Proper way to do compression test?

[timl865]

I just bought the harbor freight compression tester and need to know the right way to check compression. I have never done it before and have the dreaded cyl 3 misfire. Thanks!

[OldNuc]

To prevent washing the cylinders down with fuel remove the PCM B fuse to shut off the fuel pump, injectors, and ignition before starting the test. Remove all spark plugs and block the throttle open. Use a low charge rate battery charger to fully charge the battery before conducting the test.

Crank the engine until the gauge quits increasing and compare both the final pressure and number of compression strokes to reach the max pressure, cylinder to cylinder. You can do this test either cold or hot or warm just mention it when stating results.

For example if you find all cylinders produce a final reading of 200psi but the compression cycles are as follows: 1-8, 2-6, 3-8, 4-14 then it should be obvious that cylinder #4 has a serious problem with leakage past either the valves or compression rings.

FSM states the MINIMUM number of compression cycles is 10 and a Fully charged battery is required to obtain the full 250RPM cranking speed.

Nominal compression for a high mileage good condition DOHC is 190-220psi and SOHC is 175-205psi.


Do not screw it into the head past the point the sealing o-ring just contacts the head. Do not overtighten as it will leak.

[timl865]

Perfect! Thank you!

[onlinebiker]

Facing west whilst slaughtering a goat is always helpful too....


Ya just can't be too thorough...

[youstolemybeer]

thats only if your west of St Louis. Otherwise for proper compression test its facing North while slaughtering goats

[Silverblu02SL2]

Originally Posted by OldNuc

To prevent washing the cylinders down with fuel remove the PCM B fuse to shut off the fuel pump, injectors, and ignition before starting the test. Remove all spark plugs and block the throttle open. Use a low charge rate battery charger to fully charge the battery before conducting the test.

Crank the engine until the gauge quits increasing and compare both the final pressure and number of compression strokes to reach the max pressure, cylinder to cylinder. You can do this test either cold or hot or warm just mention it when stating results.

For example if you find all cylinders produce a final reading of 200psi but the compression cycles are as follows: 1-8, 2-6, 3-8, 4-14 then it should be obvious that cylinder #4 has a serious problem with leakage past either the valves or compression rings.

FSM states the MINIMUM number of compression cycles is 10 and a Fully charged battery is required to obtain the full 250RPM cranking speed.

Nominal compression for a high mileage good condition DOHC is 190-220psi and SOHC is 175-205psi.


Do not screw it into the head past the point the sealing o-ring just contacts the head. Do not overtighten as it will leak.

Would removing the PCM B fuse and then crank the engine result in a DTC?

[alordofchaos]

Originally Posted by Silverblu02SL2

Would removing the PCM B fuse and then crank the engine result in a DTC?

No - the PCM gets no power, so can't set a code

Pulled the PCM-B lots of times, never caused a code. Pulling the PCM-B will actually clear any stored codes.

[Silverblu02SL2]

Sorry, the question I actually meant to ask was: Will it set all readiness to "Incomplete"

[alordofchaos]

I believe it will - loses all memory of completing the readiness tests, so will set it back to "Incomplete", default fuel maps, default shift points for the auto trans, etc.

Don't have emissions testing here, so not something I ever thought about. Plus my normal commute cycle completes the tests pretty quickly, the times I checked with a reader

[OldNuc]

Originally Posted by Silverblu02SL2

Sorry, the question I actually meant to ask was: Will it set all readiness to "Incomplete"

YES IT WILL. You can attempt to disable fuel and spark independently of just pulling the PCM B fuse or you could try just pulling the PCM I as it should accomplish the same thing. Or you can move on down the tree of increasing potential mechanical damage and start pulling relays and random fuses.