Compression on 3.0L V6

[Tom58]

I just put a set of rebuilt heads on my truck. I decided to check for leaks by putting air pressure into the spark plug hole. I went to 80 psi and the valves are leaking. I can hear air and feel it if I stick my fingers into the intake and exhaust ports. I can barely feel it. It took 6 seconds for the air to bleed down to 30 psi from 80 psi. It was 17 seconds to 0 psi from 80 psi. I have never tried doing this before.

My questions are how long should it hold pressure??

And does what I am doing even tell me anything??

Thanks for help in advance.

 

[RonD]

Are the push rods and rockers installed?

If so then perhaps valves are open, cylinder being tested needs to be at it's TDC(compression stroke)

No cylinder can hold pressure, a leak down test(what you are doing) is to see what percentage of the input pressure the cylinder can hold, so if you have your compressor set for 100psi and cylinder is holding at 97psi then you have a 3% leak down.
You then add some oil to the cylinder and test again, if it holds at 99psi then rings are where the leak is.
Point being a leak down test means pressure in the cylinder all the time, not how long it holds pressure after it's turned off.

A compression test is when valves are working and you place a pressure gauge in the spark plug hole, other spark plugs removed.
Prop open throttle, turn off fuel(EFI fuse/relay).
Turn over engine with starter motor, at least 4 TDCs for the tested cylinder
This will give you the compression ability of the cylinder, 3.0l should be about 120-130psi.


These tests are more accurate if engine is warm, but still give valid data on cold engines

 

[Tom58]

The head is bolted to the block and torqued. There are no rocker arms on the engine yet. Have not bolted the right head down because of the air coming around the valves.
With the compressor set at 80 psi and the air going into the cylinder my gauge shows 79 psi.
With the compressor set at 90 psi and the air going into the cylinder my gauge shows 89 maybe 88 psi.

So am I panicing for no reason and need to assemble the engine?

 

[Rearanger]

When I did a leak down on another vehicle air flowed past the rings mostly and probably through valve seats some - constantly. The rings are not a complete seal, especially cold, dry and not at TDC. You can't hold pressure in a cylinder for very long. My compressor was set for 100psi and the gauge on the leak down tester showed 95psi for 5% leak down, at least that's how I interpreted it. My compression test on that vehicle was great.

Don't forget vacuum ports will leak air giving false noise of problem and metal conducts noise very well.

 

[RonD]

Yes, you should be fine, 80psi with 79psi in cylinder is fine with cold head/block, even warm that would be fine.

Valves and valve seats are metal to metal "seals" which can't be "air tight", same as the rings are metal to metal "seals".
These are designed to hold temporary high compression in a high heat environment.
So no need to panic


Much like the Lockheed SR-71 "Blackbird", it leaked fuel like heck until it was heated by mach + air speeds, then it sealed up nicely
I don't recommend those speeds for a 3.0l Ranger, 4.0l maybe, lol.

 

[Tom58]

Thank you for the information

Thank you for the information! I am proceding with the assembly.

 

[stmitch]

We're the valves lapped before install? That should help them seal better.