compression test

[Edgefevah]

Just did a compression test on my ranger. specs are in my profile 93/4.0/4x4/M5OD.

all cylinders were at 145 PSI, I did the test while pushing the gas peddle to the floor to get WOT.

I want to get a leak down test done soonish. but does anyone have anything to comment about 145? is this a good reading?

Haynes manual says as long as they are all within 15PSI of each other which doenst help much.

 

[jhammel85]

145 is good but I'm unsure as to why you floored the gas pedal if you are just turning it over?

 

[RonD]

Opening the throttle plate gives you a higher reading since air flow is not restricted.

145psi is fine if it was constant across all cylinders.

It has alot to do with the cranking speed of the engine when doing the test, the faster it spins the higher the psi.
To that end all spark plugs should be removed when doing a compression test, so engine cranks faster.
Throttle plate open.
It is best to do 4 or 5 compression strokes on the cylinder being tested, which is easy to hear if all spark plugs are removed.

You can also do a dry then wet test.
Dry first, write down the number
Add a teaspoon of oil to the cylinder for the wet test.
You should get an increase in psi, if not then you have leaking valve issue.

My 4.0l with 300k test at 160 dry, using above parameters

 

[jhammel85]

I honestly didn't know that. See! You learn stuff here.

 

[RonD]

Not a big deal, a compression test is mainly used to find one low cylinder, which is why there is the 10% rule, not a specific psi number.
It has no other practical value.

So 5 cylinders are 160 to 170 and 6th is 130, so there is the cylinder that is missing .

If you did the test with spark plugs in and the throttle plate closed you might get 5 cylinders with 140-150 and 6th with 120.
So result is the same you have IDed a cylinder with a problem, which was the point of the test.