A couple questions about compression testing.

[spyderdrifter]

So I was running a compression test tonight and had a couple questions about reasons for certain procedures. I did do a search, but couldn't find ant conclusive answers since most of the posts on here relate to numbers and how to test it.

So anyway, first off is I know all the how-o's say run the test with a properly warmed engine. Why does it need to be warmed up? If it is done cold, are the numbers inaccurate in comparison to a warmed engine?

Secondly, is the throttle plate being open, absolutely required? While doing my test tonight, I did one cylinder with the throttle open, as well as closed, and got the exact same psi reading. I did this simply to see if there would be a difference in psi.

These questions are simply to seek out more in depth info to be posted for the community. And its all part of my own curiosity. Thanks in advance

 

[3kgt2nv]

How to do a Compression Test

yes the engine needs to be warm so the rings properly expand to seal the cylinder walls.

yes the throttle should be open as the cylinder needs to pull air in to compress

 

[c2ypt1c]

Why does it need to be warmed up?

Thermal expansion. A cold engine can potentially mask leaks.

Secondly, is the throttle plate being open, absolutely required?

A compression test does just that - tests the compression of air in the engine. Opening the throttle lets air enter the engine easier.

EDIT: dang, I posted too slow XD

 

[spyderdrifter]

Thermal expansion. A cold engine can potentially mask leaks.



A compression test does just that - tests the compression of air in the engine. Opening the throttle lets air enter the engine easier.

EDIT: dang, I posted too slow XD

Slow isn't always bad, your answers explained more anyway. And 3kg2nv, don't take offence to that, I actually have the exact page printed up in my reference books. I appreciate your reply regardless


And even though the air flow is better with the plate open, why do I get the same numbers either way? In theory, with the plate closed, I should get a drastically lower number.

 

[c2ypt1c]

And even though the air flow is better with the plate open, why do I get the same numbers either way? In theory, with the plate closed, I should get a drastically lower number.

A post TB leak perhaps, or even residual air in the system...either way a blt is advised (when in doubt, blt)

 

[3kgt2nv]

If you did the test cold it can pull air from other cylinders thru the rings. could also have bent valves or possibly a tb that doesnt sit totally closed.

you could do a wet test and see what happens if you cant drive the car

 

[spyderdrifter]

Hmmm... that wouldn't be good. I'm gonna be getting a leak down tester this week (hopefully), and do that test at work. Wanting to learn all I can before doing it, so thanks for the answers. A post TB leak would most likely be with the gaskets right? During my rebuild, I replaced the intake manifold cadet, but not either of the TB gaskets. Those will get replaced soon, and probably the TB shaft seals too.

 

[3kgt2nv]

leakdown is a much better test to diagnose with than compression. i have seen motors that have bent valves build compression if cranked long or fast enough. leakdown will point out the issues and give you a warm/cold leak down %

a decent ld test kit is about $100 bucks but you do get to use it more than once. i actually test all my motors fairly regularly just to keep an eye on things

 

[c2ypt1c]

A post TB leak would most likely be with the gaskets right?.

Another likely place is an injector seal

PS, you've inspired my new signature

 

[spyderdrifter]

If you did the test cold it can pull air from other cylinders thru the rings. could also have bent valves or possibly a tb that doesnt sit totally closed.

you could do a wet test and see what happens if you cant drive the car

I did do the test cold, which lead me to wondering all this. I know my valves are good. The TB would be my guess for the cause of the possible air leak. I'll be repeating the compression test in the morning with the engine properly warmed up to see if there's a difference, as I'm sure there will be.


Injector seals are another thing on my list of parts to get.

 

[3kgt2nv]

a wet test will seal the rings but a true hot compression test is the best. and yeah the tb paper gaskets suck. the leak even when they are new sometimes.

 

[spyderdrifter]

I've never seen a paper TB gasket. I've always had metal ones. I only have 518 miles on my rebuild, so I really don't want to be doing a wet test..., yet

 

[Tyeler18]

I have no leaks and will pull the exact same numbers open or closed throttle. There is enough air in the manifold that it can build compression just fine. Also a warm test vs cold has gained MAYBE 5 psi at most on any motor I've ever done. It makes some difference but not enough to need to do it hot.

 

[3kgt2nv]

in that case they are definitely bad. the ss ones are once crushed they are done

stock are the paper ones which if your careful can be reused
EXTREME PSI : Your #1 Source for In Stock Performance Parts

also depending on the gasket the crush ring might sit inside the tb with the metal ones

 

[spyderdrifter]

All very good info. Thanks.

 

[3kgt2nv]

np. im recovering from spinal surgery so this is all i can do for now where my car is concerned.

 

[spyderdrifter]

No worries. I think I got some good answers to cure my curiosity for now. Hope you get better.