will pull it and rebuild with various "options".

Thanks for the suggestions, maybe I will get some gloves so I can go in a bit hotter. Thinking playtex more than mechanix

[rhjames]

Compression testing Class 101

fully charged battery or battery charger hooked up

remove all spark plugs

disconnect coil wire(s)

thread compression gauge hose into spark plug hole

turn over engine 8 times per cylinder, recording pressure for each reading

compare each of the eight cylinders to see where the "bad cylinders" are

next add a few drops of oil to each cylinder (spark plug hole)

repeat compression test as above

compare pressure readings

if the pressure readings have increased (in the bad holes), you have bad rings

if the readings do not increase (in the bad holes), you have either bad valves, head gasket issues, or cracked block/head

I do not recommend wide open throttle as it causes increased amounts of fuel to be deposited into the intake, causing a possible fuel explosion or washing the cylinders down.


secondary test would be using a leakdown tester for more specific results.


-----